Estimation of Sparse MIMO Channels with Common Support

We consider the problem of estimating sparse communication channels in the MIMO context. In small to medium bandwidth communications, as in the current standards for OFDM and CDMA communication systems (with bandwidth up to 20 MHz), such channels are individually sparse and at the same time share a common support set. Since the underlying physical channels are inherently continuous-time, we propose a parametric sparse estimation technique based on finite rate of innovation (FRI) principles. Parametric estimation is especially relevant to MIMO communications as it allows for a robust estimation and concise description of the channels. The core of the algorithm is a generalization of conventional spectral estimation methods to multiple input signals with common support. We show the application of our technique for channel estimation in OFDM (uniformly/contiguous DFT pilots) and CDMA downlink (Walsh-Hadamard coded schemes). In the presence of additive white Gaussian noise, theoretical lower bounds on the estimation of SCS channel parameters in Rayleigh fading conditions are derived. Finally, an analytical spatial channel model is derived, and simulations on this model in the OFDM setting show the symbol error rate (SER) is reduced by a factor 2 (0 dB of SNR) to 5 (high SNR) compared to standard non-parametric methods - e.g. lowpass interpolation.Comment: 12 pages / 7 figures. Submitted to IEEE Transactions on Communicatio

A semi-blind channel estimation method for multiuser multiantenna OFDM systems

A subspace-based blind method is proposed for estimating the channel responses of a multiuser and multiantenna orthogonal frequency division multiplexing (OFDM) uplink system. It gives estimations to all channel responses subject to a scalar matrix ambiguity and does not need precise channel order information (only upper bound for the orders is required). Furthermore, the scalar ambiguity matrix can be easily resolved by using only one pilot OFDM block, given that the number of users is smaller than the number of symbols in the pilot symbol block. Equalization methods are discussed based on the estimated channels. By using partial knowledge of the channels, a multipath subspace method is proposed that reduces the computational complexity. Simulations show that the methods are effective and robust.published_or_final_versio

Exploiting Diversity in Broadband Wireless Relay Networks

Fading is one of the most fundamental impairments to wireless communications. The standard approach to combating fading is by adding redundancy - or diversity - to help increase coverage and transmission speed. Motivated by the results in multiple-input multiple-output technologies, which are usually used at base stations or access points, cooperation commutation has been proposed to improve the performance of wireless networks which consist of low-cost single antenna devices. While the majority of the research in cooperative communication focuses on flat fading for its simplicity and easy analysis, in practice the underlying channels in broadband wireless communication systems such as cellular systems (UMTS/LTE) are more likely to exhibit frequency selective fading. In this dissertation, we consider a frequency selective fading channel model and explore distributed diversity techniques in broadband wireless relay networks, with consideration to practical issues such as channel estimation and complexity-performance tradeoffs. We first study a system model with one source, one destination and multiple decode-and-forward (DF) relays which share a single channel orthogonal to the source. We derive the diversity-multiplexing tradeoff (DMT) for several relaying strategies: best relay selection, random relay selection, and the case when all decoding relays participate. The best relay selection method selects the relay in the decoding set with the largest sum-squared relay-to-destination channel coefficients. This scheme can achieve the optimal DMT of the system at the expense of higher complexity, compared to the other two relaying strategies which do not always exploit the spatial diversity offered by the relays. Different from flat fading, we find special cases when the three relaying strategies have the same DMT. We further present a transceiver design and prove it can achieve the optimal DMT asymptotically. Monte Carlo simulations are presented to corroborate the theoretical analysis. We provide a detailed performance comparison of the three relaying strategies in channels encountered in practice. The work has been extended to systems with multiple amplify-and-forward relays. We propose two relay selection schemes with maximum likelihood sequential estimator and linear zero- forcing equalization at the destination respectively and both schemes can asymptotically achieve the optimal DMT. We next extend the results in the two-hop network, as previously studied, to multi-hop networks. In particular, we consider the routing problem in clustered multi-hop DF relay networks since clustered multi-hop wireless networks have attracted significant attention for their robustness to fading, hierarchical structure, and ability to exploit the broadcast nature of the wireless channel. We propose an opportunistic routing (or relay selection) algorithm for such networks. In contrast to the majority of existing approaches to routing in clustered networks, our algorithm only requires channel state information in the final hop, which is shown to be essential for reaping the diversity offered by the channel. In addition to exploiting the available diversity, our simple cross-layer algorithm has the flexibility to satisfy an additional routing objective such as maximization of network lifetime. We demonstrate through analysis and simulation that our proposed routing algorithm attains full diversity under certain conditions on the cluster sizes, and its diversity is equal to the diversity of more complicated approaches that require full channel state information. The final part of this dissertation considers channel estimation in relay networks. Channel state information is vital for exploiting diversity in cooperative networks. The existing literature on cooperative channel estimation assumes that block lengths are long and that channel estimation takes place within a fading block. However, if the forwarding delay needs to be reduced, short block lengths are preferred, and adaptive estimation through multiple blocks is required. In particular, we consider estimating the relay-to-destination channel in DF relay systems for which the presence of forwarded information is probabilistic since it is unknown whether the relay participates in the forwarding phase. A detector is used so that the update of the least mean square channel estimate is made only when the detector decides the presence of training data. We use the generalized likelihood ratio test and focus on the detector threshold for deciding whether the training sequence is present. We also propose a heuristic objective function which leads to a proper threshold to improve the convergence speed and reduce the estimation error. Extensive numerical results show the superior performance of using this threshold as opposed to fixed thresholds

Multiple antenna system and channel estimation for multiband orthogonal frequency division multiplexing in ultra-wideband systems

Multiband Orthogonal Frequency Division Multiplexing (OFDM) has been deployed for practical implementation of low cost and low power Ultra-Wideband (UWB) devices due to its ability to mitigate the narrowband interference and multipath fading effects. In order to achieve high data rates, the deployment of multiple antenna techniques into a UWB system has gained considerable research interest. In a UWB system, both the spatial and multipath diversities exist in UWB system can be exploited via the use of Multiple-Input Multiple-Output (MIMO) antenna system and Space-Time Codes (STC) by leveraging Alamouti scheme. This work shows that MIMO system outperforms Alamouti technique in providing a power combining gain in the receiver. Given that channel estimation for timefrequency multiplexed such as a multiband OFDM system is unexplored largely, this thesis also addresses this issue. In literature, most of the conventional Channel Frequency Response (CFR) estimations require either pre-storing a large matrix or performing real-time matrix inversion. In general, these requirements are prohibitive for practical implementation of UWB devices. In this thesis, the implementation issues of STC-based on Alamouti scheme are investigated for the multiband OFDM system. The research quantifies and analyses existing channel estimation in frequency domain such as Least-Square (LS) and Minimum Mean Square Error (MMSE) techniques. Consequently, low-complexity channel estimation based on Singular Value Decomposition (SVD) technique is developed for multiband OFDM system evaluates under modified Saleh-Valuenzela (S-V) channel modelling represents the realistic wireless indoor environment. This work implies that the SVD technique gives an improvement of 3-5 dB compared to LS technique. Even though SVD performs similarly to MMSE, it managed to reduce significantly the complexity by or to 57.8%

Non-orthogonal Frequency Division Multiplexing with Index Modulation

Orthogonal Frequency Division Multiplexing (OFDM) is a well-established technique in wired and wireless communications due to its high spectral efficiency compared to other multicarrier transmission schemes. However, the explosion of Internet of Things (IoT) has demanded a more spectrally-efficient technique to utilize small bandwidths, on which numerous low-power low-rate devices operate. This thesis aims to provide solutions for this problem. First, the integration of index modulation to fast-OFDM, which is a special variant of OFDM, is investigated. The highest obtainable bit rate of this system is derived, which demonstrates enhancements compared to OFDM systems in the low-power low-rate regions. Furthermore, an improved one-dimension constellation is found to optimize the overall bit error rate (BER) of this system. Numerical results show that the proposed system exhibits enhancements in both bit rate and error performance, leading to higher spectral efficiency compared to OFDM in the low-power regions. The second part of the thesis is concerned with reducing the bandwidth consumed by multicarrier transmissions. This results in the mutual orthogonality among subchannels being relaxed, yielding a Non-orthogonal Frequency Division Multiplexing (NFDM) system. The main contribution in this part includes a novel and feasible design for NFDM systems, which is capable of eliminating inter-channel interference (ICI), which is the major limitation of the conventional NFDM system. Because ICI is completely eliminated, the BER performance of the proposed system is the same as that of an OFDM system over additive white Gaussian noise channels. The power spectrum density (PSD) of the proposed system is also investigated, leading to design guidelines and tradeoffs between the PSD shape and the system's bit rate. Finally, index modulation is incorporated in the proposed NFDM systems. Thanks to our ICI-free design of NFDM, this combined system (NFDM-IM) and fast-OFDM-IM share a similar simple two-stage signal detection mechanism. Improved QAM constellations are found for NFDM-IM systems to optimize their overall BER. Obtained results show that with low modulation orders such as 8-QAM (Quadrature Amplitude Modulation), NFDM-IM systems employing the improved constellation achieve BER performance close to that of NFDM in the low BER regions. With equivalent occupied bandwidth and error performance, an NFDM-IM system with optimal 8-QAM constellation produces better spectral efficiency than the one using the conventional hexagonal constellation