Ivana Marić, Ron Dabora and Andrea Goldsmith Relaying in Networks with Multiple Communicating Pairs: Interference Forwarding Ivana Marić, Ron Dabora and Andrea Goldsmith Summary Introduction Motivation Channel Model ACHIEVEMENT DESCRIPTION Relaying in network with multiple sources has aspects not present in the relay networks: Relaying messages to one destination increases interference to others Relays can jointly encode messages from multiple sources There are many relevant encoding strategies Encoding strategies for networks with multiple sources are not well understood and developed Current approach: multihop routing Time shares between data streams (no joint encoding) Does not exploit broadcast or interference We consider smallest network that captures relaying for multiple sources: the interference channel with a relay Previous work: Sridharan, Vishwanath, Jafar and Shamai [ISIT, 2008] Rates and degrees of freedom when the relay is cognitive Sahin and Erkip [Asilomar 2007, CTW 2008] Various relaying strategies for forwarding information to intended receivers have been proposed Capacity of networks are still unknown; one of the key reasons: we don’t know how to handle and exploit interference In relay networks: Relays forward data for a single source-destination pair Cooperative strategies are well developed and known to bring gains Cooperative strategies exploit the broadcast nature of wireless medium In networks with multiple sources: The center issue is coping with interference created by simultaneous transmissions Networks with multiple sources contain broadcast, multicast, relay and interference channel elements as their building blocks ASSUMPTIONS AND LIMITATIONS: To demonstrate interference forwarding gains, we considered scenario in which the relay can observe the signal from only one source and can thus forward only one of the two messages MAIN RESULT: We determined conditions under which having a relay enhance the interference improves the performance. We also obtained capacity in the special case HOW IT WORKS: The relay forwards a message to a receiver that is not interested in that message, thus increasing the interference at that receiver. This allows the receiver to decode and cancel the interference, and decode its message in the clear channel dest1 dest2 encoder 1 encoder 2 relay Compare the rates to outer bounds Further develop strategies for forwarding in the presence of interference Consider more general scenarios in which interference enhancement needs to be combined with other relaying strategies Apply this strategy to larger networks END-OF-PHASE GOAL STATUS QUO Two messages: Rates: In networks with multiple sources, relays can help beyond forwarding useful information, by increasing interference at the receivers. This allows receivers to decode the interference and cancel it prior to decoding their desired messages Encoding: Decoding: COMMUNITY CHALLENGE NEW INSIGHTS Prize level: Capacity results for networks with multiple sources We present new relaying strategy: interference forwarding We proposed a new relaying strategy for networks with multiple sources. We showed that it can improve the rate performance and that it achieves capacity in a certain scenario. Capacity Result Gaussian Channels Assumptions Achievable Rates We define strong interference conditions as: The presence of the relay changes the strong interference conditions The relay can ‘push’ a receiver into the strong interference regime where decoding of interfering message is optimal We evaluated these results for the Gaussian channels: dest1 dest2 encoder 1 encoder 2 relay Theorem: Any rate pair (R1,R2) that satisfies (2) ? satisfied for any distribution p(x1)p(x2,x3)p(y1,y2|x1,x2,x3) (1) Conditions (2) are analogous to the strong interference conditions derived by Costa and El Gamal for the interference channel Conditions (2) imply that the flow of information from each source to the non-intended receiver is better than to the intended receiver Consequently, receivers can decode the undesired messages for ‘free’ and hence experience no interference To illustrate gains from interference forwarding, we consider the special case (shown in Figures): The relay cannot observe signal sent from source 1 Then, it can only forward message W2 thus improving rate R2 From the perspective of the other receiver, the relay is interference forwarding Can relay help also receiver 1 and improve rate R1? for any distribution p(x1)p(x2,x3)p(y1,y2|x1,x2,x3) Noise: Powers Rates in the Thm. are achieved by: Single-user encoding at the encoder 1 to send W1 Decode-and-forward at the encoder 2 and the relay to send message W2 The channel degradedness condition: (3) Theorem: When (2)-(3) hold, rates (1) are the capacity region. In strong interference, decoding both messages is optimal Insights and Future Work Comparison with Rate Splitting Numerical Results for Gaussian Channel Conclusions Without the relay, the channel reduces to the interference channel (IC) The best known rates for IC are achieved with rate splitting: Demonstrated gains from interference forwarding Interference forwarding: Can improve the performance through interference cancellation Can hurt the receiver by increasing interference Achieves capacity in a special scenario of strong interference It ‘pushes’ receiver in strong interference regime where the receiver can decode both messages We determined conditions under which decoding interference is optimal Can be realized through decode, compress -and-forward Can be combined with other encoding schemes dest1 dest2 encoder 1 encoder 2 relay Insights: When relaying for multiple sources: Jointly encode messages (network coding approach) Exploit broadcast Forward messages and interference Future work: Develop and evaluate transmission strategies that unify above approaches Analyze the general case of the interference channel with a relay Further develop strategies for relaying in the presence of interference Without the relay: interference channel in strong interference With relay, h13=0: no interference forwarding With relay, h13>0: interference forwarding Interference forwarding enlarges the rate region It facilitates interference cancellation In the case when the relay can only use interference forwarding, can the relay still help? We compare the rates achieved with and without the relay Proposition: When strong relay-rcvr1 link strong source2-relay link for any distribution p(x1)p(x2,x3)p(y1,y2|x1,x2,x3) interference forwarding outperforms rate splitting (no relaying).