Aditya Relay

8/7/2019 Aditya Relay

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Cooperative Principles and

Relay Routing

Multihop Relaying in Wimax


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Contents

y The concept of relaying

y Some relaying techniques

y Cooperative diversity principles

y Relay network entry and routing in 802.16j

y Simulation results


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Relay channel

y Origin can be traced back to Capacity theorems for the relaychannel by T Cover and AE Gamal.

y Found the theoretical upper limit of the achievable data rateof a channel with a helper node called relay.

y Relay has found its utility in modern communication systems

y Relays will manifest themselves as a reality in the IEEE802.16j standard.

y This is still under evolution.

y Motivation was to study the transformation of the relayconcepts and research results in this area into a practicalstandard implemented by a system


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Why use a relay?

y

Transmission at higher frequencies is more vulnerable to nonLOS conditions.

y The transmission power required for high data rates at large

distances is very high

y

Increasing the base station density is one optiony A relay acts as a helper node to increase coverage and

throughput

y Relay is connected to the base station through wireless

channel.y Relays are much closer to the mobile stations than the base

station, hence high data rates are possible.

y Mobile stations can transmit at lower power


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The concept of relay

Base Station Relay Station

Backhaul/Wired

network

y RS will typically cover a region up to 300 miles in diameter

and transmit at lower power level than BSy Relays do not have wired connection to backhaul

y For two hop relaying, transmission occurs in two phases

y Cooperative diversity is additional advantage of relaying.


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Relay Usage Scenarios

1) Fixed Infrastructure: To improve coverage in shadow areas

and increase throughput due to LOS communication.

2) In-Building Coverage: To fill the coverage hole inside the

building3) Temporary Coverage: Stadiums or gatherings of people

during an event. Also temporary replacement to a damaged

relay.

4) Coverage on mobile vehicle: Inside trains and buses. Acomplex relay is required that can handle quick handoffs


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Relaying Techniques

y Amplify and Forward(AF) : The RS does not decode thereceived bits. It simply forwards the received signal afteramplifying it. Hence it also amplifies noise. The two phasesneed to be of the same duration.

y

Decode and forward(DF): The RS decodes the signal andperforms error correction. The decoded data is encodedusing the same or different codebook before transmitting tothe destination. The two phases need not be of sameduration.

y Compress and forward(CF): The relay compresses thereceived signal by usingWyner-Ziv lossy source coding andforwards it to the destination.


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Relaying Techniques

y DF performs well only when the channel quality between BS

and the RS is good

y Only advantage of AF is that it is computationally less

intensive at the relay.y CF technique performs better than direct transmission in all

channel conditions even if channel between BS and RS is

degraded.

y

CF is computationally intensive.y CF is not considered forWimax relaying


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Cooperative Diversity

y Multiple paths created by use of relay can be used to exploitspatial diversity

y Antennas are distributed between the BS and RS

y Three ways of achieving diversity: Transmit, Receive and

MIMO

1. Transmit Diversity:Base Station Relay Station

Backhaul/

Wired

network


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Cooperative Diversity2. Receive Diversity

3. MIMO Diversity: Is the combination of transmit and receive

diversity.

Backhaul/

W

irednetwork

Base Station Relay Station


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Coverage and Throughput Improvement with relays

y System Model:

1) Single cell with BS in the center and Tx Power of 27.3 dB

2) 4 equi-spaced relays at distance of 10km from BS and

transmit power 20.3 dB

3) Path Loss Exponent 3.5

4) Additive Noise Power -130 dB

5) Adaptive modulation and coding (AMC) is employed

which means that the transmitter chooses a suitable

modulation scheme and coding rate based upon theinstantaneous value of SNR.

y Reference: B. Can, H. Yomo, and E. De Carvalho, Link Adaptation and Selection Methodfor OFDM BasedWireless Relay Networks,Journal of Communications and Networks, Special issue on

MIMO-OFDM and Its Applications, June 2007.


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Adaptive Modulation and Coding Scheme

Courtesy: X. Chen, L.Wan, Z. Fei, et al "The Application of the EESM and MIbased Link Quality

Models for Rate Compatible LDPC Coded OFDM System" IEEE VTC 2007-fall, IEEE 66st, 30

Spet.- 3 Oct. 2007


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Coverage and Throughput Improvement with relays

No relay used Simple relay used Cooperative DF relay used

Relay power 17.3 dB Relay power 20.3 dB


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Comparative Study of Cooperative Diversity

Schemes

y Following different schemes were tried out

1) No relaying

2) Simple relaying without cooperative diversity: Two phases

can use different AMC. Since RS is placed close to BS wecan have 64QAM 5/6 AMC for BS to RS channel

1) Tx_Amplify_Forward Diversity: Two phases use sameAMC


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Schemes

4) Tx_Decode_Forward Diversity: Two phases can use

different AMC

5) Rx_Amplify_Forward Diversity: Two phases use sameAMC

6) Rx_Decode_Forward Diversity: Two phases use same AMC


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Schemes

DF scheme performs better than AF scheme since RS is placed close to the BS.

Tx diversity seems to perform better than Rx diversity.

However in Rx diversity the BS does not transmit to the MS during the second phase.

Instead it can schedule some other users in that time. Advantage of Rx Diversity will be

seen when we do network simulations


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Relay selection algorithm for 802.16jy Network Entry for a RS and MS are almost similar

yBoth try to identify the best RS to connect to

y Each BS and RS periodically transmit UCD (Uplink ChannelDescriptor)

y Choice of relay in multihop scenario is difficult

y Assumption (802.16j): Every RS or MS is served by only a single

RS.

Courtesy: S. Ann, K.G. Lee, and H.S. Kim "A Path Selection Method in IEEE 802.16j Mobile Multi-hop Relay Networks"


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Algorithmy Consider the algorithm proposed in S. Ann, K.G. Lee, and H.S.

Kim "A Path Selection Method in IEEE 802.16j Mobile Multi-hopRelay Networks"

y Three new parameters to be added in the UCD:

1) Thrr : Minimum of the throughput among all links connecting the

RS to BS.2) BWr: Minimum of the BW of all the relays in the path including

the BS. Gives the current utilization of the relay path

3) HPr : Hop count from the RS to the serving BS

The instantaneous values of the parameters get modified

periodically. Information propagates via the UCD


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Network Entry Procedure

y The network entry procedure is started with the RS that is closestto the BS and ends at one that is farthest.

1. PHY tries to get course synchronization with the network

2. MAC attempts to acquire channel information and networkparameters.

3. It builds up a physical neighbor list and stores the relay pathparameters sent from neighbor RSs.

4. Then for each neighboring relay station it calculates its suitabilityas:

S= Thrr* BW

r/ HP

r5. It then chooses the RS or BS with maximum value of S.


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Simulation Model and Results

Single cell with BS at the center.

15 RSs with fixed positions (pre decided)

Transmit power for RS and BS = 35.3 dB

The BW associated with all RS and BS is assumed to be one i.e. full BW is

available.

Each RS/MS checks for RS/BS within 35 km of its radius.

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