A Demonstration of Video over a Cooperative PHY layer Protocol

Cooperative communication is a promising approach to improve the reliability of a received signal at the physical layer. Cooperating nodes create a virtual MIMO system that provides spatial diversity even though the nodes have a single antenna. Therefore, tremendous improvements in system capacity and delay can be achieved. An implementation approach has been pursued in this demonstration to confirm the viability and efficacy of cooperation at the physical layer. The implemented cooperative physical layer scheme is called CoopPHY and is based on Cooperative Coding. In this technique, the source node transmits only a portion of its encoded data bits based on channel conditions among cooperating nodes. Helper nodes transmit the rest of the encoded bits. The destination node, finally, combines and decodes signals received via different channels and therefore increases the decoding reliability.In this demo, a Matlab video clip is streamed from a server to a client using sequentially the CoopPHY scheme and a non-cooperative scheme. The CoopPHY protocol delivers a smooth user experience, while the video playout over the non-cooperative scheme shows frequent distortions. The demo clearly demonstrates the performance superiority of the implemented cooperative PHY layer scheme, as compared to the traditional transmission techniques that are based on a single hop transmission. Lab’s website: http://witestlab.poly.edu/ I. THE COOPERATIVE PHY PROTOCOL(COOPPHY) The implemented cooperative PHY layer scheme is called CoopPHY and is based on Cooperative Coding, a technique that achieves diversity by using distributed FEC coding [1]. The basic functionality of the protocol is illustrated in Figure 1 and summarized as follows. When a wireless node experiences a bad channel it uses cooperative communication by recruiting a relay station. The relay, which we call helper in this protocol, has a good channel with both the source and destination stations. As shown in the Figure, source node encodes a packet of length k bits using a Fig. 1: Cooperation at PHY layer. block of n = n1+n2 bits. This is done by using convolutional coding with a coding rate R=k/n, where R < 1. Based on the channel condition among the nodes(source, helper and destination), the source removes n2 bits out of the original n coding bits by using a puncturing technique. After this, the source node transmits the remaining n1 bits. The helper node receives these n1 bits and decodes with high probability due to the good channel between the source and itself. If the decoded data is correct, as determined by CRC, helper re-encodes the data using n2 bits and transmits. Destination recombines the two transmissions and decodes them. Using this technique the decoding of the frame is more robust, a fact that can lead to lower PER for the same transmission rate or to higher transmission rates for the same PER. II. IMPLEMENTATION OF THE PHY LAYER COOPERATIVE SCHEME The implementation of the CoopPHY protocol in this demo is done on the WARP platform [2]. For the upon the physical layer design we used the WARPLAB framework [3] In this framework, the PHY layer has been designed as a collection of matlab files. Main features of the design include convolutional coding with a coding rate equal to 1/2 and a memory between 3 and 7, rate compatible punctured codes for the selection of the coded bits that will be transmitted by the source and the helper, an enhanced Viterbi decoder system that is able to decode using RSSI as weights for each coded signal. III. DEMONSTRATION OF VIDEO OVER COOPPHY A. Demo Configuration The demo consists of 1 laptop and 3 WARP nodes(source, helper and destination), WARP nodes are connected to the laptop through an Ethernet cable. The basic positioning of the nodes is outlined in Figure 2. The destination node is kept behind an obstruction such that it is not in line-of-sight with the source or the helper node. Source and helper nodes are in line-of-sight with each other. The 3 WARP nodes act as a transceiver for the source, helper and destination. The logical part of the three stations resides in the laptop, in the form of matlab files.