Transmission Techniques

Traffic channels: different users are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000

Traffic channels: different frequency bands are allocated to different users, for example, AMPS

Traffic channels: different time slots are allocated to different users, for example,

GSM

FrequencyTime

Power

FrequencyTime

Power

FrequencyTime

Power

FDMA

TDMA

CDMA

User

User

User User

User User

Introduction

Time slots and frame

Capacity of cellular CDMA:

N: the number of usersS: the signal power of each userR: baseband information bit rateW: total RF bandwidthη: background thermal noise in the spread bandwidthAssume perfect power control

The number of users that can access the system is thus given as

Where W/R is called the processing gain

The principle behind the spreading of a signal is explained by the

Shannon channel capacity formula:

Bω = bandwidth in HertzC = channel capacity in bits/secondS = signal powerN = noise power

The received energy per bit is the received signal power divided by

the data rate R (bits/s) ,

Eb = Ps/R

1-Direct sequence Spread spectrum (DSSS). 2- Frequency hopping spread spectrum (FHSS). 3-Time hopping spread spectrum (THSS). 4-Hybrids Spread spectrum (HSS).

6.17

W - Spread bandwidth in HzR = 1/Tb = Date RateS - Received power of the desired signal in WJ - Received power for undesired signals like multiple access users, multipath, jammers etc in WEb - received energy per bit for the desired signal in W N0 - equivalent noise spectral density in W/Hz

00

0

NERW

NEWT

TEWN

SJ

bb

b

bb

min0max NE

RWSJ

b

)()((db)margin Jammingmin0max

dbNEdb

RW

SJ b

Are portion of a physical communications channel that is used to for a particular communications purpose. There are two groups of channels used in the CDMA system; control channels and traffic channels. Control channels are used to setup, manage, and terminate communication sessions. Traffic channels are primarily used to transfer user data but can also transfer some control information.

SyncPilot

FW Traffic(for user #1)

Paging

FW Traffic(for user #2)

FW Traffic(for user #3)

Figure 2.9: CDMA Forward and Reverse Channels

`

Cells Different Frequencies or

Codes

Base Station Fixed transceiver

Mobile Station Distributed transceivers

Downlink

UplinkHandoff

Multiple Access

CDMA：”make before break”---soft handoffOther systems: “make after break”---hard handoff

Use soft handoff, decrease drop-call rate

Definition of Coverage Areas

Location area

MSC area

PLMN area

Service area

Sectorarea

CDMA Number Planning

Cell area

Rate of reception level variation is known as the fading rate or the

Doppler rate, fd, and depends on the velocity of movement V and carrier wavelength. It can be expressed as:

W2n=Wn Wn

Wn Wn

W1 =0

W2=0 00 1

W4=

0 00 1

0 00 1

0 00 1

Walsh code

Walsh Code is one kind of orthogonal code.

1 11 0

Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0

horizontally and vertically, and then complementing the 1 diagonally

suppose we have four stations each has a sequence of chips which we designate as (A,B,C) and (D)(see the figure). Each station is assigned a code which is sequence of number called chips.