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Cooperative Principles and Relay Routing Multihop Relaying in Wimax

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Contents The concept of relaying Some relaying techniques Cooperative diversity principles Relay network entry and routing in 802.16j Simulation results

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Relay channel Origin can be traced back to Capacity theorems for the relay channel by T Cover and AE Gamal. Found the theoretical upper limit of the achievable data rate of a channel with a helper node called relay. Relay has found its utility in modern communication systems Relays will manifest themselves as a reality in the IEEE 802.16j standard. This is still under evolution. Motivation was to study the transformation of the relay concepts and research results in this area into a practical standard implemented by a system

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Why use a relay? Transmission at higher frequencies is more vulnerable to non LOS conditions. The transmission power required for high data rates at large distances is very high Increasing the base station density is one option A relay acts as a helper node to increase coverage and throughput Relay is connected to the base station through wireless channel. Relays are much closer to the mobile stations than the base station, hence high data rates are possible. Mobile stations can transmit at lower power

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The concept of relay Base Station Relay Station Backhaul/ Wired network RS will typically cover a region up to 300 miles in diameter and transmit at lower power level than BS Relays do not have wired connection to backhaul For two hop relaying, transmission occurs in two phases 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 building 3) 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. A complex relay is required that can handle quick handoffs

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Relaying Techniques Amplify and Forward(AF) : The RS does not decode the received bits. It simply forwards the received signal after amplifying it. Hence it also amplifies noise. The two phases need to be of the same duration. Decode and forward(DF): The RS decodes the signal and performs error correction. The decoded data is encoded using the same or different codebook before transmitting to the destination. The two phases need not be of same duration. Compress and forward(CF): The relay compresses the received signal by using Wyner-Ziv lossy source coding and forwards it to the destination.

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Relaying Techniques DF performs well only when the channel quality between BS and the RS is good Only advantage of AF is that it is computationally less intensive at the relay. CF technique performs better than direct transmission in all channel conditions even if channel between BS and RS is degraded. CF is computationally intensive. CF is not considered for Wimax relaying

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Cooperative Diversity Multiple paths created by use of relay can be used to exploit spatial diversity Antennas are distributed between the BS and RS Three ways of achieving diversity: Transmit, Receive and MIMO 1. Transmit Diversity: Base Station Relay Station Backhaul/ Wired network

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Cooperative Diversity 2. Receive Diversity 3. MIMO Diversity: Is the combination of transmit and receive diversity. Backhaul/ Wired network Base Station Relay Station

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Coverage and Throughput Improvement with relays 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 the instantaneous value of SNR. Reference: B. Can, H. Yomo, and E. De Carvalho, Link Adaptation and Selection Method for OFDM Based Wireless 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 usedCooperative DF relay used Relay power 17.3 dB Relay power 20.3 dB

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Comparative Study of Cooperative Diversity Schemes 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 we can have 64QAM 5/6 AMC for BS to RS channel 1) Tx_Amplify_Forward Diversity: Two phases use same AMC

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Comparative Study of Cooperative Diversity Schemes 4) Tx_Decode_Forward Diversity: Two phases can use different AMC 5) Rx_Amplify_Forward Diversity: Two phases use same AMC 6) Rx_Decode_Forward Diversity: Two phases use same AMC

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Comparative Study of Cooperative Diversity 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 802.16j Network Entry for a RS and MS are almost similar Both try to identify the best RS to connect to Each BS and RS periodically transmit UCD (Uplink Channel Descriptor) Choice of relay in multihop scenario is difficult 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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Algorithm Consider the algorithm proposed in S. Ann, K.G. Lee, and H.S. Kim "A Path Selection Method in IEEE 802.16j Mobile Multi-hop Relay Networks" Three new parameters to be added in the UCD: 1) Thr r : Minimum of the throughput among all links connecting the RS to BS. 2) BW r: Minimum of the BW of all the relays in the path including the BS. Gives the current utilization of the relay path 3) HP r : 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 The network entry procedure is started with the RS that is closest to 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 network parameters. 3. It builds up a physical neighbor list and stores the relay path parameters sent from neighbor RSs. 4. Then for each neighboring relay station it calculates its suitability as: S= Thr r * BW r / HP r 5. 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.