Wireless Communication Co-operative Communications Sharif University of Technology Wireless Communication Co-operative Communications Fall 1396 Afshin Hemmatyar

Introduction In traditional wireless communication systems, nodes are supposed to only transmit their own information. But in many occasions, if nodes start to cooperate they can improve each other’s performance significantly, especially in slow fading environments. This can be viewed as some source of spatial diversity. Similar to MIMO systems in some aspects, but now the antennas are located at other user locations, and we have distributed space time processing.

UTMS Coverage and Range Extension Examples (1) UTMS Coverage and Range Extension

Examples (2) Distributed relaying Delivering high speed data to sparse residential areas by means of roof-top relaying systems, which is facilitated by the fairly static communication topology. Distributed relaying promises an increase in data rate and link stability.

WLAN Coverage and Capacity Extension Examples (3) WLAN Coverage and Capacity Extension Wireless Local Area Networks have sporadic hot-spot coverage in offices, cafes, train stations, etc. Distributed relaying potentially increase capacity at WLAN cell edges and closes coverage holes in sufficiently dense deployment areas.

Vehicle to Vehicle Communication Examples (4) Vehicle to Vehicle Communication Automated steering within a group of cars, such as in-vehicle internet access, inter-vehicle communication, etc. The increasing density of vehicles allows the deployment of distributed relaying vehicle systems which can support above systems with low probability of outage.

Examples (5) Ad-hoc Networks Distributed relaying will be shown to increase the link stability, or alternatively decrease the link outage probability, significantly.

Examples (6) Sensor Networks Distributed relaying will be shown to decrease the power consumption per relaying sensor node.

Information Flow (1) Point to Point (Traditional) Point to Multi-point (Broadcast) Multipoint to Point (Multiple Access) Multipoint to Multi-point (General)

Information Flow (2) Direct Link (No relays between source and target) Relaying Links (relay(s) between source and target) Relaying Stages (Clusters where information passes approximately the same time)

Node Behavior Egoistic (No help) Supportive (Unidirectional help) Cooperative (Mutual help)

Cooperation Idea

Relaying Traditional relaying approach: single link connection from source to sink with the following characteristics: RF: Cannot listen and talk at the same time in the same band PHY: Transparent, regenerative or hybrid relaying mechanisms MAC: Reservation-based or randomized access schemes

Pros and Cons of Relaying Advantages: Coverage area of BS or AP can be extended. Infrastructure-less networks can be maintained. Aggregate path-loss is lower than for direct link communication. Hence, transmit power is lower and/or lower and/or data rates higher. Disadvantages Transceiver complexity may increase. Synchronization and access methods are more complex compared to traditional solutions. More traffic and hence interference is generated: also end- to-end delays are higher. Where is real application after decades of research?

Channel Scenarios

Wideband Signals and Cooperation Channel Behavior Wideband Signals and Cooperation

Cooperation Methods Amplify and Forward Decode Coded Cooperation

Decode and Forward Methods: Repetition based (repeat codeword during relaying) Channel code based (relay parity information) Space-time code based (construct ST codeword)

Coded Cooperation one practical scheme

Performance Improvement

Distributed Space-Time Coding Two main differences with a traditional STC: Relays are not co-located and do not cooperate in general. Noise at relay nodes also multiplied by channel coefficients gi.

Distributed Cooperation (1) Distributed Space-time Coding Two main differences with a traditional STC: Relays are not co-located and do not cooperate in general. Noise at relay nodes also multiplied by channel coefficients gi.

Prons and Cons of Distributed Cooperation Combining both techniques i.e. MIMO preferably in a distributed fashion, and relaying techniques, we have: Advantages: Low TX power consumption or high data rates due to MIMO. Low TX power consumption or high data rates due to relaying. Increased coverage area and no need for infrastructure. Low correlation to facilitate MIMO and hence diversity/multiplexing gains. Disadvantages: Interference and end-to-end delays are generally still high(er). Complexity of network maintenance is increased, e.g. synchronization.

Different Levels of Cooperation Use inter-cluster virtual MIMO Transmission. At receiver cluster, nodes with higher SNRs can send differential of mutual information required by destination to decode the message.