Improving channel efficiency using physical layer information in wireless systems
Date of Issue2016-03-16
School of Computer Engineering
Parallel and Distributed Computing Centre
Wireless communication has changed our world and has become an integral part of our daily life. While the wireless spectrum is a finite resource, the demand for higher communication capacity is increasing continuously and rapidly. Thus one important research direction is to improve channel efficiency - to fully utilize the limited wireless spectrum. Channel efficiency of wireless systems, nevertheless, is limited by interferences, noises, node collisions etc. This thesis focuses on two representative wireless systems - IEEE 802.11 WLANs and RFID systems, and investigates rich information in physical layer to enhance channel efficiency in wireless systems. IEEE 802.11 WLANs (Wireless Local Area Networks) is used almost everywhere, which provides Internet access for portable computing devices using radio waves. In one typical IEEE 802.11 WLAN, several nodes share the wireless media and communicate with APs (Access Points). To avoid collision, nodes transmit their own packets only when the channel is sensed to be “idle” based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol. Due to the broadcast nature of wireless media and signal attenuation in wireless environment, IEEE 802.11 wireless transmission suffers from high packet losses. Packet retransmission is a fundamental way to recover a lost packet. To extract useful information from incorrect physical layer symbols and improve retransmission efficiency, this thesis presents MISC, a packet retransmission scheme that Merges Incorrect Symbols using Constellation diversity from multiple transmissions to produce correct ones. MISC proactively creates constellation diversity by rearranging the constellation maps in retransmissions. MISC addresses practical implementation issues and makes minimum amendments to integrate into current IEEE 802.11 WLAN framework. RFID (Radio Frequency IDentification) technology has recently attracted much attention from both research community and industry. An RFID system consists of RFID readers and RFID tags. A typical RFID tag is capable of storing information, lightweight computation, harvesting ambient energy, receiving and transmitting data. When interrogated by a reader, a passive RFID tag delivers its data information by alternatively backscattering and absorbing reader’s carrier waves. The reader detects the change of signal magnitude and decodes the transmitted data. For a reader to interrogate multiple tags, current commodity RFID systems adopt Frame Slotted ALOHA collision resolution protocol and incur high communication overhead due to severe tag-to-tag collisions. Although some recent works have been proposed to support parallel decoding for concurrent tag transmissions, they require accurate channel measurements, tight tag synchronization, or modifications to standard RFID tag operations. This thesis presents BiGroup (Bipartite Grouping), a novel RFID communication paradigm that allows the reader to decode the collision from multiple COTS (commodity-off-the-shelf) RFID tags in one communication round. In BiGroup, COTS tags can directly join ongoing communication sessions and get decoded in parallel. The collision resolution intelligence is solely put at the reader side. To this end, BiGroup examines the tag collisions at RFID physical layer from constellation domain as well as time domain, exploits the under-utilized channel capacity due to low tag transmission rate, and leverages tag diversities. We investigate wireless communication systems based on Software Defined Radio (SDR) platforms. An SDR system is essentially a personal computer attached to an RF front end. Wireless signals are processed in the personal computer and transmitted/received in the RF front end. Signal processing modules such as filters, modulators/demodulators are performed by software which allows full access. To study the performance of MISC, we prototype MISC on the IEEE 802.11 based GNURadio/USRP platform and conduct extensive experimental evaluation under practical settings. Experiment results demonstrate that MISC substantially improves the IEEE 802.11 WLAN throughput. To evaluate the effectiveness of BiGroup, we build an RFID reader running BiGroup decoding scheme using GNURadio/USRP testbed and read various COTS RFID tags as well as programmable WISP (Wireless Identification and Sensing Platform) tags. Experiment results demonstrate that BiGroup greatly improves the RFID communication efficiency.