Multicarrier Frequency Hopping Spread Spectrum Techniques With Quasi-Cyclic Low Density Parity Check Codes Channel Coding

This work presents a new proposed Multicarrier Frequency Hopping Spread Spectrum (MCFH-SS) system employing Quasi-Cyclic Low Density Parity Check (QC-LDPC) codes instead of the conventional LDPC codes. A new technique for constructing the QC-LDPC codes based on row division method is proposed. The new codes offer more flexibility in terms of high girth, multiple code rates and block length. Moreover, a new scheme for channel prediction in MCFH-SS system is proposed. The technique adaptively estimates the channel conditions and eliminates the need for the system to transmit a request message prior to transmitting the packet data. The ready-to-use channel will be occupied with a Pseudonoise (PN) code and use for transmission or else, it will be banned

Techniques for improving the performance of frequency-hopped multiple-access communication systems

Imperial Users onl

Interference characterization and suppression for multiuser direct-sequence spread-spectrum system

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 175-184).In this thesis we investigate efficient modulation and detection techniques for the uplink (i.e. transmission from mobile to base station) of a DS-CDMA network. Specifically, the thesis contains three parts. In the first part, we focus on the mobile transmitter. In particular, we evaluate and compare the spectral efficiency of two promising variable rate DS-CDMA transmission techniques, multicode (MCD) and variable-spreading-gain (VSG), under the presence of multiple-access (user-to-user) interferences (MAI) and multipath interferences. The uniqueness of our study is that in bit-error-rate evaluation, instead of approximating the interference as Gaussian noise (which has been done in most of the previous studies), we incorporate both power and distribution of interferences into consideration. We show where the Gaussian assumption may give misleading answers and how our results in these cases are different from those obtained in the past. In part two and three of the thesis, we focus on the base station receiver. Specifically, we present effective joint detection techniques that have good performance-complexity tradeoff. Part two of the thesis introduces a class of novel multistage parallel interference cancellation algorithms based on stage-by-stage minimum mean-squared error (MMSE) optimization. We show that this scheme is capable of achieving significantly better performance than other algorithms with similar complexity. Part three of the thesis presents a low-complexity dual-mode multiuser detector that dynamically switches its detection mode between the matched-filter receiver and the decorrelator. We show that this detector is capable of achieving the performance of a decorrelator but with significant savings in processing power and complexity.by Mingxi Fan.Ph.D

Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Irregular Variable Length Coding

In this thesis, we introduce Irregular Variable Length Coding (IrVLC) and investigate its applications, characteristics and performance in the context of digital multimedia broadcast telecommunications. During IrVLC encoding, the multimedia signal is represented using a sequence of concatenated binary codewords. These are selected from a codebook, comprising a number of codewords, which, in turn, comprise various numbers of bits. However, during IrVLC encoding, the multimedia signal is decomposed into particular fractions, each of which is represented using a different codebook. This is in contrast to regular Variable Length Coding (VLC), in which the entire multimedia signal is encoded using the same codebook. The application of IrVLCs to joint source and channel coding is investigated in the context of a video transmission scheme. Our novel video codec represents the video signal using tessellations of Variable-Dimension Vector Quantisation (VDVQ) tiles. These are selected from a codebook, comprising a number of tiles having various dimensions. The selected tessellation of VDVQ tiles is signalled using a corresponding sequence of concatenated codewords from a Variable Length Error Correction (VLEC) codebook. This VLEC codebook represents a specific joint source and channel coding case of VLCs, which facilitates both compression and error correction. However, during video encoding, only particular combinations of the VDVQ tiles will perfectly tessellate, owing to their various dimensions. As a result, only particular sub-sets of the VDVQ codebook and, hence, of the VLEC codebook may be employed to convey particular fractions of the video signal. Therefore, our novel video codec can be said to employ IrVLCs. The employment of IrVLCs to facilitate Unequal Error Protection (UEP) is also demonstrated. This may be applied when various fractions of the source signal have different error sensitivities, as is typical in audio, speech, image and video signals, for example. Here, different VLEC codebooks having appropriately selected error correction capabilities may be employed to encode the particular fractions of the source signal. This approach may be expected to yield a higher reconstruction quality than equal protection in cases where the various fractions of the source signal have different error sensitivities. Finally, this thesis investigates the application of IrVLCs to near-capacity operation using EXtrinsic Information Transfer (EXIT) chart analysis. Here, a number of component VLEC codebooks having different inverted EXIT functions are employed to encode particular fractions of the source symbol frame. We show that the composite inverted IrVLC EXIT function may be obtained as a weighted average of the inverted component VLC EXIT functions. Additionally, EXIT chart matching is employed to shape the inverted IrVLC EXIT function to match the EXIT function of a serially concatenated inner channel code, creating a narrow but still open EXIT chart tunnel. In this way, iterative decoding convergence to an infinitesimally low probability of error is facilitated at near-capacity channel SNRs

Physical-Layer Cooperation in Coded OFDM Relaying Systems

Mobile communication systems nowadays require ever-increasing data rate and coverage of wide areas. One promising approach to achieve this goal is the application of cooperative communications enabled by introducing intermediate nodes known as relays to support the transmission between terminals. By processing and forwarding the receive message at the relays, the path-loss effect between the source and the destination is mitigated. One major limit factor for relay assisted communications is that a relay cannot transmit and receive using the same physical resources. Therefore, a half-duplex constraint is commonly assumed resulting in halved spectral efficiency. To combat this drawback, two-way relaying is introduced, where two sources exchange information with each. On the other hand, due to the physical limitation of the relays, e.g., wireless sensor nodes, it's not possible to implement multiple antennas at one relay, which prohibits the application of multiple-input multiple-output (MIMO) techniques. However, when treating multiple relays as a cluster, a virtual antenna array is formed to perform MIMO techniques in a distributed manner. %This thesis aims at designing efficient one-way and two-way relaying schemes. Specifically, existing schemes from the literature are improved and new schemes are developed with the emphasis on coded orthogonal frequency division multiplexing (OFDM) transmissions. Of special interest is the application of physical-layer network coding (PLNC) for two-phase two-way relaying. In this case, a network coded message is estimated from the superimposed receive signal at the relay using PLNC schemes. The schemes are investigated based on a mutual information analysis and their performance are improved by a newly proposed phase control strategy. Furthermore, performance degradation due to system asynchrony is mitigated depending on different PLNC schemes. When multiple relays are available, novel cooperation schemes allowing information exchange within the relay cluster are proposed that facilitate distributed MIMO reception and transmission. Additionally, smart signaling approaches are presented to enable the cooperation at different levels with the cooperation overhead taken into account adequately in system performance evaluation

Band sharing and satellite diversity techniques for CDMA.

High levels of interference between satellite constellation systems, fading and shadowing are a major problem for the successful performance of communication systems using the allocated L/S frequency bands for Non-Geostationary Earth Orbit (NGEO) satellites. As free spectrum is nonexistent, new systems wishing to operate in this band must co-exist with other users, both satellite and terrestrial. This research is mainly concerned with two subjects. Firstly, band sharing between different systems Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) has been evaluated for maximizing capacity and optimising efficiency of using the spectrum available. For the case of widened channel bandwidth of the CDMA channel, the overlapping was tested under different degrees of channel overlap and different orders of filters. The best result shows that at the optimum degree of channel overlap, capacity increases by up to 21%. For the case of fixed channel bandwidth, the optimum overlapping between CDMA systems depends on the filtering Roll-off factor and achieves an improvement of the spectrum efficiency of up to 13.4%. Also, for a number of narrowband signal users sharing a CDMA channel, the best location of narrowband signals to share spectrum with a CDMA system was found to be at the edge of the CDMA channel. Simulation models have been constructed and developed which show the combination of DS- CDMA techniques, forward error correction (FEC) code techniques and satellite diversity with Rake receiver for improving performance of interference, fading and shadowing under different environments. Voice activity factor has been considered to reduce the effect of multiple access interference (MAI). The results have shown that satellite diversity has a significant effect on the system performance and satellite diversity gain achieves an improvement up to 6dB. Further improvements have been achieved by including concatenated codes to provide different BER for different services. Sharing the frequency band between a number of Low Earth Orbit (LEO) satellite constellation systems is feasible and very useful but only for a limited number of LEOS satellite CDMA based constellations. Furthermore, satellite diversity is an essential factor to achieve a satisfactory level of service availability, especially for urban and suburban environments

Orthogonal multicarrier modulation for high-rates mobile and wireless communications

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN037085 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

PRE-FILTERING IN MC-CDMA DOWNLINK TRANSMISSIONS

Future wireless communication systems are expected to support high-speed and high-quality multimedia services. In theseapplications the received signal is typically affected byfrequency-selective fading, which must be properly counteracted toavoid a severe degradation of the system performance. MC-CDMA is a multiplexing technique that combines OFDM with direct sequence CDMA. It is robust to frequency-selective fading thanks to the underlying OFDM modulation and exploits frequency diversity by spreading the data of different users in the frequency domain. For these reasons it is considered as a promising candidate for the physical layer of future high-speed wireless communications. Recent publications show that MC-CDMA is particularly suitable for downlink transmissions, i.e., from the base station to the mobile terminals. In these applications orthogonal spreading codes are usually employed to provide protection against co-channel interference. In the presence of multipath propagation, however, signals undergo frequency-selective fading and the code orthogonality is lost. This gives rise to multiple-access interference, which strongly limits the system performance. In the past few years several advanced multi-user detection techniques have been proposed and discussed for interference mitigation. However, in spite of their effectiveness, all these methods are quite unattractive for downlink applications since they would entail high complexity and excessive power consumption at the remote units. As an alternative to multi-user detection, pre-filtering techniques can be employed in downlink transmissions to mitigate multiple-access interference and channel distortions. The idea behind pre-filtering is to vary the gain assigned to each subcarrier so that interference is reduced and the signal at the receiver appears undistorted. In this way, simple and low complex single-user detectors can be employed at the remote units, thereby moving most of the computational burden to the base station, where power consumption and computational resources are not critical issues. In general terms, the main contribution of this dissertation is threefold. First, we propose and discuss several linear and non-linear pre-filtering schemes for the downlink of MC-CDMA systems equipped with multiple transmit antennas and operating in a time-division-duplex mode. The resulting schemes are derived according to different optimization criteria and aim at combating the detrimental effects of MAI while maintaining the complexity of the remote units as low as possible. A second contribution comes from providing a unified framework for investigating pre-filtering in the downlink of both MC-CDMA and OFDMA systems. The use of a unified framework comprising both MC-CDMA and OFDMA allows a fair comparison between these multiple-access technologies under the same operating conditions. It turns out that OFDMA outperforms MC-CDMA when the system resources are optimally assigned to the active users according to the actual channel realization. As we shall see, in order to work properly, all the proposed schemes require explicit knowledge of the channel responses of the active users. In time-division-duplex systems this information can be achieved by exploiting the channel reciprocity between alternative uplink and downlink transmissions. If channel variations are sufficiently slow, the channel estimates of the active users can be derived at the base station during an uplink time-slot and reused for pre-filtering in the subsequent downlink time-slot. Thus, a third contribution comes from addressing the problem of channel acquisition in the uplink of an MC-CDMA system equipped with multiple receive antennas