Research Overview
The original Multiuser Communications programme spans direct-sequence and multicarrier CDMA, interleave-division multiple access, hybrid ultra-wideband transmission, adaptive-rate receivers and OFDMA. The common problem is the separation of simultaneous users in the presence of fading and multiple-access interference.
Code Division Multiple Access: DS-CDMA, MC-CDMA, MC-DS-CDMA
DS-CDMA spreads each user’s signal in time; MC-CDMA spreads across subcarriers, and multicarrier DS-CDMA combines both dimensions. These designs trade multipath resilience, user capacity and receiver complexity, with joint detection and space–time spreading available to suppress interference.
Selected Publications and Resources
- Multiuser Detection Assisted Time- and Frequency-Domain Spread Multicarrier Code-Division Multiple-Access
- Single- and Multi-Carrier DS-CDMA: Multi-User Detection, Space-Time Spreading, Synchronisation, Networking and Standards
- Performance of Broadband Multicarrier DS-CDMA Using Space-Time Spreading-Assisted Transmit Diversity
Multicarrier DS-CDMA transmitter
Interleave Division Multiple Access (IDMA) and Random Waveform Channel Code Division Multiple Access
IDMA distinguishes users through chip-level interleavers and supports iterative multiuser detection. The work also considers random-waveform or code-division schemes based on repeat–accumulate and LDPC codes, including structured interleaver construction to reduce storage requirements.
Selected Publications and Resources
- Three Design Aspects of Multicarrier Interleave Division Multiple Access
- Repeat Accumulate Code Division Multiple Access and Its Hybrid Detection
Multiuser MIMO IDMA transceiver
Structured interleaver design
Latin-square interleaver design
Ultra Wide Band Communications: Hybrid Direct Sequence and Time Hopping
Hybrid direct-sequence/time-hopping ultra-wideband transmission combines two spreading mechanisms. Varying their relative factors changes interference, error rate and pulse characteristics; adaptive and reduced-rank detectors are investigated for difficult multiuser channels.
Further Publications from the Original Research Page
- Reduced-Rank Adaptive Multiuser Detection in Hybrid Direct-Sequence Time-Hopping Ultrawide Bandwidth Systems
- EXIT-chart aided three-stage concatenated ultrawideband time-hopping spread-spectrum impulse radio design
Burst-by-Burst Adaptive Code Division Multiple Access
Burst-by-burst adaptation selects a data rate appropriate to changing channel conditions, using methods such as adaptive QAM and variable spreading factors. Joint detection helps estimate link quality despite multipath and multiple-access interference.
Classification of CDMA detectors
Adaptive QAM concept
Wireless Multi-user Orthogonal Frequency Division Multiplexing Systems (OFDMA)
OFDMA divides subcarriers among simultaneous users. The research considers adaptive allocation, equalisation, interference and rate decisions in multicarrier links, including trade-offs between bandwidth efficiency and implementation complexity.
OFDM transmission architecture