Development and Performance Evaluation of an Adaptive MAC Protocol for MC-CDMA Wireless LANs with QoS Support

The Institute of Electrical and Electronics Engineers, Inc. (IEEE) standards 802.11a,e are currently established as worldwide standards for Wireless Local Area Networks (WLANs). Applying Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), the Distributed Coordination Function (DCF) and Enhanced Distributed Coordination Function (EDCF) protocols deploy minimum complexity in Medium Access Control (MAC), while the use of Orthogonal Frequency Division Multiplex (OFDM) in Physical layer (PHY), promises high data rates of up to 54 Mbit/sec. The system throughput at MAC layer is though reduced both, from the overhead and in so called hot spot scenarios by the high number of collisions which limit the system’s performance and consequently its Quality of Service (QoS) achievements. A possible improvement, is the application of a new multiple access method, namely Multi-Carrier Code Division Multiple Access (MC-CDMA). MC-CDMA is a novel, high capacity, multicarrier modulation scheme, which is developing to a key radio transmission technology for future WLANs. Besides the fact that its Bit Error Rate (BER) performance (in fully loaded indoor communication systems) is due to the spreading gain better
than that of Orthogonal Frequency Division Multiple Access (OFDMA), when low or no mobility scenarios are considered, MC-CDMA can reduce the probability of collisions, due to its inherent characteristic of operating parallel, from spreading sequences distinguished, channels in the same frequency channel. Thus the system throughput enhances and the delay reduces. The system throughput profits further as spreading improves the ratio of frame length to guard intervals, thus implicitly reducing the overhead.