Wireless communications has become an essential part of everyday life, not only the number of mobile users has increased exponentially in the rich and poor world alike, but the merger of mobile phones and computers in the form of smart phones and the emergence of tablets and cloud computing has drastically opened the way for many new position-based, data, and multimedia applications.
However these new wireless revolution require a massive increase in users’ data rate which is expected to reach several tens of Mbps. unlike wired communication where one can easily increase capacity by adding more fibre. Wireless communication uses limited resources (time, bandwidth) and battery-powered mobile-held devices that are limited in transmit power as well. Therefore, Maximising Bandwidth efficiency, exploiting all available degrees of freedom (time, frequency, and space), minimising power consumption and adaptation to the fading nature of wireless channels are the main research objectives enabling improved performance and increased capacity.
One key technology that is widely acknowledged to fulfil these objectives is multiple-input-multiple-output (MIMO) systems. Spatial multiplexing (SM) schemes that use MIMO architecture and layered space-time decoding at the receiver, promise enormous spectral efficiency in a range of 4-40 bits/sec/Hz and much improved error performance over conventional single input single output (SISO) systems without consuming extra bandwidth or transmit power. However SM-MIMO is limited by the lesser number of transmit and receive antennas, the amount of feedback required, and by channel correlations due to insufficient antenna separation at the terminals and/or poor scattering environment. As a result, the sum rate capacity and bit error rate (BER) performance are significantly degraded and users with highly correlated channels may not be served which reduces the user capacity. To address these problems, we propose a novel collaborative modulation multiple access (CMMA) scheme based on collaborative precoding using a simple common pilot feedback. CMMA provides a non-orthogonal network access without subdivision in time, frequency, or code. Moreover, unlike spatial multiplexing, CMMA can operate with a single receive antenna and is resilient to correlation levels among users’ channels. CMMA assigns users with unique collaborative modulation sets jointly designed at the BS to insure that the received composite signal is part of a higher constellation set with a rate equal to the sum rate of all individual users and with a structure that aims to maximize the minimum Euclidian distance among constellation points. The receiver periodically broadcasts a simple common pilot to provide Channel state information at each transmitter (CSIT) to enable precoding which insures that each user’s received signal belongs to the modulation set assigned for that user. A simple non-coherent Maximum likelihood detector (ML) jointly decodes the composite signal eliminating the need for pilots in transmitted packets to reduce overhead and increase net data rate. We also propose a new CMMA selective diversity scheme that requires no pilot-aided channel estimation at the receiver and no change in the precoding algorithm at transmitters while maintaining resilience to any correlation level.
Summary and Contributions
1. A new collaborative modulation multiple access schemes using a simple feedback channel was introduced and analyzed.
2. CMMA uses synchronized pre-coded individual modulated signal carefully designed and fading-compensated to form a composite higher modulation constellation
3. Decoding is achieved by only using a simple single-stage ML receiver with no need for pilots or CSI at the receiver.
4. CMMA composite Constellation design offers a consistent and power-efficient performance that is adaptive to channel fading fluctuations, channel correlation , and changes in group size
5. CMMA offers a flexible way that can accommodate users with different rates and error protection capability.
6. Simulation results shows that CMMA offers better BER performance and higher capacity than power-limited single user equivalent or the non precoding multiuser approach with CSI at the receiver.
7. A new selective diversity combining technique for multiuser transmission was introduced and analysed, this techniques provides a simple algorithms to achieve diversity gain in MIMO-CMMA systems without any additional requirement on the users in the transmitters side compared with SIMO-CMMA, however results shows that the diversity gain that can be achieved highly depends on the number of users within the CMMA group and the diversity gain diminishes with increasing number of receive antennas.