Background

In cooperative systems, the relay helps a direct source-destination link. Cooperative communication with multi-antenna relays can significantly increase the reliability and speed. However, cooperative MIMO detection would impose considerable complexity overhead onto the relay if a full detect-and-forward (FDF) strategy is employed. In order to address this challenge, we propose a novel cooperative partial detection (CPD) strategy to partition the detection task between the relay and the destination. CPD utilizes the inherent structure of the tree-based sphere detectors, and modifies the tree traversal so that instead of visiting all the levels of the tree, only a subset of the levels, thus a subset of the transmitted streams, are visited. Based on this methodology, the destination combines the source signal and the partial relay signal to perform the detection step.

More information can be found here: Cooperation Paper

Using the WARPLab platform, we show that with the CPD approach, the relay can avoid the considerable overhead of MIMO detection while helping the source-destination link to improve its performance.

WARPLab Experiments of Cooperative Partial Detection

In this setup, multiple WARP boards are connected to a host PC through an Ethernet switch. A set of boards are designated as the transmitters and a set of boards are designated as the receivers through the WARPLab framework. To transmit, raw samples (I/Q) values are generated in MATLAB and uploaded to the transmit boards via Ethernet. When the host PC sends a trigger signal to all nodes via Ethernet, the transmit boards modulate the raw samples in the FPGA transmit buffers to the 2.4 GHz frequency signals, and transmit them through the radio boards of the transmit boards. The receive boards capture the incoming RF signals, downconvert them to the baseband samples and store them in the receive buffers of the FPGA. The host PC, in MATLAB, reads and process the values in the receive buffers via Ethernet. Note that the relay's baseband physical layer is a highly parallel architecture, and therefore, can effectively utilize FPGA's resources. As part of the future work, we will transfer parts of these processing from the host PC to the FPGA.

Our test setup is a 2 x 2 three node cooperative system. A total of three WARP boards are connected to a host computer through an Ethernet switch. Experiments are conducted using an Azimuth ACE 400 WB wireless channel emulator. The emulator can support up to a 4 x 4 setup, and has four inputs and four outputs and 16 bidirectional links. For the 2 x 2 full MIMO relay setup, we use 2 inputs, 4 outputs and 12 paths. For the first time slot, we designate one node as the source, one node as the relay, and one node as the destination. Four forward links are used to connect the source node to the relay node. Similarly, four other links are used to connect the source node to the destination node. Since all processing is done at the host computer, we can use the reverse link (which can be independent from the forward link) for the relay to destination link. We designate one node as the relay and one node as the destination and connect the two nodes with four reverse links.