Dispensing with channel estimation: differentially modulated cooperative wireless communications

As a benefit of bypassing the potentially excessive complexity and yet inaccurate channel estimation, differentially encoded modulation in conjunction with low-complexity noncoherent detection constitutes a viable candidate for user-cooperative systems, where estimating all the links by the relays is unrealistic. In order to stimulate further research on differentially modulated cooperative systems, a number of fundamental challenges encountered in their practical implementations are addressed, including the time-variant-channel-induced performance erosion, flexible cooperative protocol designs, resource allocation as well as its high-spectral-efficiency transceiver design. Our investigations demonstrate the quantitative benefits of cooperative wireless networks both from a pure capacity perspective as well as from a practical system design perspective

Fractionally sampled decorrelating detectors for time-varying rayleigh fading CDMA channels

In this dissertation, we propose novel decorrelating multiuser detectors in DSCDMA time-varying frequency-nonselective and frequency-selective fading channels and analyze their performance. We address the common shortcomings of existing multiuser detectors in a mobile environment, such as detector complexity and the error floor. An analytical approach is employed almost exclusively and Monte Carlo simulation is used to confirm the theoretical results. Practical channel models, such as Jakes\u27 and Markovian, are adopted in the numerical examples. The proposed detectors are of the decorrelating type and utilize fractional sampling to simultaneously achieve two goals: (1) the novel realization of a decorrelator with lower computational complexity and shorter processing latency; and (2) the significant reduction of the probability of error floor associated with time-varying fading. The analysis of the impact of imperfect power control on IS-95 multiple access interference is carried out first and the ineffectiveness of IS-95 power control in a mobile radio environment is demonstrated. Fractionally-spaced bit-by-bit decorrelator structures for the frequency-nonselective and frequency-selective channels are then proposed. The matrix singularity problem associated with decorrelation is also addressed, and its solution is suggested. A decorrelating receiver employing differentially coherent detection for an asynchronous CDMA, frequency-nonselective time-varying Rayleigh fading channel is proposed. A maximum likelihood detection principle is applied at the fractionally spaced decorrelator output, resulting in a significantly reduced error floor. For coherent detection, a novel single-stage and two-stage decision feedback (DF) maximum a posteriori (MAP) channel estimator is proposed. These estimators are applicable to a channel with an arbitrary spaced-time correlation function. The fractionally-spaced decorrelating detector is then modified and extended to a frequency-selective time-varying fading channel, and is shown to be capable of simultaneously eliminating MAI, ISI, and path cross-correlation interference. The implicit equivalent frequency diversity is exploited through multipath combining, and the effective time diversity is achieved by fractional sampling for significant performance improvement. The significance of the outcome of this research is in the design of new lower complexity multiuser detectors that do not exhibit the usual deficiencies and limitations associated with a time-varying fading and multipath CDMA mobile environment

Performance Analysis and Optimization of Tc-DTR IR-UWB Receivers over Multipath Fading Channels with Tone Interference

International audienceIn this paper, we analyze the performance of a particular class of transmitted-reference receivers for impulse radio ultra wideband communication systems, which is called chip-time differential transmitted-reference (Tc-DTR). The analysis aims at investigating the robustness of this receiver to single-tone and multi-tone narrowband interference (NBI) and comparing its performance with other non-coherent receivers that are proposed in the literature. It is shown that the Tc-DTR scheme provides more degrees of freedom for performance optimization and that it is inherently more robust to NBI than other non-coherent receivers. More specifically, it is analytically proved that the performance improvement is due to the chip-time-level differential encoding/decoding of the direct sequence (DS) code and to an adequate design of DS code and average pulse repetition time. The analysis encompasses performance metrics that are useful for both data detection (i.e., average bit error probability) and timing acquisition (i.e., false-alarm probability Pfa and detection probability Pd). Moving from the proposed sem-analytical framework, the optimal code design and system parameters are derived, and it is highlighted that the same optimization criteria can be applied to all the performance metrics considered in this paper. In addition, analytical frameworks and theoretical findings are substantiated through Monte Carlo simulations

Study of efficient transmission and reception of image-type data using millimeter waves

Evaluation of signal processing and modulation techniques for transmission and reception of image type data via millimeter wave relay satellite

Towards an enhanced noncoherent massive MU-MIMO system

PhD ThesisMany multiple-input multiple-output (MIMO) downlink transmission schemes assume channel state information (CSI) is available at the receiver/transmitter. In practice, knowledge of CSI is often obtained by using pilot symbols transmitted periodically. However, for some systems, due to high mobility and the cost of channel training and estimation, CSI acquisition is not always feasible. The problem becomes even more difficult when many antennas are used in the system and the channel is changing very rapidly before training is completed. Moreover, as the number of transmit/receive antennas grows large, the number of pilot symbols, system overheads, latency, and power consumption will grow proportionately and thereby the system becomes increasingly complex. As an alternative, a noncoherent system may be used wherein the transmitter/receiver does not need any knowledge of the CSI to perform precoding or detection. This thesis focuses on the design of a noncoherent downlink transmission system to jointly improve the performance and achieve a simple low complexity transmission scheme in three MIMO system scenarios: low rate differential spacetime block coding (STBC) in a downlink multiuser (MU-MIMO) system; high rate differential algebraic STBC in a downlink MU-MIMO system; and differential downlink transmission in a massive MU-MIMO system. Three novel design methods for each of these systems are proposed and analysed thoroughly. For the MIMO system with a low rate noncoherent scheme, a differential STBC MU-MIMO system with a downlink transmission scheme is considered. Specifically, downlink precoding combined with differential modulation (DM) is used to shift the complexity from the receivers to the transmitter. The block diagonalization (BD) precoding scheme is used to cancel co-channel interference (CCI) in addition to exploiting its advantage of enhancing diversity. Since the BD scheme requires channel knowledge at the transmitter, the downlink spreading technique along with DM is also proposed, which does not require channel knowledge neither at the transmitter nor at the receivers. The orthogonal spreading (OS) scheme is employed to have similar principle as code division multiple access (CDMA) multiplexing scheme in order to eliminate the interference between users. As a STBC scheme, the Alamouti code is used that can be encoded/decoded using DM thereby eliminating the need for channel knowledge at the receiver. The proposed schemes yield low complexity transceivers while providing good performance. For the MIMO system with a high rate noncoherent scheme, a differential STBC MU-MIMO system that operates at a high data rate is considered. In particular, a full-rate full-diversity downlink algebraic transmission scheme combined with a differential STBC systems is proposed. To achieve this, perfect algebraic space time codes and Cayley differential (CD) transforms are employed. Since CSI is not needed at the differential receiver, differential schemes are ideal for multiuser systems to shift the complexity from the receivers to the transmitter, thus simplifying user equipment. Furthermore, OS matrices are employed at the transmitter to separate the data streams of different users and enable simple single user decoding. In the OS scheme, the transmitter does not require any knowledge of the CSI to separate the data streams of multiple users; this results in a system which does not need CSI at either end. With this system, to limit the number of possible codewords, a sphere decoder (SD) is used to decode the signals at the receiving end. The proposed scheme yields low complexity transceivers while providing full-rate full-diversity system with good performance. Lastly, a differential downlink transmission scheme is proposed for a massive MIMO system without explicit channel estimation. In particular, a downlink precoding technique combined with a differential encoding scheme is used to simplify the overall system complexity. A novel precoder is designed which, with a large number of transmit antennas, can effectively precancel the multiple access interference (MAI) for each user, thus enhancing the system performance. Maximising the worst case signal-to-interference-plus-noise ratio (SINR) is adopted to optimise the precoder for the users in which full power space profile (PSP) knowledge is available to the base station (BS). Also, two suboptimal solutions based on the matched and the orthogonality approach of PSP are provided to separate the data streams of multiple users. The decision feedback differential detection (DFDD) technique is employed to further improve the performance. In summary, the proposed methods eliminate MAI, enhance system performance, and achieve a simple low complexity system. Moreover, transmission overheads are significantly reduced, the proposed methods avoid explicit channel estimation at both ends.King Fahad Security Collage at the Ministry of Interior - Saudi Arabia

Two-group decodable distributed differential space-time code for wireless relay networks based on SAST codes 2

Space-time code can be implemented in wireless relay networks when all relays cooperate to generate the code at the receiver. In this case, it is called distributed space-time code. If the channel response changes very quickly, the idea of differential space-time coding is needed to overcome the difficulty of updating the channel state information at the receiver. As a result, the transmitted signal can be demodulated without any knowledge of the channel state information at the relays or the receiver. In this paper, development of new low decoding complexity distributed differential space-time codes is considered. The developed codes are designed using semiorthogonal algebraic space-time codes. They work for networks with an even number of relays and have a two-group decodable maximum likelihood receiver. The performance of the new codes is analyzed via MATLAB simulation which demonstrates that they outperform both cyclic codes and circulant codes