Jos Akhtman

Power Versus Bandwidth-Efficiency in Wireless Communications: The Economic Perspective

We carry out a comprehensive analysis of a range of wireless network efficiency considerations. Firstly, we explore the properties and the implications of the power- versus bandwidth-efficiency criteria. Secondly, we perform a detailed top-down analysis of a typical commercial wireless network, which emphasizes the inherent differences between the aforementioned two efficiency metrics, while demonstrating that the appropriate choice of the network optimization criterion can have a profound effect on the overall network performance. Finally, we address the issue of resource management and its impact on the definition of the overall system efficiency.

Decision Directed Channel Estimation Aided OFDM Employing Sample-Spaced and Fractionally-Spaced CIR Estimators

In this letter we characterize the substantial difference between two channel estimation approaches, namely the sample-spaced (SS) and the fractionally-spaced (FS) channel impulse response (CIR) estimators. The achievable performance of decision-directed channel estimation (DDCE) methods employing both the SS- and the FS-CIR estimators is analyzed in the context of an OFDM system. The performance of the two estimation methods is compared and it is shown that the DDCE scheme employing the projection approximation subspace tracking (PAST)-aided FS-CIR estimator outperforms its SS-CIR estimator-based counterpart

Peak-to-average power ratio reduction for OFDM modems

A novel technique of reducing the peak-to-average power ratio (PAPR) of multi-carrier (MC) modulated signals is proposed. The method advocated is spectrally efficient, has a low implementational complexity and hence it is also suitable for low-power, portable implementation. Moreover, it was designed to be compatible with existing standard-based orthogonal frequency division multiplex (OFDM) systems. As an example of the performance of the proposed scheme, the amplifier back-off requirement in a terrestrial digital video broadcast (DVB-T) system can be reduced to 12 to 6 dB, while satisfying the of out-of-band emission specifications imposed by the Federal Communication Commission (FCC) spectral mask.

Heterogeneous Networking: An Enabling Paradigm for Ubiquitous Wireless Communications [Point of View]

It is a widely accepted notion that the long-term aim of the technological progress in the field of wireless communications is constituted by the provision of ubiquitous connectivity between individuals, as well as a multiplicity of existing and future pervasive devices which increasingly permeate our environment. In this context the currently dominant cellular (a combination of metropolitan, local, and personal area networks-MANs, LANs, and PANs, respectively), as well as the emerging mobile ad hoc network (MANET) architectures constitute the two major contenders for the role of systemic paradigm, which would facilitate the realization of ubiquitous wireless networking. Against this background, we would like to maintain that in order to achieve the required level of integration, performance and efficiency, the future telecommunications technology may have to infer cues from the structural characteristics of the human society itself. Correspondingly, in this paper we would like to consider some of the relevant aspects of the societal constitution, which may hold crucial clues to identifying the promising avenues towards further technological advancement. More specifically, in this paper we explore the principle of heterogeneous wireless networking, constituted by the fusion of classic cellular and ad hoc network topologies. Specifically, we would like to maintain, that the heterogeneous network inherits the vital complementary characteristics of both aforementioned architectures, and thus has the potential of attaining the levels of performance and efficiency required by the future wireless communications.

An Optimized-Hierarchy-Aided Approximate Log-MAP Detector for MIMO Systems

In this paper we propose a novel space division multiplexing (SDM) detection method. The proposed technique constitutes a list search method and may be regarded as an advanced extension of the sphere decoder (SD). Our method may be employed in the so-called over-loaded scenario, where the number of transmit antenna elements exceeds that of the receive antenna elements. Furthermore, it is suitable for high-throughput, non-constant modulus modulation schemes, such as 16 and 64-QAM. We introduce a series of optimization rules which facilitate a substantial reduction in computational complexity. More specifically, we demonstrate that the method proposed, which we refer to as the soft-output optimized hierarchy (SOPHIE)-aided SDM detector exhibits the near-optimum performance of log-MAP SDM detector in all considered scenarios. The associated computational complexity, which we control using two complexity-control parameters, is substantially lower than that imposed by all previously proposed methods

Generic reduced-complexity MMSE channel estimation for OFDM and MC-CDMA

The performance of a decision-directed channel estimation scheme is analyzed in the context of both OFDM and MC-CDMA systems. A difficulty associated with the employment of the least squares (LS) approach to the problem of a posteriori channel estimation in the context of a MC-CDMA system is described and a suitable MMSE-based estimator is proposed instead. It is demonstrated that the computational complexity associated with the MMSE estimation method proposed is relatively high in comparison to that of the conventional LS technique. Thus a low-complexity version of the MMSE estimator is proposed. The MC-CDMA system using a low-complexity MMSE estimator proposed is shown to outperform the corresponding OFDM-based scheme.

REDUCED-COMPLEXITY MAXIMUM- LIKELIHOOD DETECTION IN MULTIPLE-ANTENNA-AIDED MULTICARRIER SYSTEMS

In this contribution we explore a novel Optimized Hierarchy Reduced Search Algorithm (OHRSA)-aided space-time processing method, which may be regarded as an advanced extension of the Complex Sphere Decoder (CSD) method. The algorithm proposed extends the potential application range of the CSD method, as well as reduces the associated computational complexity.

Iterative Receiver Architectures for MIMO-OFDM

This paper proposed a turbo-detected multi-antenna-multi-carrier receiver scheme. Following the philosophy of the turbo processing, the turbo MIMO-OFDM receiver comprises a succession of detection modules, namely the channel estimator, the space-time detector and the decoder, which iteratively exchange soft bit-related information and thus facilitate a substantial improvement of the overall system performance. This paper analysed the achievable performance of the iterative system proposed with the aim of documenting the various design trade-offs, such as the achievable error-rate performance, the attainable data-rate as well as the associated computational complexity. Specifically, virtually error-free performance was reported for a rate-frac12 turbo-coded 8times8-QPSK-OFDM system, exhibiting an effective throughput of 8middot2middot frac12 = 8 bits/sec/Hz and having a pilot overhead of only 10%, at SNR of 7.5dB and normalized Doppler frequency of 0.003, which corresponds to a mobile terminal speed of about 65 km/h.

Advanced Channel Estimation for MIMO-OFDM in Realistic Channel Conditions

An advanced decision-directed channel estimation scheme is proposed, which is suitable for employment in a wide range of multi-antenna multi-carrier systems as well as for communications over the entire range of practical channel conditions. In particular, we consider a MIMO-OFDM system operating in a mobile wireless multipath channel, which exhibits frequency-selective Rayleigh fading and is characterized by a time-variant power delay profile. Both the mean square error as well as the bit error rate performances achieved by the proposed system are documented. Specifically, we report a virtually error- free performance of a rate 1/2 turbo-coded 8times8-QPSK-OFDM system, exhibiting an effective throughput of 8middot2middot1/2=8 bits/sec/Hz, while having a pilot overhead of only 10%, at an SNR of lOdB and an OFDM-symbol-normalized Doppler frequency of 0.003, which corresponds to the mobile terminal speed of about 65 km/h.

Iterative Near-Maximum-Likelihood Detection in Rank-Deficient Downlink SDMA Systems

In this paper, a precoded and iteratively detected downlink multiuser system is proposed, which is capable of operating in rank-deficient scenarios, when the number of transmitters exceeds the number of receivers. The literature of uplink space division multiple access (SDMA) systems is rich, but at the time of writing there is a paucity of information on the employment of SDMA techniques in the downlink. Hence, we propose a novel precoded downlink SDMA (DL-SDMA) multiuser communication system, which invokes a low-complexity near-maximum-likelihood sphere decoder and is particularly suitable for the aforementioned rank-deficient scenario. Powerful iterative decoding is carried out by exchanging extrinsic information between the precoders decoder and the outer channel decoder. Furthermore, we demonstrate with the aid of extrinsic information transfer charts that our proposed precoded DL-SDMA system has a better convergence behavior than its nonprecoded DL-SDMA counterpart. Quantitatively, the proposed system having a normalized system load of , i.e., 1.333 times higher effective throughput facilitated by having 1.333 times more DL-SDMA transmitters than receivers, exhibits a ldquoturbo cliffrdquo at an of 5 dB and hence results in an infinitesimally low bit error rate (BER). By contrast, at , the equivalent system dispensing with precoding exhibits a BER in excess of 10%.