Cooperative wireless networks Cooperative wireless networks based - PowerPoint PPT Presentation

The 2004 International Workshop The 2004 International Workshop on Wireless Ad- -hoc Networks hoc Networks on Wireless Ad INFOCOM Dpt Dpt. . INFOCOM University of Rome University of Rome “La Sapienza Sapienza” ” “La Cooperative wireless networks Cooperative wireless networks based on based on distributed space time coding distributed space time coding S. Barbarossa Barbarossa, L. , L. Pescosolido Pescosolido, D. , D. Ludovici Ludovici, L. , L. Barbetta Barbetta, G. , G. Scutari Scutari S. Oulu, Finland June 3, 2004 , Finland June 3, 2004 Oulu

Context Context Romantik Project Project Romantik R es es O O urce urce M M anagement and anagement and A A dva dva N N ced ced T T ransceiver ransceiver Algor Algor I I thms thms for for R Multihop Networ Networ K K s s Multihop Partners: : Partners - Universitat Politecnica de Catalunya - Universitat Politecnica de Catalunya - University of Bristol - University of Bristol - University of Rome “La Sapienza Sapienza” ” - University of Rome “La - - DUNE DUNE - - INTRACOM INTRACOM - FUJITSU Lab. of Europe LTD - FUJITSU Lab. of Europe LTD - Telenor - Telenor

Overview Overview • Cooperative communications Cooperative communications • • Cooperation coding gain Cooperation coding gain • • Distributed space Distributed space- -time coding strategies time coding strategies • - Orthogonal STC ( OSTC ) Orthogonal STC ( OSTC ) - - Full Rate Full Diversity ( FRFD ) Full Rate Full Diversity ( FRFD ) - - V V- -BLAST BLAST - • Performance Performance • • Conclusion Conclusion •

State of the art State of the art Cooperative transmission and DSTC cooperation among users in a wireless network cooperation among users in a wireless network [A.Sendonaris,E.Erkip,B.Aazhang A.Sendonaris,E.Erkip,B.Aazhang ’98] ’98] [ can increase the capacity in the uplink multi- -user user can increase the capacity in the uplink multi channel channel coding strategies essentially based on time coding strategies essentially based on time repetition code and orthogonal channels with a repetition code and orthogonal channels with a [J.N.Laneman,G.W.Wornell J.N.Laneman,G.W.Wornell ’02,’03] ’02,’03] [ loss in terms of rate loss in terms of rate transmission strategy optimization in case of transmission strategy optimization in case of [S.Barbarossa,G.Scutari [ S.Barbarossa,G.Scutari ‘03] ‘03] source and relay know the channel source and relay know the channel approach merging the idea of cooperation with approach merging the idea of cooperation with space time coding for flat fading and frequency space time coding for flat fading and frequency [P.A.Anghel,G.Leus,M.Kaveh P.A.Anghel,G.Leus,M.Kaveh ‘03] ‘03] [ selective channels selective channels comparison of different DSTC techniques based comparison of different DSTC techniques based [S.Barbarossa,G.Scutari S.Barbarossa,G.Scutari ‘04] ‘04] [ on D&F and A&F strategies in case of detection on D&F and A&F strategies in case of detection errors at the relay nodes errors at the relay nodes

Cooperation coding gain Cooperation coding gain Example: • Circle of radius R centered around 0 where there are n radio nodes • C(x 0 ,r) = circle of radius r, centered around the point x 0 • Radio nodes distributed uniformly and independently of each other within C(0,R) • p =r 0 2 / R 2 = probability of finding a node within a circle of radius r 0   n p = − − = k n k (1 p ) P{k dots out of n lie in C(x,r 0 ) included in C(0,R)}   k   Bernoulli distribution

Cooperation paradigms Cooperation paradigms Coordinated multihop networking Old paradigm source and relays transmit as a R 1 time-repetition code, distributed D through space S R 2 R 3 New paradigm cooperation between source and relays R 1 creates a virtual MIMO system D S R 2 R 3 use space-time coding instead of repetition coding

Distributed STC vs. STC vs. conventional conventional STC STC Distributed 1. Probability of finding a (reliable) relay 1. Probability of finding a (reliable) relay 2. Synchronization 2. Synchronization 3. Decision errors at the relay nodes 3. Decision errors at the relay nodes

Probability of finding reliable relays Probability of finding reliable relays Average error rate: Probability of finding K relays ∞ ∑ = + P p ( ) k P k ( 1) e r e 0 = k 0 # relays error probability of a 0 10 multiple transmit antenna having k+1 Tx antennas -1 10 -2 10 Average BER coding gain may be -3 10 Coding gain considerable if relay k = 0 coop -4 10 density is high 1 2 0 5 10 15 20 25 30 SNR (dB)

Cooperation coding gain Cooperation coding gain c ∝ P p 0 (0) At high SNR e r SNR • diversity gain = 1 (i.e. no gain) 2 πρ = r G e • coding gain : 0 c Gain depends on node density and on power used to discover a relay Coding gain increases by increasing the transmit power or decreasing the bit rate

Cooperation protocol Cooperation protocol Three main phases: 1. relay discovery ( RD ) 2. transmission from Source to the Relays ( S2R ) 3. coordinated transmission from (Source,Relays) to the Destination ( SR2D )

Resource Discovery Resource Discovery • each source sends orthogonal pseudo-noise codes to verify whether there are available neighbors • radio nodes available as relays compute SNIR for each source • potential relays retransmit an ACK signal back only to those sources whose SNIR exceeds a given threshold • each source receives the ACK and SNIR from all potential relays and it decides which relay to use this operation has to be repeated at least this operation has to be repeated at least once every channel coherence time once every channel coherence time

Resource Discovery Resource Discovery How far should be source and relays ? If source and relays are very close • less power wasted in S2R time slot • less synchronization problems in SR2D time slot • less interference between different source/relay pairs but … it is less likely to find sufficiently reliable relays

Multi- -user scenario user scenario Multi R R 4 4 S2R transmission R 8 R S 1 S R 1 R 1 R R 8 1 1 1 R 2 R 2 R R 2 2 R 7 R S 2 S 7 2 R 9 R 9 S 3 S 3 R 3 R 3 R R 3 3 R R 6 6 R 5 R 5 SNIR constraint induces a circle of radius D There is real cooperation between S i and R i D imax SNIR constraint induces a circle of radius All sources transmit simultaneously imax only if S i is inside the circle of radius D imax around Ri around every potential relay Ri Ri around every potential relay interference occurs among the active links

Multi- -user scenario user scenario Multi R 4 R 4 D 3 D 3 SR2D transmission R 8 R S 1 S R 1 R 8 1 1 R 2 R 2 D 1 D R 7 R S 2 S 1 7 2 R 9 R 9 S 3 S 3 R 3 R 3 R R 6 6 R 5 R 5 D 3 D 3 virtual transmit arrays are induced between virtual transmit arrays are induced between source- -relay pairs and destinations relay pairs and destinations source

S2R transmission S2R transmission for each source S S i for each source i if S S i has found its own relay R R i in Relay Discovery phase if i has found its own relay i in Relay Discovery phase then S S i transmits information to R R i in a dedicated S2R slot then i transmits information to i in a dedicated S2R slot Rate loss resulting from the insertion of S2R slot Increasing constellation order, insertion loss decreases but … the probability of finding a relay decreases Constellation order as trade- -off rate loss / probability of finding relay off rate loss / probability of finding relay Constellation order as trade

SR2D transmission SR2D transmission for each pair ( for each pair (S S i i ,R ,R i i ) ) S i and R R i transmit information to D D i inducing a virtual array using dedicated resources S i and i transmit information to i inducing a virtual array using dedicated resources no interference among different SR2D transmissions & Distributed Space Time Coding strategy & D&F: relays decode the received symbols and retransmit them

Alternative DSTC strategies Alternative DSTC strategies max rate gain, limited receiver V-BLAST (1) complexity low diversity gain max diversity gain, low rate Orthogonal STC (2) min receiver complexity max diversity gain and max rate Full Rate Full Diversity (3) gain, but high receiver complexity potential max diversity gain and max Trace-orthogonal STC (4) rate, as a function of complexity (3) Giannakis et al., El Gamal et al. [‘03] (1) Foschini [‘96] (2) Tarokh et al. [‘96] (4) Barbarossa [‘04]

Rate loss wrt wrt no no- -coop system coop system Rate loss • TDMA context • T S2R = duration of S2R time slots • T SR2D = duration of SR2D time slots NT η = SR D 2 • T s = symbol duration in all time slots + T NT S R 2 SR D 2 • Q = constellation order used in S2R • M = constellation order used in SR2D Rate reduction factor • S2R slot is shared among N source-relay pairs by decreasing T S2R higher SNIR constraint Rate loss can be reduced: at the relay by increasing N the right choice as a trade-off rate / performance