Paul A. Anghel

Exact symbol error probability of a Cooperative network in a Rayleigh-fading environment

In a distributed spatial diversity wireless system, not all antennas are located at one station as in classical transmit diversity systems, but are dispersed at different, possibly mobile, stations in the network. Transmit diversity is created when the selected stations assist a sender by relaying its information signal to the destination. In this letter, we present an exact average symbol error rate analysis for the distributed spatial diversity wireless system with K amplifying relays in a Rayleigh-fading environment. The average symbol error rate formula allows us to clearly illustrate the advantage that the distributed diversity system has in overcoming the severe penalty in signal-to-noise ratio caused by Rayleigh fading. Using simple bounds on the probability of error, we show that the cooperative network presented in this letter achieves full diversity order.

Multi-user space-time coding in cooperative networks

Multiple antennas at the receiver and transmitter are often used to combat the effects of fading in wireless communication systems. However, implementing multiple antennas at mobile stations is impractical for most wireless applications due to the limited size of the mobile unit. We emulate spatial diversity using mobile relay stations, which cooperate by retransmitting the information received from a mobile station to a destination station. We propose an Alamouti based cooperative system with two relay stations and we provide an approximate formula for the average symbol error probability of this system in a Rayleigh fading environment.

On the performance of distributed space-time coding systems with one and two non-regenerative relays

Spatial diversity can be induced by using wireless relay stations, which cooperate by amplifying and retransmitting the information received from a source to a destination station. In this context we propose a distributed space-time coding (DSTC) system based on the Alamouti codes. We characterize the symbol error rate of systems with one and two non-regenerative relays using bounds and high signal-to-noise ratio (SNR) approximations. The asymptotic (high SNR) symbol error probability formulas are used to optimize the power allocation in the DSTC system. Furthermore, using the asymptotic symbol error probability formulas we argue that the DSTC system has at least 1.5 times the diversity achieved by point-to-point transmissions with the same bandwidth. Simulations show not only that the DSTC outperforms the amplify-and-forward cooperative system with orthogonal transmissions, but also convolutional encoded one-hop transmissions with the same information rate as the DSTC system. Assuming full channel knowledge at the source and the relays, we find an optimum cooperative system by minimizing the bit error rate of the DSTC system with one and two non-regenerative relays subject to fixed transmit energy constraints at each radio. Numerical results show that the DSTC system with two relays performs very close to the optimum cooperative system.

Distributed Space-Time Cooperative Systems with Regenerative Relays

This paper addresses some of the opportunities and the challenges in the design of multi-hop systems that utilize cooperation with one or two intermediate regenerative relays to provide high-quality communication between a source and a destination. We discuss the limitations of using a distributed Alamouti scheme in the relay channel and the additional complexity required to overcome its loss of diversity. As an alternative to the distributed Alamouti scheme, we propose and analyze two error aware distributed space-time (EADST) systems built around the Alamouti code. First, using a bit error rate based relay selection approach, we design an EADST system with one and two regenerative relays that rely on feedback from the destination and we show that the proposed system improves on the distributed Alamouti scheme. In addition, we prove that the proposed one relay EADST system collects the full diversity of the distributed MISO channel. Second, we introduce an EADST system without feedback in which the relaying energies depend on the error probabilities at the relays. Numerical results show that both EADST systems perform close to the error probability lower bound obtained by considering error-free reception at the relays

Relay assisted uplink communication over frequency-selective channels

Based on block-orthogonal frequency division multiple access (block-OFDMA) and distributed space-time coding, we provide a novel design for a multi-user uplink communication system with fixed wireless relay stations. The relays can decode the information symbols received from the mobile users in the system and forward this information to the base station. We do not favor a system that enforces cooperation between users and relays at all times. We propose an adaptive scheme for a Rayleigh fading environment in which users and relays cooperate depending on the average power of the channels in the network. The proposed adaptive cooperative scheme increases the performance and the flexibility of the OFDMA uplink cellular system.

On the diversity of cooperative systems

The paper addresses some of the opportunities and the challenges that could arise in the design and analysis of multi-hop systems that utilize cooperation among several intermediate regenerative relays to provide reliable high-quality communication between a source and a destination. This mode of communication can be viewed as a network-embedded distributed, or extended MIMO system. Using the infrastructure provided by the uplink cellular system, we develop and analyze a distributed space-time coding (DSTC) system based on the Alamouti design. We show that with limited feedback from the base station, a DSTC system with two relays, which act as tetherless antennae, is able to induce and collect diversity in a distributed MIMO setup.

Multi-carrier multiple access is sum-rate optimal for block transmissions over circulant ISI channels

We establish that practical multiple access based on finite size information blocks transmitted with prescribed power and with loaded multicarrier modulation, is optimal with respect to maximizing the sum-rate of circulant intersymbol interference (ISI) channels, that are assumed available at the transmitter. Circulant ISI channels are ensured either with cyclic prefixed block transmissions and an overlap-save reception, or, with zero-padded block transmissions and an overlap-add reception. Analysis asserts that sum-rate optimal multicarrier users could share one or more subcarriers depending on the underlying channels. Optimal loading is performed by specializing an existing iterative low-complexity algorithm to circulant ISI channels.

Generalized multicarrier CDMA: unification and linear equalization

Relying on block-symbol spreading and judicious design of user codes, this paper builds on the generalized multicarrier (GMC) quasisynchronous CDMA system that is capable of multiuser interference (MUI) elimination and intersymbol interference (ISI) suppression with guaranteed symbol recovery, regardless of the wireless frequency-selective channels. GMC-CDMA affords an all-digital unifying framework, which encompasses single-carrier and several multicarrier (MC) CDMA systems. Besides the unifying framework, it is shown that GMC-CDMA offers flexibility both in full load (maximum number of users allowed by the available bandwidth) and in reduced load settings. A novel blind channel estimation algorithm is also derived. Analytical evaluation and simulations illustrate the superior error performance and flexibility of uncoded GMC-CDMA over competing MC-CDMA alternatives especially in the presence of uplink multipath channels.

Wideband generalized multicarrier CDMA over frequency-selective wireless channels

Relying on symbol blocking and judicious design of user codes, this paper builds on a generalized multicarrier (GMC) quasi-synchronous CDMA system capable of multiuser interference (MUI) elimination and intersymbol interference (ISI) suppression with guaranteed symbol recovery, regardless of the wireless frequency-selective channels. GMC-CDMA provides a unifying framework for multicarrier (MC) CDMA systems and offers flexibility in full load (maximum number of users allowed by the available bandwidth) as well in reduced load settings. Analytic evaluation and simulations illustrate that GMC-CDMA outperforms competing MC-CDMA alternatives especially in the presence of uplink multipath channels.

Load-Adaptive MUI/ISI-resilient generalized multi-carrier CDMA with linear and DF receivers

A plethora of single-carrier and multi-carrier (MC) CDMA systems have been proposed recently to mitigate intersymbol interference (ISI) and eliminate multiuser interference (MUI). We present a unifying all-digital Generalized Multicarrier CDMA framework which enables us to describe existing CDMA schemes and to highlight thorny problems associated with them. To improve the bit error rate (BER) performance of existing schemes, we design block FIR transmitters and decision feedback (DF) receivers based on an inner-code/outer-code principle, which guarantees MUI/ISI-elimination regardless of the frequency-selective physical channel. The flexibility of our framework allows further BER enhancements by taking into account the load in the system (number of active users), while blind channel estimation results in bandwidth savings. Simulations illustrate the superiority of our framework over competing MC CDMA alternatives especially in the presence of uplink multipath channels.