Multi-Carrier Burst Contention (MCBC)

Medium Access Control continues to be a hot topic and the current trend of everything wireless is raising issues that call for thorough investigation and improved designs. Our interests lie in scalability and efficiency as well as low-power and reliable Quality of Service. We are interested in designing, modeling and implementing schemes capable of coping with large and dense networks and rapid topology changes - increasingly common characteristics of present and future wireless networks - while striving for low-complexity in order to achieve a low-power consumption system.

MCBC hardware implementation We designed the Multi-Carrier Burst Contention or MCBC, a new class of protocols using a randomized two-dimensional and cross-layer leader election scheme. During contention, it exploits both the frequency and the time domains to achieve throughputs close to the theoretical limits even for networks with hundreds (and, theoretically, even thousands) of active nodes. The contention spans multiple rounds and is mapped onto random and unmodulated subcarriers of an underlying OFDM physical layer, requiring only minimal changes to a standard OFDM PHY (e.g. IEEE 802.11a/g/n), and supporting adhoc as well as infrastructure configurations.

MCBC has been implemented in hardware on an FPGA platform using a multi-core design, providing a testbed for current and future research. We have also developed an MCBC simulator which is available for download along with our dot11agn simulator. The Verilog and C++ source code for the hardware implementation and software simulator respectively will be made available in the future. An analytical model that accounts for realistic factors such as channel fading and cumulative signal strength, is also available.

MCBC contention (slide extract) Most advantages of MCBC stem from the two-dimensional leader election scheme designed to converge very rapidly in order to minimize collision probability and overhead. The scheme is randomized in both frequency and time domains to achieve a steep descent in number of contenders. Simulations also show that the feedback mechanism employed in each contention round works equally well in ad-hoc as well as infrastructure configurations. The algorithm ensures statistical fairness and its non-adaptive nature makes it resilient to changes in network topology. Overall, the collision probability is kept below 5% after only 3 rounds with several hundreds of active contending nodes.

Performance in fading channels As with almost all new designs, MCBC is no exception regarding challenges, many of which are inherent to wireless access schemes employing individual OFDM subcarriers. The most important challenges are the frequency spectrum shift due to local oscillator drift and offset (Doppler shift is a negligible problem), time desynchronization, frequency selective fading, timing constraints and others. The solutions we developed for these challenges (as well as for other implementation issues) strive for maintaining low complexity.

Some of the design features of MCBC that were attained are high throughput, scalability, low overhead and delay, low complexity, resilience to topology changes and the ability to perform equally well in both ad-hoc and infrastructure communication. Reliable Quality of Service at the MAC layer with support from the PHY was another design feature. Current and future research includes interoperation with IEEE 802.11, full QoS implementation and adaptive and cooperative extensions to improve efficiency and fairness in asymetric channel conditions.

[1] B. Roman, I.Wassell and I. Chatzigeorgiou, "Scalable Cross-Layer Wireless Access Control Using Multi-Carrier Burst Contention," in IEEE Journal on Selected Areas in Communications, vol. 29, no. 1, pp. 113-128, Jan 2011.

[2] B. Roman, I. Chatzigeorgiou and I. Wassell and F. Stajano, "Evaluation of Multi-Carrier Burst Contention and IEEE 802.11 with Fading During Channel Sensing," in the 20th IEEE Intl. Symposium on Personal Indoor and Mobile Radio Communications Conference, PIMRC'09, Tokyo, Sep 2009.

[3] B. Roman, F. Stajano, I. Wassell and D. Cottingham, "Multi-Carrier Burst Contention (MCBC): Scalable Medium Access Control for Wireless Networks," in the 9th IEEE Intl. Wireless Communications and Networking Conference, WCNC'08, Las Vegas, Mar 2008