WiMax Module v11

8/13/2019 WiMax Module v11

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IPlanner WiMax (802.16) ModuleVersion 0.1

Chu Rui Chang

Core RF Engineering

September, 2005


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Table of Contents

Chapter 1 Technology Backgrounds

1.1 OFDM Technology

1.2 Useful Terms and Values1.2.1 Inter-Tone Spacing and Orthogonal Conditions

1.2.2 Cyclic Prefix (CP) and Useful Symbol Time

1.2.3 802.16D and 802.16E Systems

1.2.4 Channel Bandwidth, FFT Size, Sampling Frequency

1.2.5 Channel, Sub-channel, Sub-carrier (tone)

1.2.6 WiMax Frame Durations

1.2.7 WiMax Overheads

1.2.8 OFDM vs. OFDMA

1.2.9 FUSC vs. PUSC in OFDMA

Chapter 2 Details on WiMax (802.16) OFDM & OFDMASystems

2.1 Definition of Signal-to-Interference & Noise Ratio (CINR)

2.2 Resource Allocations: Comparing CDMA, 1xEV-DO, OFDM and OFDMA2.2.1 CDMA / 1xRTT

2.2.2 DO Forward link

2.2.3 WiMAX Forward & Reverse Link

2.2.3.1 802.16 E System (S-OFDMA)

2.2.3.2 802.16 D System (OFDM)

2.3 S-OFDMA Forward Link2.3.1 Forward Link FUSC

2.3.2 Forward Link PUSC

2.3.3 Forward Link Power Allocations

2.4 S-OFDMA Reverse Link

2.4.1 Reverse Link PUSC

2.4.2 Reverse Link Power and Bandwidth Allocations

2.5 Modeling the WiMax Scheduler2.5.1 1xEV-DO Scheduler

2.5.2 WiMax Scheduler

2.5.2.1 Proportional Fairness Scheduler2.5.2.2 Linear Biased Scheduler

2.5.2.3 Other Schedulers

2.5.2.4 Difference between D and E Scheduler

2.6 Handoffs in 802.16 E2.6.1 Hard Handoff

2.6.2 Fast-BTS-Switching (FBSS) Handoff

2.6.3 Soft Handoff


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Chapter 3 How to Model WiMax in iPlanner?

3.0 Items need to be Modeled3.0.1 Call Models

3.0.2 Maximum Supported Number of Active Users per Sector per Carrier

3.0.3 Coverage and Link Budget3.0.4 Per-User and Per-Sector Throughput

3.1 How to model WiMax Forward Link in iPlanner?3.1.1 802.16E S-OFDMA Forward Link & Scheduler

3.1.1.1 What is the maximum achievable forward rate for a single user?

3.1.1.2 Forward throughput with multiple users per sector

3.1.1.3 An example with Frequency Reuse of N=1 (FUSC)

3.1.1.4 An example with Frequency Reuse of N=3 (FUSC)

3.1.2 802.16D OFDM Forward Link & Scheduler

3.1.3 Forward Link Coverage

3.1.4 Forward Link Out-of-Cell Interference for OFDMA

3.1.4.1 Instant Interference Power Density vs. Average Interference PowerDensity within a channel bandwidth

3.1.4.2 Data Activity Factor

3.1.4.3 Forward interference contributed from multiple cells

3.2 How to Model WiMaX Reverse Link in iPlanner?3.2.1 802.16E S-OFDMA Reverse Link & Scheduler

3.2.1.1 What is the maximum achievable reverse rate for a single user?

3.2.1.2 Reverse throughput with multiple users per sector

3.2.2 802.16D OFDM Reverse Link & Scheduler

3.2.3 Reverse Link Coverage

3.2.4 Reverse link Out-of-Cell Interference for OFDMA

3.2.4.1 Instant Interference Power Density vs. Average Interference PowerDensity within a channel bandwidth

3.2.4.2 Reverse interference contributed from multiple cells

3.3 How to Model Handoffs for 802.16 E in iPlanner?3.3.1 Hard handoff

3.3.2 FBSS handoff

3.3.3 Soft handoff

Chapter 4: IPlanner WiMAX Module Roadmaps

4.1 First Release of WiMaX Module4.2 Future Releases of WiMaX Module

References


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Chapter 1 Technology Backgrounds

1.1 OFDM Technology

One of the major advantages of OFDM (Othrogonal-Frequency-Division-Multiplex) isthat it is extreme robust under multipath environment.

The basic operating principle is that the transmit channel is divided into a large number(N>>1) of parallel sub-channels. The data stream from the source is split into each sub-

channel. Because of this 1:N split, the data rate of each sub-channel becomes 1/N as the

main string, consequently, the symbol duration in the sub-channel becomes N timeslonger than the symbol duration from the source. Also each sub-channel is transmitted via

a very narrow bandwidth so the fading is basically flat within the sub-channel (no

frequency-selective fading). The longer symbol duration and flat fading make OFDM

very robust under multipath fading with little or no inter-symbol-interference (ISI), as

shown in Fig. 1.

largeT

smallT

0

11

T 2T

Channel Input

Channel Output

0 T 2T

0 T 2T

Two-ray model = rms delay spread2

Delay

Received

Power

T small negligible intersymbol interference

large significant intersymbol interference,which causes an irreducible error floor

T

largeT

smallT

0

11

T 2T

Channel Input

Channel Output

0 T 2T

0 T 2T

largeT

smallT

0

11

T 2T

Channel Input

Channel Output

0 T 2T

0 T 2T


Delay

Received

Power


Delay

Received

Power

T small negligible intersymbol interference

large significant intersymbol interference,which causes an irreducible error floor

T

Fig. 1 Time domain Inter-Symbol-Interference (ISI) caused by delay spread

This can be illustrated by an example. Assume the data rate of the main data stream is 1

Mbps. If this data stream is transmitted via one carrier, the bit duration in this channel


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will be 1*10^(-6) sec. If the multipath delay spread is comparable to 1*10^(-6) sec, at the

receiver, the symbols from the multipath will be superimposed on top of the symbols inthe main path, causing inter-symbol-interference (ISI).

On the other hand, if the carrier is divided into 1000 sub-carriers (in parallel, as shown in

Fig. 2), then each sub-carrier will only be transmitting 1/1000 of the data rate, i.e., 1 kbps(1000 sub-channels adding together results 1 Mbps). The bit duration of each sub-carrier

will become 1000 times longer, or 1*10^(-3) sec. Then the multipath delay spread will

have to be 1000 times larger in order to cause the same level of ISI.

Serial

To

Parallel

Sub-Carrier #N, Rate = R/N

Sub-Carrier #1, Rate = R/N


Data Carrier

Rate = R

Serial

To

Parallel

Sub-Carrier #N, Rate = R/N



Data Carrier

Rate = R

Fig. 2 Splitting a carrier into N (N>>1) parallel sub-carriers, data rate from each

sub-carrier will be 1/N as much as the main carrier, and the symbol duration will be

N times longer.

Conceptually, the OFDM transmitter and receiver are equivalent to many narrow-band

transmitters and many narrow-band receivers operating in parallel, as show in Fig. 3.

Note that each narrow-band receiver has a channel filter that only allows the signalenergy within this channel to go through. Therefore, the signal-to-noise ratio (SNR), or

carrier-to-interference and noise ratio (CINR), of OFDM is defined as SNR per sub-

carrier, or SNR per tone1.

However, an OFDM transmitter and receiver in real-life do not really work like Fig. 3. It

is only conceptually equivalent to Fig. 3.

There are two disadvantages of the schemes in Fig.3:(1)Requires a large number of narrow Tx filters and Rx filters (filter banks) at the

transmitter and receiver. If a system were really implemented like this, it wouldbe too expensive and too complicated

1Because the bandwidth of one sub-carrier is so narrow, one often calls a sub-carrier as one tone. From

now on, a tone or one sub-carrier is used interchangeably.


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(2)Spectrally inefficient, because no filters can be infinitely sharp, so the guardbands between the channels must be sufficiently large (i.e., there must besufficient frequency separations between f1 and f2, between f2 and f3, ) in

order to avoid adjacent channel interference (like AMPS channels)

Fig. 3 Conceptually, OFDM transmitter and receiver are equivalent to many

narrow-band transmitters and many narrow-band receivers operating in parallel.

The beauties of OFDM are that:

(1)It does not need the complicated and expensive filter bands, instead digital-signal-processing techniques (FFT) are used so that under idea condition there will be no

adjacent channel interference from neighboring sub-carriers, as if the system doeshave very sharp filter bands.

(2)Not only one does not need guard band between the sub-carriers, the sub-carrierscan overlap (Fig.4). If fact, as long as the frequency separation between adjacent

sub-carriers satisfy the orthogonal relation (in Fig.5 and defined in Eq.[1]), thereis no adjacent channel interference between sub-carriers (these sub-carriers are

said to be orthogonal to each other).

RF

Transmitte

R/N b/s

D(t)

f0

f1

fN-1

dN-1(t)

QAM filter

R/N b/sQAM filter

R/N b/sQAM filter

(t)d0

(t)d1

Bandlimitesignal

f0 f1 f2

f0

fN-1

f1

N-1

Receive

RF

filter

QAM

QAM

QAM

filterf1

f0

filter

fN-1


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serial

to

parallel

converter

QAM RF

f0

f1

fN-1

Time-Limited

Signals

(Block Processing)

R b/s

d(n) D(t)

d(0)

d(1)

d(N-1)

OFDM increases the spectral efficiency by allowing subchannels

to overlap.

ff0 f1 f2 f3 f4 f5

}t]D(t) = Re d(n) exp [-j nn=0N-1

How do we separate the subchannels in the receiver?f

f serial

to

parallel

converter

QAM RF

f0

f1

fN-1

Time-Limited

Signals

(Block Processing)

R b/s

d(n) D(t)

d(0)

d(1)

d(N-1)

OFDM increases the spectral efficiency by allowing subchannels

to overlap.

ff0 f1 f2 f3 f4 f5

}t]D(t) = Re d(n) exp [-j nn=0N-1

How do we separate the subchannels in the receiver?f

f

Fig. 4 A basic OFDM Transmitter. The adjacent sub-carriers are allowed to overlap,

thus results much higher spectrum efficiency.

A guard in terval can vi rtually eliminate ISI

Subchannel separation choose fn = f0 + nf, with f = in tegrate over NT, then d(m) = d(m)

1

NT^

(or, interblock interference) lower spectralor power efficiency.

parallel

toserial

converter

QAM

f0

f1

fN-1

d(0)

d(1)

d(N-1)

RF

parallel

toserial

converter

QAM

f0

f1

fN-1

d(0)

d(1)

d(N-1)

RF

Fig.5 A basic OFDM Receiver, and the orthogonal condition


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(3)Fig.6 shows the comparison between OFDM and conventional multi-carrierdeployment. The adjacent channels of a conventional narrow-band system cannever overlap or intolerable adjacent-channel-interference will result. For

example, in AMPS system, a 7/21 frequency re-use will have adjacent channels

21 AMPS channels apart. However, adjacent sub-carriers of OFDM do overlapand no adjacent channel interference from sub-carriers (orthogonal).

Fig. 6 Comparison between OFDM and conventional multi-carrier modulation.

Conventional multi-carrier modulation must have guard bands between carriers.OFDM allows adjacent sub-carriers to overlap.

Because many narrow sub-carriers are involved, the overall transmit spectrum of one

OFDM carrier looks like the Tx spectrum of a spread-spectrum CDMA system (Fig.7,

Fig. 8)

Fig. 7 The power spectrum of OFDM signal looks like a spread spectrum CDMA

spectrum.


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Fig. 8 Measured OFDM Tx spectrum from BTS.

Although the Tx spectrum of one OFDM carrier looks like Tx spectrum of a CDMA

carrier, the two systems do not operate the same way.

In CDMA/DO, every channel occupies 1.25 MHz of spectrum. Each channel / usermay only use a different fraction of power (power control) or a different fraction of

service time (time division in DOs forward link), but the channel bandwidth is fixed.

In OFDM, each channel / user can use different bandwidth (i.e., a different number oftones), different power, and different percentage of service times. So OFDM system

has a higher degree of freedom. This makes OFDM more flexible and can be better

optimized for different types of services, but it also makes OFDM system morecomplicated to model.

Note that OFDM itself is a special kind of Frequency-Division-Multiplex (except it isOrthogonal Frequency-Division-Multiplex). But one can add further Time-Division or

Code-Division or both on top of OFDM, making it a Time-Division OFDM or a Code-

Division OFDM, or a Time and Code Division OFDM. OFDM can also work by itselfwithout further TDM or CDM.

Similar to Code-Division-Multiple-Access (CDMA) and Time-Division-Multiple-Access

(TDMA), for OFDM, there is also OFDMA, which is what 802.16e (the mobility versionof WiMax) uses. WiMax Module of OFDMA should use OFDMA.


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1.2 Useful Terms and Values

In this section, some of the key parametersused in WiMax are defined. Some of them aregeneric to OFDM, but others are specific to WiMax.

1.2.1Inter-Tone Spacing and Orthogonal Conditions

An OFDM system must satisfy the following condition in order for the tones to beorthogonal

[1]F

T

=1

, orT

F1

=

where T = time duration of an OFDM symbol (useful portion, to be defined later), and

F = frequency separation between two neighboring tones, as shown in Fig. 9.

Fig. 9 Orthogonal Condition: Relation between Inter-Tone Spacing and Symbol

Duration.

Orthogonal condition is general to OFDM. That is, any OFDM system must satisfy this

condition.

1.2.2 Cyclic Prefix (CP) and Useful Symbol Time

Cyclic-Prefix (CP) is a guard time within an OFDM symbol, with CP the actual useful

symbol duration CPSymboluseful TTTT == (Fig. 10).


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CP is actually a copy of the last portion of the data symbol appended to the front of the

symbol during the guard interval. The main purpose of adding CP to OFDM symbols isto combat the effect of multipath. Without CP, multipath will mess up the orthogonal

condition and cause Inter-Symbol-Interference (ISI). With CP, as long as the magnitude

of multipath delay < CP duration, there will be no ISI. Therefore, the duration of CP must

be designed to be > maximum multipath delay, which is typically around 10microseconds. However, the amount of overhead increase with the increase of CP (thus

useful symbol time reduces), therefore it is important to optimize the CP: large enough to

cover the maximum multipath spread, but small enough to not significantly reduce theuseful symbol time.

Fig. 10 OFDM Symbol Duration, CP Duration and Useful Symbol Duration

The total symbol duration T_symbol= (T_CP+ T_Useful) = T_Useful * (1 + T_CP/ T_Useful). CP

is general to all OFDM systems, however, 802.16 standard defines 4 different (T_CP /T_Symbol) values for WiMax:

(T_CP/ T_Useful) = 1/32, T_Symbol = 1.03125 * T_Useful

(T_CP/ T_Useful) = 1/16, T_Symbol = 1.0625 * T_Useful

(T_CP/ T_Useful) = 1/8, T_Symbol = 1.125 * T_Useful(T_CP/ T_Useful) = 1/4, T_Symbol = 1.25 * T_Useful

Depends on the multipath characteristics, usually values or 1/16 to 1/8 are most commonin WiMax. In iPlanner, use 1/8 as the default value, but do allow users to use other values

defined in the standard.

1.2.3 802.16 D and 802.16 E Systems

There are two major WiMax systems:

(1) 802.16 Dsystem (also called 802.16-2004) is for fixed and nomadic terminals only.

There is no handoff capability. That means although a terminal can move (withreasonable speed up to 40 ~ 50 km/hr), the connection can be maintained only if the


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terminals stay within the same cell. Once terminals move out of the sector, the connection

must drop. The terminal will re-scan for new frequencies and re-establish a newconnection with the new sector, but the call will be interrupted.

(2) 802.16 Esystem is for mobility networks. Vehicle traveling speed can be over 120

km/hr and the system supports hard handoff, fast-switching handoff and soft handoff, just

like cellular terminals.

The E system is not backward compatible with D system. D system uses 256-tone OFDM.

The E system purposely avoids 256-tone option, so it does not have to be backward

compatible with D. E-System uses 128-, (no 256), 512-, 1024 or 2048-tone OFDMA.

The difference between OFDM and OFDMA is explained in 1.2.8.

1.2.4 Channel Bandwidth, FFT Size, Sampling Frequency

802.16 D System (OFDM)

There are three sets of channel bandwidth values in the D system:

The first set contains a set of channel bandwidth values that are multiple of 1.5 MHz, i.e.,

1.5 MHz 3 MHz (= 2x1.5 MHz) 6 MHz (= 4x1.5 MHz) 12 MHz (= 8x1.5 MHz) 24 MHz (= 16x1.5 MHz)

The second set of channel BWs that are multiple of 1.75 MHz:

1.75 MHz 3.5 MHz ( = 2x1.75 MHz) 7 MHz ( = 4x1.75 MHz) 14 MHz (=8x1.75 MHz) 28 MHz (= 16x1.75 MHz)

The third set of channel bandwidth values that are multiple of 2.5 MHz:

2.5 MHz 5 MHz (= 2x 2.5 MHz) 10 MHz (= 4x 2.5 MHz) 15 MHz (= 6x 2.5 MHz)


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The number of tones per channel is called FFT2Size N_FFT. For D-system (OFDM), the

N_FFT is always 256, taking out the DC tone and 55 guard tones, N_used = 200 for allchannel bandwidth values (Fig. 12). The bandwidth of each tone is = (Channel

Bandwidth) / 200. Since the N_FFTis fixed, the wider the channel bandwidth, the wider

the tone bandwidth.

The following table summarizes common channel bandwidth for OFDM.

802.16 E System (OFDMA)

The most commonly used channel bandwidth values in E system are multiple of 1.25MHz (except there is no 2.5 MHz in E system):

1.25 MHz 5 MHz (= 4x 1.25 MHz) 10 MHz (= 8x 1.25 MHz)

2FFT = Fast Fourier Transform.


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20 MHz (= 16x 1.25 MHz)

For E system, the N_FFTsize scales with the channel bandwidth (the so-called Scalable-OFDMA), that means the wider the channel bandwidth, the higher the N_FFT (by the

same ratio), so the bandwidth of each tone is always the same regardless of channel

bandwidth.

For 1.25 MHz channel bandwidth, it uses 128 tones (N_FFT= 128) For 5 MHz channel bandwidth, it uses 4x128 tones (N_FFT= 512) For 10 MHz channel bandwidth, it uses 8x128 tones (N_FFT= 1024) For 20 MHz channel bandwidth, it uses 16x128 tones (N_FFT= 2048)

WiMax (D or E) uses a sampling factor of 8/7. The inter-tone spacing for a given channel

bandwidth and N_FFTis:

[2]FFTN

BWChannel

FloorF

80008000

_

7

8

=

In E system, first the channel bandwidth is chosen. From the channel bandwidth onedetermines FFT size. From equation [2] and [1] the symbol duration can be calculated.

The following table gives the common parameters for 802.16 E, which should be used as

default values for iPlanners 802.16 E Module:

Table 1: Common 802.16E OFDMA parameter values

ChannelBW [MHz]

N_FFT(# of Tones)

F [kHz](Inter-ToneSpacing)

T [msec](UsefulSymbol

Duration)

T_Symbol[msec]

(Total Symbol

duration when

1/16 CP used)

T_Symbol[msec]

(Total Symbol

duration when

1/8 CP used)

1.25 128

5 512

10 1024

20 2048

11.16 kHz 0.0896 msec 0.0952 msec 0.1008 msec

As was mentioned previously, for Scalable-OFDMA, the inter-tone spacing F and thesymbol duration T are both constant regardless of the channel bandwidth.

1.2.5 Channel, Sub-Channel, Sub-Carrier (Tone)

This is a summary of what was discussed in previous sections.


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Channel: The largest frequency unit. One channel bandwidth can be Nx1.25 MHz (for E

systems), or Mx1.5MHz, Jx2.5 MHz or Kx1.75 MHz (for D systems). N, M, J, K areintegers starting from 1.

For D-system (OFDM), one channel always contains 256 tones.

For E-System (S-OFDMA), one channel contains many sub-channels, each sub-channel in turn contains many sub-carriers (tones).

Sub-Channel: Only used in E-system (S-OFDMA) and in reverse link of D system with

sub-channelization. The forward link of D-system does not use sub-channels.

Sub-channel is the finest frequency unit that can be allocated to a user by the scheduler.

That means one or more sub-channels can be allocated to a user, but not a fraction of asub-channel. One sub-channel contains multiple sub-carriers (tones). The number of

tones per sub-channel is different for different sub-carrier allocation mode, which will be

discussed later. The tones belonging to the same sub-channels are not necessarily

adjacent. Using adjacent tones has no advantage (except may be conceptually simpler)because it offers no frequency diversity. To provide frequency diversity, which is

important under mobility condition, the tones should be randomlized. 802.16 E-sysytemuses a permutation scheme to randomlize the tones for each sub-channel.

Sub-Carrier (Tone): The finest frequency unit in OFDM or OFDMA. Inter-tone spacingand symbol duration must satisfy Eq.[1] for all tones within a channel to be orthogonal.

Note that

For S-OFDMA, channel bandwidth may vary from 1.25 MHz all the way up to 20MHz, but the N_FFTscales with the channel bandwidth (as shown in Table 1) so the

bandwidth of a sub-channel and the bandwidth of a sub-carrier will never change. Thebandwidth of a sub-carrier must satisfy Eq.[1], which is fixed for the fixed symbol

duration. For a given sub-carrier allocation mode, the number of tones per sub-

channel is also fixed, which means the bandwidth of a sub-channel is fixed.

For OFDM, it is N_FFT= 256 that is fixed. If the channel bandwidth increases, thebandwidth of a tone increases with the channel bandwidth.

1.2.6 WiMAX Frame Durations

WiMAX defines 7 frame duration values, from 2.5 ms to 20 ms, as shown in Table 2 (D

or E has the same set of frame durations, as defined in Table 232 from the 802.16Dstandard or Table 384b from the 802.16E standard). Frame duration of 5 ms is mostcommonly used. Therefore, 5 ms should be the default value for iPlanner, however, do

allow users to use other values defined in Table 2.

The details of WiMax frame structure will be discussed in the next chapter, this chapter

only defines the common parameters and list their values.


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Table 2 Common parameters defined in WiMax

1.2.7 Overheads

Since WiMax resources has two dimensions, Frequency and Time, we discuss the % of

overhead in each dimension.

Overheads in Frequency Domain

We first discuss the E system, then discuss the differences between D and E systems.

For E-system, although each OFDMA channel can contain 128, 512, 1024 or 2048 tones,

not all of these tones can be used to carry traffic. Traffic tones = Total # of tones

(Guard Tones) (overheads tones). The tones can server different purposes:


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(1) Guard Band: In order to satisfy the out-of-channel emissions, there must be guard

bands on both sides. That means some of the tones located on both edges must be turnedoff and cannot transmit. The actual number of tones that are actually on is therefore =

(total # of tones) (# of guard tones).

(2) There is a DC tone in the down link.

(3)Pilot Tones. Under mobile environment, mechanisms are needed to provide time andfrequency domain synchronization and track the frequency shift (due to Dopplereffect). WiMax uses pilot tones for both forward link and reverse link (Fig. 11).

(4)When calculate power per tone, one should use N_used. When calculate throughput,one should use number of traffic tones.

Fig. 11 gives an example illustrating tone-allocations for guard band, pilot and data

traffics for E-system (OFDMA).

Fig. 11 Overheads in Time Domain (E-System). Only those black arrows are traffic

tones.

E-System also added the option of Full-Usage of Sub-Carriers (FUSC) and Partial-Usageof Sub-Carrier (PUSC). Depends on different channel bandwidth and whether it is FUSCor PUSC, the % of overheads are different. The details will be discussed in the next

section.

Fig. 12 gives channel structure in frequency space for D-system. The total number of

tones that are on = 256 56 = 200 = N_used. Taking out 8 pilot tones, the total number

of data tones = 192.


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Fig. 12 Pilot, Guard Tones and Traffic tones in 802.16D-System. The total data

tones are divided into 16 sub-sets, which can form 16 sub-channels in the reverse

link (12 data tones each).


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Overheads in Time Domain

As was mentioned previously, the most common frame duration is 5 msec. Within this 5

msec, part of the frame must be used for other purpose, only a portion of the 5 ms can be

used for carrying traffic.

(1) TDD or FDD: WiMAX can support Frequency-Division-Duplex (FDD) or Time-

Division-Duplex (TDD).

FDD requires a paired spectrum allocation, one for up link, one for down link. Forthis reason, many operators do not prefer FDD option. However, for FDD, adown-link (DL) frame is 100% for DL transmission, an up link (UL) frame is

100% for UL, no guard time is needed so bandwidth is used more efficiently.

Also the time synchronization for FDD is not so critical.

TDD only requires one single block of frequency, for both UL and DL. For thisreason, many operators prefer TDD option. However, for TDD, a frame is divided

into DL sub-frame and UL sub-frame.o There must be a guard time between DL sub-frame and UL sub-frame,

called TTG; and another guard time between the UL sub-frame and DL

sub-frame, called RTG. During these guard times neither BTS nor mobileis transmitting.

o These guard times serve two purposes: (a) allow the transmitter to haveenough time to turn on or turn off; (b) take care of the effect of

propagation delay so the Tx energy from a remove transmitter will notcause interference to a receiver (Fig. 30).

o Time synchronization for TDD system is very critical. Without time

synchronization, severe BTS-to-BTS interference or mobile-to-mobileinterference will occur if one BTS / mobile is transmitting but the otherBTS / mobile is receiving. As a result, GPS is almost always required for

TDD ssytems.

For TDD option, part of the frame is used for TTG (typically 0.2 ms) and part offrame is used for RTG (typically < 0.1 ms). The steps to determine TTG + RTGare: first determine symbol duration, then determine # of symbols needed for

overhead, # of symbols needed for DL and UL sub-frames (there are some rules

which will be discussed later), then

(TTG + RTG) = (Frame Duration) (# of DL Symbols) (# of UL Symbols)

(2) Preamble, DL_MAP, UL-MAP, etc are all overhead. In additional to carry data traffic,

a frame must also carry preambles and other control / signaling messages. All of these are

considered overheads and must be subtracted from the total frame duration to get the %of time a frame is actually carrying data traffic. Fig. 13 shows an example in OFDMA,

how a TDD frame is divided between data traffic and overheads (only areas that are

covered by bursts are used for carrying data traffics, all others areas are overheads).Note that:


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The DL control channels (e.g. DL MAP, FCH) are shared. Also UL controlchannel (UL_MAP) is in DL sub-frame, not in UL-sub-frame. However, UL sub-

frame has a control channel for UL Ranging.

The sizes of DL-MAP and UL-MAP are flexible, unlike CDMA where the controlchannels are fixed sizes with dedicated Walsh codes assigned.

No hard constraint on the number of DL/UL bursts assigned per frame

Dynamic Resource Allocation is a purely frame-by-frame resource allocation, i.e.no dedicated traffic channels on either DL or UL even for real-time services

There is no dedicated control channel for UL resource request. UL resourcerequest is performed by contention-based random access.

o It is costly to assigned dedicated control channel for OFDMA since eachdedicated control channel will occupy hard overhead in terms of sub-carriers and OFDMA symbols, regardless whether there is request to be

sent.

Fig. 13 Overheads in Time Domain for OFDMA. Only those areas covered by

bursts are for carrying data traffics.

Fat-pipe operation is similar to 1xEV-DV/DO or HSD(U)PA

o The exception is 802.16e has additional degrees of freedom in frequencydomain, OFDMA symbol domain and spatial domain. The tradeoff is between

scheduling flexibility and control signaling overhead.

Each frame can be dynamically partitioned into different permutation zones, e.g.PUSC, FUSC, AMC etc.


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o This allows for better inter-sector interference control by applying PUSCmode for MSS in cell edge, and for taking advantage of frequency selectivefading using AMC mode for MSS in low speed.

Fig. 14 shows an example in OFDM of how a TDD frame is divided between data traffic

and overheads. Again, only DL Bursts and UL Bursts are used for carrying DL and ULdata, all the rests are overheads.

Fig. 14 Overhead in Time Domain for OFDM.

1.2.8 OFDM vs. OFDMA

The main difference between OFDM and OFDMA is sub-channelization, OFDM does

not support sub-channelization; OFDMA does.

The forward link of D system uses OFDM. The reverse link uses OFDMA if it supports

sub-channelization. For iPlanner, we can assume all mobiles support sub-channelization

feature in the reverse link.

Both the forward link and reverse link of E system use Scalable-OFDMA.

The forward link of 802.16D system does not use sub-channelization. The entire 256-tonechannel (minus guard tones) must be used. Therefore, modeling the forward link of D


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system is very similar to modeling the forward link of 1xEV-DO. When multiple users

exist in a sector, the scheduler time divides the channel to serve each user.

D systems reverse link can use OFDM, just like the forward link. But due to the

limitation of handset Tx power level, most of time reverse link coverage is limited. For

this reason, most D systems use sub-channelization, i.e., dividing one channel into 16sub-channels.

If the handset is located very close to the BTS, it can use the entire 16 sub-channelsso the reverse throughput is maximized;

If the handset is located far from the BTS, and if all sub-channels are used, the C/I pertone may become too low to support even the lowest rate. In this case, the handset can

concentrate the power to a smaller number of sub-channels, which increases theenergy per sub-channel. As a result, the mobile can move farther away and still be

able to have service.

In effect, sub-channelization is trading reverse throughput for extra coverage. The

smaller the number of sub-channels used, the thinner the pipe, the lower the reversethroughput, but the higher the C/I per tone so the longer the reverse range. If all

handset power is concentrated in only one sub-channel, the power density per sub-channel is increased by 16 times compared to the case when all 16 sub-channels are

used, so the range can be significantly extended. Of course, the reverse throughput is

also dropped because the pipe size drops to 1/16 as much as before.

Due to the capability of sub-channelization, each user may use a different bandwidth

value (different number of sub-channels), therefore, it becomes OFDMA instead ofOFDM. OFDMA adds one more dimension to the reverse link scheduler:

(1)Bandwidth used by each user (# of sub-channels used)

(2)% of service time allocated to each user

(3)The peak rate achieved for this particular user

The 802.16 E-system uses scalable-OFDMA. For E systems, both the forward andreverse scheduler work with three dimensions (1) (3) mentioned above.

In summary:

An OFDM scheduler is similar to a DO scheduler, the only variable to be scheduled is% of service time.

An OFDMA scheduler must add one more variable, that is, the % of channelbandwidth allocated to each user.

1.2.9 FUSC and PUSC

The 802.16 E (S-OFDMA) defines two kinds of sub-channel allocation modes: Fully-

Used-Sub-Channelization (FUSC) and Partially-Used-Sub-Channelization (PUSC).


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FUSC: All sub-channels are allocated to the transmitter. FUSC is used for DL only.

PUSC: Only part of the sub-channels are allocated to the transmitter. UL must use PUSC.

DL may use PUSC or FUSC.

Note that FUSC or FUSC are defined on the transmitter side only.

D system always uses frequency-reuse of 3 (N=3). It cannot support N = 1 because itcannot do soft handoff. In this case, the total available spectrum is divided into 3 sets, one

for alpha-sector, one for beta-sector and one for gamma-sector. There is no concept of

PUSC or FUSC in D system.

However, in E system there is an option for PUSC or FUSC. It is very confusing which

one corresponds to N =1 and which one corresponds to N = 3.

N = 1 will require soft handoff and / or Fast-BTS_Switch (FBSS) handoff, otherwisecell edge performance will not be acceptable. Initially E system may or may not havethese handoff capabilities, so it is highly likely that N = 1 is not supported initially.

A true N = 1 is deployed by FUSC. As an example, if the channel bandwidth is 5MHz, every sector uses the same channel of 5 MHz, FBSS and soft handoff must be

supported. The total spectrum usage is 5 MHz. This situation is very similar to UMTS.Except the reverse link always use PUSC, and the maximum total bandwidth the

reverse link can use is 5 MHz, the same as forward link.

FUSC can also be used to deploy a true N = 3. As an example, if the bandwidth of

one channel is 5 MHz, there will be a total of 15 MHz of spectrum required. This 15MHz of spectrum is divided into three 5 MHz channels (F1, F2 and F3). Alpha-sectoruses F1, Beta-sector uses F2 and Gamma-sector uses F3. Hard handoff must be used

for mobile to move from one sector to the other. Reverse link always use PUSC. Thechannel bandwidth used by the reverse link is the same as the channel bandwidth used

in forward link, i.e., if the forward link is using F1, then reverse link will also use F1,

etc.

Forward link PUSC causes most confusion. Some time people call it virtual- orpseudo- N = 1, others call it virtual- or pseudo- N = 3, depends on how one looks

at it.

o From a channel usage point of view, PUSC should be considered as N = 1,because there is only one channel used, and the cells can support FBSS andsoft handoff. Reverse link theoretically can use the entire channel bandwidth.

So if one looks at it from the channel usage point of view, it is N = 1 because

every sector uses the same channel.

o However, from a sub-channel usage point of view, PUSC should beconsidered N=3 because in the forward link, total sub-channels of the BTS


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are divided into three disjointed sub-sets. One subset is for alpha-sector, one

for beta-sector and one-for gamma-sector. For this reason, the total number ofsub-channels in forward link PUSC is always a multiple of 3, as shown in

Table 5. The fact that the neighbor sectors do not use the same set of tones

makes the PUSCs forward link interference like the situation of N = 3.

Note that channel configuration may change with time within a frame. The standarddefines many possible zones (in time dimension) within a frame. The DL sub-channeloverhead (in time dimension) must always be PUSC. However, forward data traffic

portion of the sub-frame can be PUSC, FUSC, AMC, etc. UL sub-frame must start

with PUSC, the rest part can be PUSC or AMC, as shown the following figure.

Fig. 15 Mandatory Zones and Optional Zones in a frame.


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Chapter 2 Details on WiMax (802.16) OFDMA

System

2.1 Definition of Signal-to-Interference+Noise Ratio

(CINR)

In WiMax, the CINR is defined on a per-tone basis (Fig. 16).

Frequency

Power Density

Thermal Noise Density

Out-of-Cell Interference

BW of one Tone

BW of one ChannelFrequency

Power Density

Thermal Noise DensityThermal Noise Density

Out-of-Cell InterferenceOut-of-Cell Interference

BW of one Tone

BW of one Channel

Fig. 16 Definition of CINR in WiMax. Thermal noise density is a constant across

the channel. However, out-of-cell interference will not likely be a constant across the

channel.

Signal is received in-cell energy (from the home BTS or from the home mobile)within one traffic tone (for scalable-OFDMA, bandwidth = f = 11.16 kHz as given

in Table 1)

Noise is total thermal noise energy within one tone (11.16 kHz)

Interference is the sum of all energies from out of cell within one tone.

o On the reverse link, this is the sum of all out-of-cell interference contributedfrom all mobiles Tx power from neighboring cells. No in-cell interference(Fig.37).

o On the forward link, this is the sum of all out-of-cell interference contributedfrom the Tx power of all neighboring BTSs (Fig. 35).


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This C/I definition is very different from the C/I definition in CDMA or DO, in whichboth signal and interference and noise are all defined on a per 1.25 MHz basis.

Another important difference is that

CDMA only has orthognality in the forward link (via Walsh codes) within the celland within the same multipath. That is, forward link energies within the same cell and

the same multi-path are orthogonal. Out-of-cell energy are interference. In-cellenergies that do not belong to the same multi-path (with path delay differences > 1

chip) are also interference.

There is no orthognality in the CDMAs reverse link. All mobile Tx power from othermobiles, whether they are in-cell mobiles or out-of-cell mobiles, are interference.

OFDM and OFDMA has orthogonality in both forward link and in reverse link. There

is no multi-path interference as long as Cyclic-Prefix (CP) > Multi-path delay. Thatmeans Tx power within the cell does not contribute to interference. Only energy from

out-of-cell causes interference.

OFDM and OFDMA can achieve much higher CINR than CDMA or DO. Themaximum C/I value a DO system can achieve is about 13 dB due to the finite Rho

Factor. The OFDM/OFDMA can achieve 30+ dB CINR.

The following table shows the simulation result under AWGN of achieved peak rate per

tone as a function of CINR:

Table 3 Simulation Results of Achieved Peak Rate per Tone vs. CINR

CINR [dB]

[Stationary]

CINR [dB]

[Mobility](0.7 dB

Penalty)

Modulation

Order

Coding

Rate

Bits carried

per unit-area**

Rate

contributed byone unit-area in

5 ms Frame

-3.59 -2.89 2 (QPSK) 1/5 2/5 0.08 [kbps]

-2.43 -1.73 2 (QPSK) 1/4 1/2 0.1 [kbps]

-0.84 -0.14 2 (QPSK) 1/3 2/3 0.133 [kbps]

1.26 1.96 2 (QPSK) 1/2 1 0.2 [kbps]

3.21 3.91 2 (QPSK) 2/3 4/3 0.266 [kbps]

4.14 4.84 2 (QPSK) 3/4 3/2 0.3 [kbps]5.17 5.87 2 (QPSK) 4/5 8/5 0.32 [kbps]

6.75 7.45 4 (16QAM) 1/2 2 0.4 [kbps]

9.16 9.86 4 (16QAM) 2/3 8/3 0.53 [kbps]

10.25 10.95 4 (16QAM) 3/4 3 0.6 [kbps]

11.44 12.14 4 (16QAM) 4/5 16/5 0.64 [kbps]

13.95 14.65* 6 (64QAM) 2/3 4 0.8 [kbps]

15.68 16.38* 6 (64QAM) 3/4 9/2 0.9 [kbps]


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16.80 17.5* 6 (64QAM) 4/5 24/5 0.96 [kbps]

*: 64 QAM can be achieved under stationary condition; it is unlikely achieved under mobility

condition

**: One unit-area = (one tone) * (one symbol)

Table 3 is based on 802.16 E system using S-OFDMA, i.e., the bandwidth of one tone =11.16 kHz (Table 1). For 802.16 D system, the bandwidth of one tone scales with the

channel bandwidth, so the values from Table 3 cannot be directly used.

However, note that Table 3 is based on Shannons channel capacity formula, which says

that the rate supported by one channel is proportional to the channel bandwidth, and the

rate increases as the channel SNR increases, using the following relation (Equation [3]):

[3]

+=

N

SBRate 1log*

Therefore, for the same SNR value, the wider the channel bandwidth, the higher thesupported data rate, and vice versa. If the bandwidth of one tone in D system is different

from 11.16 kHz, the supported rate per-tone per-symbol can be obtained by a simply

equal-area approach:

To achieve the orthogonal condition, Eq.[1] says that the symbol-duration T mustinversely scale with the tone bandwidth F [= (Channel Bandwidth) / 256]. If thechannel bandwidth increases, the symbol-duration must decrease accordingly.

One unit-area is (one tone = F ) * (one symbol=T). Assume for E-system the unit-area is A_e, for D-system the unit-area is A_d, if the peak rate supported per unit-area

for E-system is known (=R_e), then the peak rate per unit-area for D-system can be

calculated as:

E

E

D

D RA

AR =

A simple example3can be used to illustrate the above equal-area scaling.

o Assume for X-system the tone bandwidth F = 10 kHz, and symbol-durationT = 0.1 ms. A frame-duration = 2 ms, so one frame-duration can contain 20

symbol-durations. A 1 MHz channel contains 100 tones. A frame of 1 MHz

by 2 ms can contain 100*20 = 2000 (unit-areas)

o Assume for Y-system the channel bandwidth is twice = 2 MHz, but N_FFTisthe same. So the tone bandwidth F = 20 kHz. To maintain orthogonalcondition, the symbol-duration becomes half as before: T = 0.05 ms. The

frame-duration is the same = 2 ms. With everything else being equal,

according to Shannons theory, the throughput per frame from Y-system

3This is for illustration purpose only. The values used here have nothing to do with the values used in a

real WiMax System.


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should be twice as much as the throughput per frame for X-system. A frame-

duration = 2 ms, so one frame-duration can now contain 40 symbol-durations.A 2 MHz channel contains 100 tones. A frame of 2 MHz by 2 ms can contain

100*40 = 4000 (unit-areas)

o Since one unit-area of (one tone)*(one symbol) in X-system is the same asone unit-area of (one tone)*(one symbol) in Y-system, using equal-area

argument, both unit-areas support the same data rate. Therefore, the

throughput per frame in Y-system, containing twice as many unit-areas, is

exactly twice as much as the throughput per frame in X-system, as indicatedby Shannons theory. The following figure illustrates the concept.

Frame Duration = 2 m s Frame Duration = 2 ms

Channel

BW

=1 MHz

Channel

BW

=2 MHz

One unit-area of (1 tone)*(1 symbol) in X-system

=

One unit-area of (1 tone)*(1 symbol) in Y-system

=

X System

Y System


Channel

BW

=1 MHz

Channel

BW

=2 MHz


=


=


Channel

BW

=1 MHz

Channel

BW

=2 MHz


=


=


Channel

BW

=1 MHz

Channel

BW

=2 MHz


=


=

X System

Y System

Fig. 17 Illustration of equal-area scaling. Y-System having twice as much channel

bandwidth should support twice as much throughput.

Table 3 specifies the peak rate achieved per unit-area in 5 ms frames. To obtain what is

the achieved throughput per user, one has to calculate how much total area (i.e., howmany symbols per second and how many traffic tones) this user can get. This will depend

on what % of resources allocated by the scheduler to this user, and for a particular

channel configuration. These details are discussed in Chapter 2.5. The picture will

become clear after the discussions of channel structures (Chapter 2.3 and 2.4) and thediscussions of forward and reverse schedulers (Chapter 2.5).


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2.2 Resource Allocations: Comparing CDMA, 1xEV-

DO and OFDM and S-OFDMA

2.2.1 CDMA / 1xRTT

There is only one dimension in order to model a CDMA/1xRTT voice link resource, that

is, power. CDMA technology uses power control.

Each CDMA link will always occupy 1.25 MHz of bandwidth, this is a constant andwill never change.

Each active CDMA voice link is always on (with a voice-activity-factor). Noconcept of scheduling.

Voice rate is always VAF*9.6 kbps. Data rate, within a burst, is fixed at 9.6, 2x9.6,4x9.6, 8x9.6 or 16x9.6 kbps.

Therefore, to model a CDMA link, the only variable is: how much traffic powerrequired by the traffic channel in order to meet a particular Eb/Nt target for that rate.

2.2.2 1xEV-DO Forward Link

The reverse link of DO is very similar to that of 1xRTT. It is also a continuous

transmission and also supports soft handoff. However, DOs forward link is very

different. It gives 100% of power to the user that is currently under service, so power is aconstant. It is the achieved rate (DRC) that is changing (rate control). Also it has a

scheduler to determine who is served next and for how long.

There are two dimensions in order to model a DO forward link, they are

(1)The achievable peak rate (= DRC) which is a function of forward link SNR;

(2)The only forward link resource the scheduler can control is the % of service timethe BTS is serving a particular mobile, which is a function of the number ofsimultaneous active users per sector.

On the other hand, the forward power is a constant (= 100% of PA power). The DO link

will always occupy 1.25 MHz of bandwidth.

2.2.3 WiMAX Forward & Reverse Link

2.2.3.1 E system (S-OFDMA)

OFDMA is more complicated than DO + 1xRTT combined together. First of all, to

model a WiMax link, it requires at least 4-dimensions. They are: (1) Bandwidth; (2) % of

Service Time; (3) Power and (4) Achievable Peak Rate (equivalent to DRC). There is aforward scheduler and a reverse link scheduler. Consequently, a mobile may or may not


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transmit at the same time or using the same amount of bandwidth in the forward link as in

the reverse link. WiMax has so much flexibilities, which makes the technology morepowerful, but also makes it extremely complicated to model.

We now discuss each of these 4 dimensions.

(1)Bandwidth: In OFDMA, the allocated bandwidth to each user is usually a fraction ofthe channel bandwidth. Suppose one WiMax channel bandwidth is 5 MHz. This

channel is divided into multiple sub-channels. Each sub-channel is further sub-divided into a large number of tones. Although a tone is the smallest frequency unit

in OFDM, the smallest frequency unit the scheduler can allocate to each user is one

sub-channel.

On the forward link, the BTS may give all sub-channels to a user, but it may alsoonly give 1 or a subset of sub-channel(s) to a user.

On the reverse link, a mobile can transmit via the entire channel bandwidth or via1 or a few sub-channels. This allows a mobile to trade throughput for extracoverage.

a. If the mobile is located near cell edge, it can transmit all of its power in 1or a few sub-channels(s), consequently the power density per sub-channelis higher so SNR per-tone can be higher. This allows the mobile to go

farther away from the BTS while still be able to maintain the connection.

b. If the mobile is located near the center, it is allowed to use the entirechannel BW to achieve a higher reverse link throughput.

As a result, bandwidth is no longer a constant like the cases for CDMA / DO.Instead, it is a new variable. One mobile may use different amount of bandwidth

from another. Also for the same mobile, forward link may use different amount ofbandwidth from the reverse link.

(2)% of Service Time: Because there is a scheduler for the forward link and a schedulerfor the reverse link, depends on the forward / reverse loading condition, QoS, call

models,, etc., a mobile may or may not transmit @ forward link at the same time as@ reverse link.

If a sector supports N active users at the same time, then no two users will beallowed to collide in the time-frequency space. That is, if multiple users use thesame set of frequencies, then they cannot use the same time slot; if multiple users

are transmitting at the same time slot, then they cannot use the same set of

frequencies.

This is somewhat similar to the scheduler in DOs forward link, except DOscheduler only needs to work with one dimension: the time-dimension; but

OFDMA scheduler must simultaneously work with two dimensions: time-

dimension and frequency-dimension.

(3)Power: WiMax power allocation can be similar to DO, or similar to 1xRTT. TheWiMax reverse link has power control, just like CDMA. However, it can also behave


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like DOs forward link. If the up link traffic is full-queue, the mobile will transmit

maximum power to achieve the highest reverse link throughput. This is similar toDOs forward link. On the other hand, if the up link traffic is constant-rate (like

VoIP), then reverse link will behave very similar to 1xRTT. The mobile will only

transmit enough power to support the reverse rate required for VoIP.

In the forward link:a. If there is only one single full-queue user in the sector, the system can give

100% of traffic power to this user to achieve the maximum throughput.This is similar to DO.

b. On the other hand, if a sector contains many constant-rate users (e.g.,VoIP), the system will allocate just enough bandwidth (smaller number of

sub-channels and smaller % of service time) to these user to maintain theVoIP rate.

c. WiMax standard does not mention forward link power control, thereforeone can assume that there is no forward link power control. But the

forward link scheduler can still change each users throughput byallocating different % of resources (= area in frequency-time space).

d. The standard does not define how a scheduler will work. This gives eachvendor complete freedom of designing its own scheduler. Therefore,

different vendors WiMax system will likely behave differently.

(4) Achieved Rate: Once Tx power level and path loss is given, the achieved SNR per-

tone can be calculated. Once SNR is known, the achievable peak rate is also known. Thisis very similar to DRC vs. SNR in DO, except WiMax has rate control on both forward

and reverse links. DO only has rate control on the forward link.

Since resources in OFDMA have two-dimensions, to model what % of resources is given

to each user, one needs to work with a two-dimensional Area4, with one dimension

being Time (the minimum unit is 1 symbol duration) and another dimension beingFrequency (the minimum unit is 1 tone, but in terms of scheduling, the minimum

frequency unit is one sub-channel), as shown in Fig. 18.

If there is only one full-queue user in the sector, this user may use the entire area, andthe achieved throughput = (Achieved Peak Rate)*(Total Area for data traffic).

If there are N (N>1) full-queue users in the sector, then these users share the total area

available for traffic, and one particular users achieved throughput

= (Achieved Peak Rate for this user)*(% of traffic area allocated for this user).

4The formal name of area used in 802.16 standard is called Data Region. In this document area and

Data Region is used interchangeably.


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Frequency

Dimension

Time

Dimension

Symbol Duration

Sub-

Channel

This Area is for User B

This Area is for User A

This Area is for User C

This Area is for Overhead

Frequency

Dimension

Time

Dimension

Symbol Duration

Sub-

Channel

This Area is for User B

This Area is for User A

This Area is for User C

This Area is for Overhead

Fig. 18 A sub-frame (can be DL sub-frame or UL sub-frame) can be represented by

an area. The overhead occupies part of the area. The rest can be shared by all users

in the sector.

2.2.3.2 D system (OFDM)

There are two main differences in D System:

(1) D Systems Channel structure is different (Fig. 12), one channel contains 256 tones(N_used = 200). E system does not use 256 tones, to avoid the requirement of backward

compatibility with D system.

(2) D systems forward link does not use sub-channels. Therefore, the forward linkscheduler has one less dimension to worry about, it is similar to the forward link in DO.

Although reverse link of D also use sub-channelization, the sub-channel structure in Dand sub-channel structure in E are different. One sub-channel in reverse link of D

contains 192 / 16 = 12 data tones. A sub-channel in E uses a tile structure, which contains

4 * 6 = 24 tones (Fig. 22), among which 16 are data tones. Reverse link tile structure isdiscussed in Chapter 2.4.


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2.3 802.16 E Forward Link

As was mentioned previously, the two most commonly used sub-channel configurations

in E Systems forward link are FUSC or PUSC. Additionally, there are several other

optional configurations, but they will not likely be implemented in the early releases of E

products, so they will not be implemented by the first release of iPlanner.

2.3.1 Forward Link FUSC

For forward link FUSC, each sub-channel contains 48 data traffic tones. These 48 tones

are randomized via permutation schemes5 in order to gain frequency diversity. But for

iPlanner modeling purpose, the main important thing is that

For FUSC, One Sub-Channel = 48 Data Traffic Tones.

The following table summarizes the details of sub-channel structure for channel

bandwidth of 1.25, 5, 10 and 20 MHz. The information contained in this table isimportant to determine the forward link throughput under FUSC.

Table 4 Forward Link Distributed Sub-Carrier Permutation for FUSC

Parameters Value

Channel

Bandwidth[MHz]

1.25 5 10 20

Tot. # of Tones(FFT Size)

128 512 1024 2048

Number of

Guard Tones

21(11 left, 10 right)



345(173 left, 172

right)

Number of used

Tones

(N_used)*

107 427 851 1703

Number of

Tones used for

Data Traffic*

96 384 768 1536

Number ofSub-Channels

Per Sector

2 8 16 32

Number of

Data Tones

per Sub-

Channel

48

5Permutation has similar functionalities as frequency hopping, the main purpose is to randomize the sub-

carriers to gain frequency diversity, so these 48 tones are not adjacent.


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*: N-used is used to determine the power density per tone. Number of traffic tones is

used to determine the forward link traffic throughput.

The minimum time resolution for forward traffic under FUSC is 1 OFDM symbol. That

means the data traffic portion of the DL frame can be even or odd number of frames

(divisible by 1).

The finest resolution the scheduler can allocate to a user is 1 slot6. One slot for FUSC is

an area of one Sub-Channel (48 tones) by one OFDM Symbol = 48 tone-symbol.

From Fig. 15, all DL sub-frames must start with a PUSC configuration, so even for FUSC,

the beginning part (overheads) of the DL sub-frame is PUSC. After the initial part, therest of DL sub-frame is zone-switched into FUSC, so the data traffic portion of the DL

frame is FUSC. Since frames from all BTSs are synchronized, when calculating CINR, it

is the data portion that counts. So if the data portion is FUSC, the CINR must becalculated according to FUSC.

2.3.2 Forward Link PUSC

In DL PUSC mode, total sub-channels of the BTS are divided into three disjointed setsthat can be allocated to the alpha-, beta- and gamma- sector of the same BTS. The

adjacent sectors from neighbor BTSs do not use the same set of frequencies so the out-of-

cell interference is significantly reduced. A permutation mechanism is designed to

minimize the probability of hits between adjacent / sectors / cells by reusing sub-carriers.

The main advantage of PUSC is reduced interference, because the forward interference

produced from PUSC is like the situation of N = 3. The disadvantage is that for the samechannel bandwidth, the amount of bandwidth allocated to each sector is much smaller.

If the traffic densities among three sectors are uniform, each sector gets 1/3 as muchbandwidth, the frequency planning in this case is easy. On the other hand, if the traffic

densities among three sectors are not uniform, the service provider can allocate more sub-

channels to the sector with the heaviest loading. However, this uneven sub-channelallocation involves re-planning of the frequencies of the neighboring cells in order to

make sure that the adjacent sectors do not use the same set of frequencies. Theoretically,

it is possible to have a dynamic sub-channel allocation scheme so these sub-channels are

like pooled resources among three sectors, but this requires the entire network to be very

smart in order to make sure that neighboring sectors never use the same set of sub-carriers.

PUSC defines a new unit of (frequency * time) space called a cluster. A cluster consists

of 14 adjacent tones in frequency dimension (2 pilot tones + 12 data tones) and twoOFDM symbols in time dimension, as shown in Fig. 19:

6A slot in 1xEV-DO has a time dimension, and the value is 1 / 600 (sec) = 1.6667 msec. However, a slot in

WiMax has the dimension of an area in (Frequency * Time) space.


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Fig. 19 A Cluster defined in PUSC consists of an area of 14 tones by 2 OFDM

symbols.

Because of this cluster structure, the smallest time resolution for PUSC DL frame is 2

OFDM symbols. That means the data portion of DL frame duration must be even numberof symbols (divisible by 2).

In frequency dimension, one PUSC sub-channel contains two clusters (= 2 * 14 = 28)tones, taking out the 4 pilot tones, the # of data tones per sub-channel is 24. That means:

For PUSC, One Sub-Channel = 24 Data Traffic Tones.

Although the 14 tones (12 traffic + 2 pilots) within a cluster are adjacent, each sub-

channel contains 2 clusters, these two clusters are not adjacent in frequencies so each

sub-channel does have frequency diversity.

The following table summarizes the details of forward PUSC sub-channel for channel

bandwidth of 1.25, 5, 10 and 20 MHz.

Table 5 Forward Link Distributed Sub-Carrier Permutation for PUSC

Parameters Values

Channel

Bandwidth

[MHz]

1.25 5 10 20

Tot. # of Tones

(FFT Size)

128 512 1024 2048

Number of

Guard Tones

43

(22 left, 21 right)

91

(46 left, 45 right)

183

(92 left, 91 right)

367

(184 left, 183

right)

Number of used

Tones

85 421 841 1681

Number ofTones used for

Data Traffic

72 360 720 1440

Number of

Clusters per 6 30 60 120


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Sector

Number of

Sub-Channels

Per BTS

3 15 30 60

Number of

Clusters perSub-Channel

2

Number ofTones (data +

Pilot) per Sub-

channel

28

Number of

Data Tones

per Sub-

Channel

24

From the above table, one can see that 1.25 MHz PUSC has only 3 sub-channels. Thischannel pipe-size is too thin and the Erlang efficiency will be poor. A channel BW of 5

MHz will be much more efficient.

Note that the first part of DL sub-frame always starts with PUSC (Fig. 15). If the data

portion of the DL sub-frame is also PUSC, there is no need for zone switching.

One slot for PUSC is an area in (frequency * time) space of one Sub-Channel (24 data

tones) by two OFDM Symbols. Comparing to a slot in FUSC, because PUSC has onlyhalf as many data tones per sub-channel, the time dimension is doubled. Consequently, in

PUSC, the area in (frequency * time) space of 1 slot is still the same as FUSC.

Fig. 20 shows an example of mapping OFDMA slots into sub-channels (frequency) and

symbols (time) in the down link in PUSC mode. Note that Sub-Channel i and Sub-

Channel (i+1) are not necessarily physically adjacent.


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Fig. 20 How to map data slots into the (Frequency * Time) space to form a Data

Region in DL PUSC.

2.3.3 Forward Power Allocation

1xEV-DO uses Rate Control which gives 100% of power for an active user at a giventime. Therefore, forward link power never changes, it is the achievable rate that is

changing depends on the RF condition.

On the other hand, 1xRTT uses Power Control. The BTS only gives a certain maximum

allowed power to a user (PTxMax). The actual power allocated to a user at any given

time is different, but the maximum power is not allowed to go over PTxMax.

Forward power control is not mentioned in the WiMax standard, therefore we can assume

that 802.16 forward link does not have power control. However, OFDMA can allocate a

smaller bandwidth to a particular user. If the PA power is uniformly allocated to eachsub-carrier, by allocating a smaller number of sub-carriers to a user, the total power

allocated to this user is also reduced. On the other hand, the forward link of D system

(OFDM) does not use sub-chanelization, so the situation will be like DO.


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The next question is how much power to allocate to pilot tones and how much power to

allocate to data tones. The simplest approach is the equal power allocation, i.e., one pilottone has the same power as one data traffic tone. Then all tones have the equal power.

The pilot tones are used for CINR estimation. If the pilot tone has the same power density

as data tones, then CINR of pilot tones will be the same as CINR of data tones.

For equal power allocation, the total power allocated to a user is proportional to the total

bandwidth given to this user.

It is possible to allocate different power to pilot tones from data traffic tones. As long as

the difference is a constant, one can still estimate traffic CINR from pilot CINR. For

example, if pilot tones are always 1 dB stronger than traffic tones, than traffic CINR willalways be 1 dB lower than pilot CINR.

In iPlanner, use equal power allocation as default, but do allow users to change thedefault and give pilot tones more or less power compared to traffic tones.


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2.4 802.16 E Reverse Link

2.4.1 Reverse Link PUSC

Fig. 15 shows that uplink can use PUSC or AMC mode. AMC is not modeled in the first

release, so only PUSC will be modeled.

Uplink PUSC defines another new unit for area in the (Frequency * Time) space calleda Tile. A tile consists of 4 adjacent tones in frequency dimension and three OFDM

symbols in time dimension, as shown in Fig. 21. The only exception is the case for 20

MHz channel bandwidth, in that case there are only 3 tones per tile (Table 6).

Fig. 21 Definition of an UL Tile.

Left: for 1.25, 5 and 10 MHz channel BW; Right: for 20 MHz channel BW.

Although a tile uses adjacent tones, a sub-channel contains 6 tiles chosen from a

permutation scheme. This permutation scheme tries to randomize frequencies by

choosing tiles that are not adjacent to each other, as a result, each sub-channel containing6 tiles does have frequency diversity.

Because of this tile structure, the smallest time resolution for UL frame is 3 OFDM

symbols. That means the data portion of UL sub-frame duration must be a multiple of 3symbols. For example, the UL sub-frame data portion can be 12, 15, 18, symbols, but

cannot be any number that is not divisible by 3.

One slot in UL PUSC is an area of 1 sub-channel (in frequency dimension) by 3 OFDM

symbols (in time dimension).

Each UL sub-channel contains 6 tiles. For 1.25, 5 and 10 MHz channel bandwidth, one

sub-channel contains 6 * 4 = 24 tones, only part of them are data tones. For 20 MHz

channel bandwidth, one sub-channel contains 6 tiles = 6 * 3 = 18 tones, only part of themare data tones.

To calculate the average number of data tones per UL sub-channel, one must average it

over three symbols, because the number of traffic tones per sub-channel is different atdifferent symbol time (Fig. 21, 22).


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Fig. 22 UL Sub-Channel Structure: 6 Tiles per Sub-channel.

Top: One Slot = area of one sub-channel (= 24 tones, only 16 are data tones) in

frequency space and 3 symbols in time space, for channel bandwidth of 1.25, 5 and

10 MHz.

Bottom: for channel BW = 20 MHz, one slot is still one sub-channel (= 18 tones, only

16 tones are data tones) by 3 symbols, the number of traffic tones per slot is the

same, but the number of pilot tones is smaller.

For 1.25, 5, ad 10 MHz channel bandwiths:

In the 1stsymbol it is 12 pilots tones + 12 data traffic tones; In the 2nd symbol it is 24 data traffic tones with no pilot tones; In the 3rd symbol it is 12 pilots tones + 12 data traffic tones (same as the first

symbol)

On the average (averaged over 3 symbols), the number of traffic tones per sub-channel = (12 + 24 + 12) / 3 = 16. The averaged number of pilot tones (averaged

over 3 symbols) = (12 + 0 + 12) / 3 = 8.

One slot = (16 Data Tones / sub-channel) * (3 symbols) = 48 (Data Tones) *(Symbols)If channel bandwidth = 20 MHz, each tile has only 3 tones. Out of these 3 tones, on the

average there are (3 + 2 + 3) / 3 =3

22 data traffic tones and 1/3 pilot tones. So each sub-

channel contains 6 tiles = 16 data traffic tones in frequency dimension. One slot alsocontains 3 symbols in the time dimension. So 1 slot is an area of = (16 data tones / sub-

TimeDimension

FrequencyDimension

Frequency Dimension(# of Tones)

TimeDimension(# of


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channel)*(3 symbols) = 48 (data tones) * (symbols), the same as the case for channel

bandwidth of 1.25, 5 and 10 MHz.

The total number of data traffic tones within a UL sub-frame will be used to calculate UL

throughput.

The following table summarizes the details of sub-channel and sub-carrier allocation for

UL PUSC for channel bandwidth values of 1.25, 5, 10 and 20 MHz.

Table 6 Reverse Link Distributed Sub-Carrier Permutation (Always PUSC)

Parameters Values

Channel

Bandwidth

[MHz]

1.25 5 10 20

Tot. # of Tones

(FFT Size)

128 512 1024 2048

Number of

Guard Tones

31

(16 left, 15 right)

103

(52 left, 51 right)

183

(92 left, 91 right)

367

(184 left, 183

right)

Number of used

Tones

97 409 841 1681

Number of

Tones per Tile 4 3

Number of

Tiles per Sub-

Channel

6

Number oftones (data +

pilot) per Sub-Channel

24 18

Number of

Data Traffic

Tones per

Sub-channel

16 16

Number of

Sub-Channels 4 17 35 92

From Table 6, one can see that the total number of sub-channels in the reverse link is notdivisible by 3 (unlike the situation in forward link PUSC). Since sub-channel is the finestfrequency unit that can be allocated by the scheduler, there is no easy way to allocate

exactly 1/3 of the total channel bandwidth to each sector to emulate N=3 frequency

reuse in the reverse link7.

7Using BW = 1.25 MHz as an example, there is a total of 4 sub-channels. If each sector uses 1 sub-channel,

then it uses 25% of total bandwidth; if a sector uses 2 sub-channels, it uses 50% of bandwidth. The

scheduler cannot allocate a fraction of sub-channels.


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We now compare a slot for forward link FUSC, forward PUSC and reverse link PUSC.

Because in UL a sub-channel contains less number of data traffic tones (averaged over 3

symbols = (12+24+12)/3 = 16), the time dimension is increased to three. Consequently,

the area (frequency * time) of 1 slot in all three cases are the same:

DL FUSC: 1 slot = (48 traffic tones)*(1 symbol) = 48 (tone*symbol)

DL PUSC: 1 slot = (24 traffic tones)*(2 symbols)= 48 (tone*symbol)

UL PUSC: 1 slot = (16 traffic tones)*(3 symbols)= 48 (tone*symbol)

Fig. 23 shows an example of mapping OFDMA slots into sub-channels (frequency) and

symbols (time) in the up link. Note that Sub-Channel i and Sub-Channel (i+1) are notnecessarily physically adjacent.

Fig. 23 Data Region (=area in Frequency * Time Space) in UL PUSC. Note that

the minimum time resolution is 3 symbols.

2.4.2 Reverse Link Power and Bandwidth Allocations

In reverse link, WiMax offers both power control and rate control.

For constant-rate traffics (like VoIP) that do not need high reverse rate, the rate isfixed but mobiles Tx power is varying (power control). In this case, the mobile will


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transmit just enough power to achieve the required Eb/Nt for the required VoIP data

rate. Power control reduces reverse link interference.

For full-queue kind of traffics, the mobile will transmit maximum power whichresults maximum reverse CINR and the maximum achieved reverse link rate. In thiscase, the mobiles Tx power is fixed (=Max power), the achieved rate is varying (rate

control). The pro of rate control is that it produces maximum reverse link throughput;the con is that it also produces maximum reverse link interference.

Additionally, the sub-channelization feature allows mobiles to trade used channel

bandwidth with service range (Fig. 24).

If this mobile is located very close to the BTS (so CINR of each UL tone can be veryhigh even if the total mobile power is spreaded to all tones), the mobile can use all

UL traffic tones (i.e., all UL sub-channels) to achieve the maximum reverse rate.

If this mobile is located far from the BTS, so that if the total power is spreaded to alltones, the achieved CINR per tone will be too poor to support the minimum rate, themobile can use a smaller number of sub-channels so the power can be concentrated to

a smaller number of tones so each tone can have a higher CINR. The reduced numberof sub-channels will result a smaller pipe size which means lower overall UL

throughput, but the mobile can be moved farther away from the BTS, thus trading UL

throughput with extra coverage.

Power Density

Per Tone

Distance to BTS4 Subchannels 3 Subchannels 2 Subchannels 1 Subchannel

Power Density

Per Tone

Distance to BTS4 Subchannels 3 Subchannels 2 Subchannels 1 Subchannel

Figure 24 Reverse Link Sub-channelization: Coverage vs. Rate trade-offs.

For power allocation between pilot and data tones, use equal power allocation as defaultin iPlanner, but do allow users to change the default and give pilot tones more or less

power compared to traffic tones.


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2.5 Modeling the WiMax Scheduler

Since iPlanner has implemented the effect of Proportional Fairness Scheduler for DO, it

is useful to compare a DO scheduler with a WiMax Scheduler.

2.5.1 1xEV-DO Scheduler

In DOs forward link, each mobile determines what is the peak rate (DRC) the current airlink condition can support based on the measured C/I value. Once the DRC value is send

to the BTS, the BTS must honor the rate requested. The only parameter the BTS cancontrol is: Time. That is, when to serve this user and what % of service time to be

allocated to this user.

The PF Scheduler is fair, in the sense that over a long period of time, the % of servicetime to each active user is approximately equal. Therefore if there are N users in a sector,

each user gets about 1/N as much service time (the so-called 1/N effect).

However, over a short period of time, if there are N users in the sector, the scheduler

tends to serve users that are on good RF condition. Because of this, there is a multi-user

diversity gain.

When there are N users in a sector, the average per-user throughput is approximately

= (DRC / N)*(Multi-User-Diversity-Gain)

2.5.2 WiMax Scheduler

The WiMax standard only specifies there will be forward and reverse link schedulers, but

it does not specify what or how to implement forward / reverse Schedulers. Therefore the

scheduler is very product specific, each vendor is free to implement its own scheduler.

Currently, the detailed information on schedulers of Nortels WiMax system is not yet

available, since the product is not yet available. Therefore, in this document we can onlydescribe two simple schedulers. They are the simplest to implement, when actual system

is ready, it may use different schedulers.

Like DO, WiMax mobile / BTS also determine what are the fwd / rev peak rates the

current air link condition can support, based on the measured fwd / rev CINR valuesmeasured from the mobile / BTS. Peak rates are equivalent to DRC, except WiMax can

have rate control in both directions, DO only has rate control in the forward direction.

Once the fwd / rev peak rates are determined, what the scheduler can control is when tosend a burst to this user, and how large is the area of the burst in (Frequency * Time)

space (Fig. 13, Fig. 18).


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2.5.2.1 Proportional Fairness Scheduler

Assume there are N active users in a sector. A PF scheduler will try to give every use

equal % of total available area for traffic. However, if there are multiple users in a

sector, a PF scheduler tends to server users who is currently at peak of the signal

(consequently it also has a multi-user-diversity-gain MUDG), although over the long runit servers every user with approximately equal % of time.

The per-user throughput = (% of traffic areas) * (peak rate supported based on CINR)* (Multi-User-Diversity-Gain)

The per-sector throughput = Sum of (per-user throughput) for all users

With PF scheduler, assume there are N full-queue users in a sector, within one second,

assume the total area (= (Total Service Time)*(Total # of Traffic Tones) ) = A, then

each user gets about the same % of resources, or A/N as much area. The per-userthroughput will be (Peak_Rate) * (% of Area). So, per-user-throughput can be graphically

illustrated as a Volume = (Area) * (Height), with height been (Achieved PeakRate)*(MUDG), area been % allocated resources in 2-dimensions, as shown in Fig. 25,26.

( ) ( )HeightArea

MUDGCINRrTonePeakRatepeN

AtPerUserTpu

*

*)(*

=

=

Area = (% of Area allocated to a user) in Frequency * Time space

Height = Peak_Rate (CINR) * (Multi-User-Diversity-Gain)

Frequency Dimension

Time Dimension

User #1 User #2

User #4 User #5

User #3

User #6

Total # of Traffic Frames

Total Number

of

Traffic Tones

Frequency Dimension

Time Dimension

User #1 User #2

User #4 User #5

User #3

User #6

Frequency Dimension

Time Dimension

User #1 User #2

User #4 User #5

User #3

User #6

Total # of Traffic Frames

Total Number

of

Traffic Tones

Fig. 25 Example showing 6 active users per sector. The fair scheduler gives

approximately 1/6 of the total resources (equal area) to each user.


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User #1User #2

User #3

User #4

User #5

User #6

Height

=

Achieved

Peak RateOf this

UserUser #1

User #2

User #3

User #4

User #5

User #6

Height

=

Achieved

Peak RateOf this

User

Fig. 26 Each users achieved throughput is represented by a Volume, where the

Height is the (achieved peak rate based on the CINR) * MUDG; the Area is (%

of resource allocated to a user) by the scheduler.

2.5.2.2 A Linear Biased Scheduler

It is possible to trade fairness with throughput. If the scheduler allocates a higher %

of resources to users who are currently under good RF conditions (so can achieve higher

peak rates) and less % of resources to users who are currently under poor RF conditions(so can achieve lower peak rates), the aggregate sector throughput will be higher. The

drawback is that the scheduler is less fair, and a user under poor condition will experience

even worse throughput.

As an example, assume there are two users, User A and User B.

User A is under extremely good RF condition, so the peak rate it can achieve is 10

Mbps

User B is under bad RF condition so the peak rate it can achieve is 0.1 Mbps.

Under a Fair-Scheduler, each user will get 50% of resources:

o The per-user-Throughput of User A = 50% * 10 Mbps= 5 Mbps

o The per-user-Throughput of User B = 50% * 0.1 Mbps = 0.05 Mbps

o The aggregate sector throughput = 5 + 0.05 = 5.05 Mbps.

Suppose the scheduler is biased, it gives 75% of resources to User A, and only 25%of resources to user B.

o The per-user-Throughput of User A = 75% * 10 Mbps= 7.5 Mbps

o The per-user-Throughput of User B = 25% * 0.1 Mbps = 0.025 Mbps

o The aggregate sector throughput = 7.5 + 0.025 = 7.525 Mbps.

o The aggregate sector throughput becomes much higher, but User B gets onlyhalf as much throughput as before.


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One can use a simple linear approach by defining a variable called degree of fairness =

Max-to-Min-Ratio, i.e., the ratio of maximum % of resources to min % of resources(Fig. 27).

For a fair scheduler, this ratio is 1.

For a linear biased scheduler, this ratio > 1. For the example given above, the ratio =0.75 / 0.25 = 3.

% of

Resource

Allocat ion

Max

Min

RF Conditi on

CINR [dB]

Fair Scheduler

Biased Scheduler

% of

Resource

Allocat ion

Max

Min

RF Conditi on

CINR [dB]

Fair Scheduler

Biased Scheduler

Fig. 27 Illustration of Linear Biased Scheduler (Orange) vs. Fair Scheduler (Green)

Fig. 28 shows the per-user throughput as a result of the biased scheduler. Note that the

difference of the volumes becomes much more significant compared to the case of Fair

Scheduler. A user under a good RF condition gets a double gain: it has a higher peakrate, also gets higher % of resources. A user under a poor RF condition gets a doublehit: it has a lower peak rate, also gets lower % of resources.

User #1User #2

User #3

User #4User #5

User

#6

Height

=

Achieved

Peak Rate

Of thisUserUser #1

User #2

User #3

User #4User #5

User

#6

Height

=

Achieved

Peak Rate

Of thisUser

Fig. 28 Illustration of per-user throughput (= Volume) under biased scheduler.


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With sub-channelization, some users use fat pipes, some users use medium pipes,

and others use thin pipes. Therefore, the scheduler must work with a mix of fat pipesand thin pipes.

For a fair scheduler, each user get the same area, therefore, the thin pipes will be

served for a longer duration (in time) than fat pipes, as shown in Fig. 29.

Fig. 29 Fair scheduler gives each user the same area, therefore thin pipes will

be allocated more service time than fat pipes.

On the other hand, if all pipes (regardless of sizes) are allocated with equal % ofservice time, then the fat pipes will have larger areas than the medium pipes, which

will have larger areas than the thin pipes, then this is a linear biased scheduler with

Max-to-Min-Ratio = (Fat Pipe Size)/(Thin Pipe Size).

2.5.2.3 Other Schedulers

A commercial WiMax system will likely have some kinds of priority schedulers to give

higher priority to real-time applications. However, the detail of such priority scheduler is

unknown for the time being, so it cannot be implemented. The two examples given inprevious two sections are the simplest to implement, and are mainly used for full-queue

applications. Only these two schedulers will be implemented initially, most complicated

schedulers are left to future releases.

Time

Frequency

Fat Pipe

Medium Pipe

Thin Pipe

Time

Frequency

Fat Pipe

Medium Pipe

Thin Pipe


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2.5.2.4 Difference Between D Scheduler and E Scheduler

Most discussions in 2.5 are for 802.16 E systems. As far as scheduler is concerned, the

scheduler for D system can be considered as a special case of the schedulers discussed

above.

Forward Link of D System As was mentioned previously, forward link of D system(OFDM) does not us sub-channelization, therefore, the channel bandwidth is not a

variable, the only variable is % of service time. So the scheduler has one lessdimension, and two-dimensional resources become one-dimensional. The situation is

very similar to 1xEV-DO. The PF Scheduler works almost identical as DO. Thelinear biased scheduler simply allocate higher % of service time to users at good RF

conditions.

Reverse Link of D System Reverse link of D system also uses OFDMA with sub-

channelization, except the channel structure is different from System Es channelstructure. It is not a scalable-OFDMA, and the reverse link does not use Tile Structure.

For full-queue user, how many sub-channels a mobile should use is a function of the

reverse link budget. If a mobile is located very close to the BTS, it will use all 16 sub-

channels. If it is located on cell edge, depends on its reverse link budget limit, it mayuse less than 16 sub-channels, the number of used sub-channels can drop all the way

down to 1. Fair Scheduler and Biased Scheduler work the same way, except to D

systems channel structure.


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2.6 Handoffs in 802.16 E

802.16 D system cannot do handoff. 802.16 E system defined three kinds of handoff:

1. Hard Handoff (HO)

2. Fast-BTS-Switching Handoff (FBSS)3. Soft Handoff (SHO)

Hard handoff is used in N=3 frequency-reuse, and is a mandatory feature, since N = 3 is

mandatory.

N = 1 frequency reuse is optional. If N = 1 is used, the system must support at least one

of FBSS or SHO, preferably both. The cell edge reliability would be very poor if N = 1 is

used but the system only supports hard handoff. Since N = 1 is optional, both FBSS and

SHO are also optional.

The main reason that both FBSS and SHO are optional is because it is more challengingto deploy them in OFDMA systems (compared to CDMA systems). Consequently, it ishighly possible that early versions of 802.16 E systems only use N = 3 with hard handoff.

N = 1 with FBSS and SHO will probably not be supported until later releases.

2.6.1 Hard Handoff

Hard handoff is well-defined in the standard. However, most sections discussed in thestandard are how mobile scan for neighbor list, cell-reselection, HO decision &

initialization, hand-shaking messaging, ranging, HO process.etc. these dynamic

processes cannot be modeled by a static tool like iPlanner, so they are not important from

an tool implementation point of view.

What iPlanner will model is to produce a contour plot for the footprints of the best-server

for F1, F2 and F3, and assume hard handoff is 100% efficient (no dragging handoff), somobiles that are physically located in the best-server footprint of F1 will belong to sector

1, mobiles that are physically located in the best-server footprint for F2 will belong to

sector 2, , etc.

The details of implementing hard handoff in iPlanner are given in Chapter 3.3.1.

2.6.2 Fast-BTS-Switching Handoff (FBSS)

One key difference between FBSS and hard handoff is that FBSS is fast (that is why it iscalled fast switching). Because it is fast, FBSS does not afraid of ping-ponging (very

little hysteresis is needed), thus it has practically no handoff dragging. In fact, because it

can quickly switch to the best-server, there is a selective diversity gain.

The reason FBSS is fast compared to hard handoff is that FBSS has an Active Set (just

like the situation of s

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WiMax Module v11