Onur Oguz

Rate-optimized power allocation for DF-relayed OFDM transmission under sum and individual power constraints

We consider an OFDM (orthogonal frequency division multiplexing) point-to-point transmission scheme which is enhanced by means of a relay. Symbols sent by the source during a first time slot may be (but are not necessarily) retransmitted by the relay during a second time slot. The relay is assumed to be of the DF (decode-and-forward) type. For each relayed carrier, the destination implements maximum ratio combining. Two protocols are considered. Assuming perfect CSI (channel state information), the paper investigates the power allocation problem so as to maximize the rate offered by the scheme for two types of power constraints. Both cases of sum power constraint and individual power constraints at the source and at the relay are addressed. The theoretical analysis is illustrated through numerical results for the two protocols and both types of constraints.

Improved OFDM transmission with DF relaying and power allocation for a sum power constraint

In this paper we consider an OFDM (orthogonal frequency division multiplexing) transmission scheme with a relay. Symbols sent by the source may or may not be retransmitted during a second time slot by the relay, which is assumed to be of the decode-and-forward (DF) type. For each relayed carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem in order to maximize the rate offered by the scheme. Compared to [VDLZ08], the second time slot is better used. The source is allowed to transmit a new symbol during this second time slot when the relay is inactive. The optimization is performed for a sum power constraint. The theoretical analysis is illustrated by numerical results.

Power Allocation for Improved DF Relayed OFDM Transmission: The Individual Power Constraint Case

We consider an OFDM (orthogonal frequency division multiplexing) point to point transmission scheme improved by a relay. For each carrier, symbols sent by the source may be retransmitted during a second time slot by the relay, which is assumed to be of the Decode-and-Forward (DF) type. For each relayed carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem in order to maximize the rate offered by the scheme. Compared to [1], the second time slot is better used. The source is allowed to transmit a new symbol during this second time slot when the relay is inactive. For this improved protocol, the optimization has been conducted for a sum power constraint and reported in [2]. The present paper is devoted to the case of individual power constraints at the source and at the relay. The theoretical analysis is illustrated by numerical results.

Rate-Optimized Power Allocation for OFDM Transmission with Multiple DF/Regenerative Relays and an Improved Protocol

We consider an OFDM (orthogonal frequency division multiplexing) point to point transmission scheme improved by means of multiple relays. For each carrier, symbols sent by the source during a first time slot, may be retransmitted during a second time slot by the relays, which are assumed to be of the Decode-and-Forward (DF) type. For each relayed carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem in order to maximize the rate offered by the scheme. Similarly to the protocol proposed in [1], the source is allowed to transmit a new symbol during the second time slot when none of the relay is assisting. The contraints of decodability at the relays are properly handled. The optimization is conducted for a constraint on the sum of powers at the source and at the relays. Next to the optimized solution, a suboptimum method based on relay selection is proposed and discussed. The theoretical analysis is illustrated by numerical results.

Distributed Space-Time-Frequency Block Codes for Multiple-Access-Channel with Relaying

In this paper, we investigate diversity gain in coding for a 2-user multiple-access-channel (MAC) with cooperating transmitters-the MAC with relaying. We propose a simple distributed space-time-frequency block coding (D-STFBC) scheme and analyze the offered diversity gain. In particular, we show that full diversity (order 3) is possible if collaboration is well enough and rigorous signal processing is assumed both at the transmitters and the receiver. Bit-error-rate (BER) analysis and curves are provided for illustrative purposes.

Lowcomplexity iterative detection in the presence of nuisance parameters

This work addresses the bit-wise optimal data detection problem when unknown nuisance parameters influence the observation at the receiver. For an arbitrary communications system, the optimal maximum a-posteriori detection problem is first defined as a marginalization of a joint distribution which statistically models the interaction of available sets of variables/parameters. Then, using a factor graph representation with an accompanying sum-product message passing algorithm, it is shown that the marginalization can be performed iteratively. To alleviate complexity due to the marginalization over continuous natured nuisance parameters, variational Bayesian approximation is introduced and it is shown that, if the nuisance parameters are constant for a period of time, the receiver has linear complexity.

Advanced Transmitter and Receiver Design

This chapter addresses the design of receivers for multiple-input multiple-output (MIMO) communication. It covers the decoding of error-correcting codes, the mitigation of intersymbol and cross-antenna interference, and the synchronization, that is, the estimation of channel parameters. It presents the techniques that are based on the turbo principle. The turbo principle originates from the turbo decoding algorithm, which has shown how to approximate an intractable optimal decoder with a simpler, iterative decoder. It also presents the receivers that use the same design idea, referred to as the turbo principle, to carry out interference mitigation and synchronization. This results in high-performance iterative receivers that address decoding globally by iterating between tasks, and that approximate closely the optimal but intractable decoders. It introduces turbo equalization, a technique to mitigate intersymbol interference and extends turbo equalization to MIMO channels, where the interference comes both from adjacent symbols and from other antennas.

A novel adaptive iterative detection technique for joint estimation and detection

Reliability of the communication systems depends hugely on the receiver performance where the synchronization and detection tasks need to be performed. Classically these two tasks are attended separately resulting in simple yet non-optimal receivers. During the last decade, a family of iterative receivers has been introduced to approximate the optimal solution to joint estimation and detection problem by means of the turbo principle. With the help of graph theory and the belief propagation framework these receiver structures are unified and soft information driven schemes emerged, leading to more reliable detection. The essence of these schemes lays in factorization of a global function whose marginal corresponds to the objective function and obtained via simple message passing algorithms. On the other hand, as far as our knowledge, all of the proposed structures pursue a specific factorization while devising their respective schemes. In this work utilizing a different factorization, we introduce a novel iterative receiver for joint equalization/detection problem. With the aid of Variational Bayesian approximation we show that the complexity can be reduced without sacrificing the error performance drastically.

Design and performance analysis of a novel trellis coded space-time-frequency OFDM transmission scheme

In this paper a new transmit diversity technique is proposed for wireless communications over frequency selective fading channels. The proposed technique utilises orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading subchannels on which first space-frequency coding and then additional space-time coding are applied resulting in a space-time-frequency OFDM (STF-OFDM) scheme. This scheme provides a quasi-orthogonal 4 x 4 transmission matrix but relax the constraint on channel fading parameters to be constant over four time or frequency slots into two time and two frequency slots. The pairwise error probability of the new scheme is evaluated and new code design criteria are obtained to improve the error performance. 64-, 128- and 256-state 4-PSK trellis codes for STF transmission scheme are generated based on these criteria and their frame error performances in an OFDM system are evaluated by computer simulations. Copyright (c) 2007 John Wiley & Sons, Ltd.

A novel space-time-frequency coded system design for OFDM over fast fading channels

The paper is concerned with a transmitter diversity technique for wireless communications over frequency selective fading channels. The proposed technique utilizes orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading subchannels on which space-frequency processing is first applied and then additional space-time coding is applied in order to increase the diversity. In order to improve the performance of the system, a convenient trellis coding scheme is investigated and design criteria are established. Performance of the proposed space-time-frequency OFDM (STF-OFDM) transmitter diversity technique has been examined by computer simulations and compared with existing schemes.