Communication over interference in wireless networks
by Kaishun Wu
Ph.D. Computer Science and Engineering
xvi, 137 p. : ill. ; 30 cm
Wireless technologies grow rapidly and benefit almost every aspect of our daily lives. In a typical multiple-user environment, different users may severely interfere with each other. How to reduce the coordination overhead in order to improve the efficiency of the wireless networks becomes a big challenge....[ Read more ]
Wireless technologies grow rapidly and benefit almost every aspect of our daily lives. In a typical multiple-user environment, different users may severely interfere with each other. How to reduce the coordination overhead in order to improve the efficiency of the wireless networks becomes a big challenge.
Unlike the wired counterpart, a wireless link is easily affected by environment changes and surrounding wireless activities. Determining the instant link conditions (or qualities) is essential for most protocol designs and application developments in wireless communications. In previous studies, link-level metrics are utilized to reflect the link conditions such as Received Signal Strength Indication (RSSI), Signal-to-Noise Ratio (SNR) and Signal-to-Interference plus Noise-Ratio (SINR). In practice, however, these metrics exhibit many limitations and could be misleading. As they are often the statistic measurements over the packet transmission while the link conditions may vary dramatically, the packet-level metrics are unable to indicate the instant link condition. Motivated by this, we propose to use more fine-grained information from the lower layer of the network protocol stack. A chip is an accessible element at the physical layer for many wireless standards such as IEEE 802.15.4 and 802.11b. In these standards, information bits are repackaged as certain sequences of chips before being transmitted over the air. As the chip duration is much shorter, it is more capable to capture the instant channel changes. Analyzing the chip-level error characteristics brings a new measure for the upper-layer network design such as the network diagnosis, transmission power control, routing, localization and topology control.
With the interesting observation that by generating intended patterns, some simultaneous transmissions, i.e., ”interference”, can be successfully decoded without degrading the effective throughput in original transmission. An extra and ”free” coordination channel is thus designed based on the coding redundancy in DSSS. Based on this idea we propose a DC-MAC to leverage this ”free” channel for efficient medium access in a multiple-user wireless network. I also theoretically analyze the capacity of this channel under different environments with various modulation schemes.
However, the previous Side Channel design is based on the coding redundancy in DSSS which cannot work in OFDM-based WLANs. I then propose a new communication model where the control frames can be ”attached” to the data transmission. Thus, control messages and data traffic can be transmitted simultaneously and consequently the channel utilization can be improved significantly. I implement the idea in OFDM-based WLANs called hJam, which fully explores the physical layer features of the OFDM modulation method and allows one data packet and a number of control messages to be transmitted together.