Communication over interference in wireless networks

The Hong Kong University of Science and Technology
Department of Computer Science and Engineering


PhD Thesis Defence


Title: "Communication over interference in wireless networks"

By

Mr. Kaishun Wu


Abstract

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.


Date:			Thursday, 2 June 2011

Time:			10:00am - 12:00noon

Venue:			Room 3494
 			Lifts 25/26

Chairman:		Prof. Qingping Sun (MECH)

Committee Members:	Prof. Lionel Ni (Supervisor)
 			Prof. Shing-Chi Cheung
 			Prof. Qian Zhang
                      	Prof. Furong Gao (CBME)
                         Prof. Jiannong Cao (Computing, PolyU.)


**** ALL are Welcome ****