Cooperative Principles and Relay Routing Multihop Relaying in Wimax
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- Slide 1
- Cooperative Principles and Relay Routing Multihop Relaying in
Wimax
- Slide 2
- Contents The concept of relaying Some relaying techniques
Cooperative diversity principles Relay network entry and routing in
802.16j Simulation results
- Slide 3
- 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
- Slide 4
- 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
- Slide 5
- 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.
- Slide 6
- 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
- Slide 7
- 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.
- Slide 8
- 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
- Slide 9
- 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
- Slide 10
- Cooperative Diversity 2. Receive Diversity 3. MIMO Diversity:
Is the combination of transmit and receive diversity. Backhaul/
Wired network Base Station Relay Station
- Slide 11
- 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.
- Slide 12
- 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
- Slide 13
- 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
- Slide 14
- 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
- Slide 15
- 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
- Slide 16
- 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
- Slide 17
- 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"
- Slide 18
- 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
- Slide 19
- 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.
- Slide 20
- 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.