This paper considers a device-based Fog Radio Access Network (F-RAN), where users’ smart devices (denoted by F-UEs) can be exploited to cache popular files so as to communicate them when requested by other F-UEs among them. This architecture restricts the involvement of the central baseband processing unit (BBU) to serving those F-UEs not immediately served by their peers, thus offloading the BBU's fronthaul spectrum and increasing the overall capacity of the network. This paper aims to further maximize fronthaul offloading in this architecture using network coding (NC). The latter exploits both the cached and previously received files by different F-UEs as side information to serve more requesting F-UEs in each transmission from their peers or the BBU so as to maximize fronthaul offloading. Being half-duplex devices (they can only either send or receive in any given time), the problem of BBU fronthaul offloading in this setting is formulated over two transmission phases on an NC graph. After showing that this problem is NP-hard, the paper proposes two novel heuristic algorithms to solve it in real-time. In addition, a lower-bound on the offloading gain of these algorithms is derived for a special file placement case, their asymptotic optimality is proven, and their complexities are analyzed. Simulation results both show that these proposed algorithms perform closely to the optimal solution and quantify the significant offloading gains achieved by them.

In this paper, we investigate the system performance of a multi-cell multi-user (MU) hybrid millimeter wave (mmWave) multiple-input multiple- output (MIMO) network adopting the channel estimation algorithm proposed in [1] for channel estimation. Due to the reuse of orthogonal pilot symbols among different cells, the channel estimation is expected to be affected by pilot contamination, which is considered as a fundamental performance bottleneck of conventional multicell MU massive MIMO networks. To analyze the impact of pilot contamination on the system performance, we derive the closed-form approximation expression of the normalized mean squared error (MSE) of the channel estimation performance. Our analytical and simulation results show that the channel estimation error incurred by the impact of pilot contamination and noise vanishes asymptotically with an increasing number of antennas equipped at each radio frequency (RF) chain deployed at the desired BS. Thus, pilot contamination is no longer the fundamental problem for multi-cell hybrid mmWave systems.

In this paper, we investigate the system performance of a multi-cell multi-user (MU) hybrid millimeter wave communications in a multiple-input multiple-output (MIMO) network. Due to the reuse of pilot symbols among different cells, the performance of channel estimation is expected to be degraded by pilot contamination, which is considered as a fundamental performance bottleneck of conventional multi-cell MU massive MIMO networks. To analyze the impact of pilot contamination to the system performance, we first derive the closed-form approximation of the normalized mean-squared error of the channel estimation algorithm proposed by Zhao et al. over Rician fading channels. Our analytical and simulation results show that the channel estimation error incurred by the impact of pilot contamination and noise vanishes asymptotically with an increasing number of antennas equipped at each radio frequency chain at the desired BS. Furthermore, by adopting zero-forcing precoding in each cell for downlink transmission, we derive a tight closed-form approximation of the average achievable rate per user. Our results unveil that the intra-cell interference and inter-cell interference caused by pilot contamination over Rician fading channels can be mitigated effectively by simply increasing the number of antennas equipped at the desired BS.

This paper studies the problem of offloading the fronthaul of Fog Radio Access Networks (F-RANs) with smart user equipments (denoted by F-UEs) that can cache files and cooperate with each other to retrieve their requested files. This cooperation between the F-UE devices will reduce the load on the fronthaul of the central baseband processing unit (BBU), thus increasing the overall capacity of the network. By smartly employing network coding (NC), this paper aims to minimize the number of transmissions required from the BBU given a realistic half-duplex transmission scenario. In this setting, F-UE devices can only send or receive at a time, and thus must receive their requested files in maximum two time-slots in order to achieve high quality of experience (QoE). The above problem is first formulated over an NC graph. Being NP-hard, two heuristic algorithms are proposed to solve the problem in real-time. Simulation results show that these proposed heuristics perform closely to the optimal solution. They also demonstrate the significant fronthaul offloading gains achieved by our proposed algorithms.

In random machine-to-machine (M2M) networks, the network topology along with the variation of the wireless channel are the main sources of uncertainty that affect the network performance. Unlike most prior works, which assume a reference receiver such as aggregator located at a certain fixed point, this paper discusses a more general case where the intended receiving node can be any randomly-located node distributed uniformly in a circle. Each machine node transmits a backlog of data in time duration based on slotted-synchronous ALOHA multiple access control (MAC) scheme. A closed-form expression for the order statistics of the interference power produced by a randomly located interfering node for Nakagami-m fading channel model is obtained. Due to the dependency of the inter-node distances in uniform deployment, deriving a closed-form expression for outage probability from randomly-selected node's perspective is demanding. Using the order statistics notion, a tractable lower bound is derived for the outage probability seen from any randomly-located node's perspective inside the network region. The numerical results validate the accuracy of analytical ones and tightness of the lower bound for the broad range of the network parameters.

Using stochastic geometry, the downlink (DL) and uplink (UL) coverage probabilities are derived for co-channel packet multi-tier heterogeneous cellular networks (HCNs). The spatial locations of base stations (BSs) as well as user equipments (UEs) are modeled as independent spatial homogeneous Poisson point processes. The decoupled association is evaluated where the UE may connect to different BSs in the UL and DL transmissions. Unlike most of the existing works, the packet transmission slots are not synchronized, that is, the starting and ending points of the slots are not aligned. We investigate fundamental performance metrics of dynamic packet HCN for two traffic models, namely, the slotted-asynchronous and exponential-interarrival. Furthermore, tight lower bounds for the DL and UL coverage probabilities for two traffic models are obtained. The derived bounds are tight especially in high data rate regimes. The analysis provided herein enables us to determine the performance limits of packet-based HCNs with possible asynchronous time-slots. Simulation results are conducted to verify the analytical derivations. Furthermore, the performance comparison between pure synchronous and asynchronous packet-based systems is provided. The results confirm that the synchronous case outperforms the asynchronous one in terms of the UL and DL coverage probabilities at the cost of higher computational complexity.

This paper proposes a new technique to concatenate 1-bit compressed sensing with a convolutional channel encoder for transmission of sparse signals over a memoryless AWGN channel. At the reconstruction part, an iterative decoder, referred to as turbo-CS decoder, has been proposed. At the turbo-CS decoder, the sparse signal is decoded through iterations between an a posteriori probability decoder and a softin/soft-out 1-bit compressed sensing decoder. By numerical experiments, we show that the turbo-CS decoder outperforms the state-of-the-art algorithms for 1-bit compressed sensing reconstruction in the presence of AWGN channel by more than 10 dB in terms of signal reconstruction performance.

Using stochastic geometry, downlink (DL) coverage probability in slot-based two-tier heterogeneous cellular networks (HCNs) is studied. Two traffic patterns namely slotted-asynchronous and exponential-interarrival are introduced to capture user activity, and the DL coverage probability based on max-SINR association is derived. In contrast to many works, the transmission slots of base stations (BS) are not synchronous. This causes a packet transmitted in one slot to experience varying levels of interference during the time slot interval. The SINR statistical distribution during one slot is derived for both traffic patterns. The numerical results are presented to illustrate the analytical ones. The comparison between asynchronous and conventional synchronous HCNs is provided to illustrate the asynchronous effect on DL performance. It is shown that to meet high coverage requirements, synchronizing slots from different BSs help to boost the coverage performance. Furthermore, it is shown that densifying networks with low-power BSs improves coverage performance and the improvement is more pronounced at higher path-loss exponents.

This work proposes asynchronous interleaved scan architecture (AISA) intended for the internal structure of online Built-in Self-test for Null Convention Logic (NCL) circuits. NCL is a robust asynchronous paradigm which can target devices for long-life missions and hard-to-access environments. However, on-line self-test methods adapted to match the characteristics of this important asynchronous method are not available currently. Existing test hardware for NCL is based on synchronous test elements. Such test hardware when used for on-line testing could introduce metastability issues and reduce the reliability of the highly robust NCL devices. We propose a scan architecture specifically designed for NCL circuits and demonstrate how this can be integrated into an on-line BIST architecture. Results for timing of the scan cell are shown with an average area overhead of 10%, which is only 35% of that of previous work [1]. Furthermore we do not need clock trees or double latches for sync/async interfaces.

In heterogeneous networks (HetNets), mobile users are proactively offloaded to small cells by association bias. To alleviate the signal-to-interference-plus-noise-ratio (SINR) degradation to offloaded users, resource partitioning and transmit power reduction are proposed in Long Term Evolution-Advanced (LTE-A), which is known as enhanced inter-cell interference coordination (eICIC). In this paper, we develop a tractable framework for performance analysis of eICIC with joint cell association, resource partitioning, and transmit power reduction by using stochastic geometry. We derive the downlink coverage probability of the network. Numerical results are provided and we give some valuable insights and guidelines for eICIC in cochannel HetNets.

Random wireless networks with finite number of nodes distributed uniformly in a circular-shaped finite region are considered. A closed-form expression for the complimentary cumulative distribution function (CCDF) of Euclidean distance between a randomly selected node and its ith nearest node is derived. In contrast to most of the existing literature, the node observed is not necessarily required to be located at the centre of the network region nor located at fixed point. Moreover, the joint distribution of the distances between the ith and jth nearest nodes is derived. Finally, a tractable and quite tight lower bound on outage performance of the uniform network based on the dominant interferer node is derived. Using our derived lower bound, we can determine the maximum number of nodes that should be deployed to satisfy the required QoS. Computer simulation illustrates the validity of the theoretical analysis.

Random wireless networks with finite number of nodes distributed uniformly in a square-shaped finite region are considered. The transmission channels are assumed to experience Nakagami-m fading. A closed-form expression for the cumulative distribution function (CDF) of the interference power imposed on two randomly-located communicating nodes is derived. In contrast to most of the existing literature, the receiving node is not necessarily required to be located at the center of the network region. Two packet traffic patterns, viz., slotted-synchronous and slotted-asynchronous are considered and closed-form expressions for the CDF of the interference is obtained for each traffic pattern. Computer simulation illustrates the validity of the theoretical analyses.

In this paper, we consider the 1-bit compressive sensing reconstruction problem in a scenario that the sparsity level of the signal is unknown and time variant, and the binary measurements are contaminated with the noise. We introduce a new reconstruction algorithm which we refer to as Noise-Adaptive Restricted Step Shrinkage (NARSS). NARSS is superior in terms of performance, complexity and speed of convergence to the algorithms already introduced in the literature for 1-bit compressive sensing reconstruction from the noisy binary measurements.

We consider compressive sensing as a source coding method for signal transmission. We concatenate a convolutional coding system with 1-bit compressive sensing to obtain a serial concatenated system model for sparse signal transmission over an AWGN channel. The proposed source/channel decoder, which we refer to as turbo CS, is robust against channel noise and its signal reconstruction performance at the receiver increases considerably through iterations. We show 12 dB improvement with six turbo CS iterations compared to a non-iterative concatenated source/channel decoder.

Asynchronous design is predicted to have a significant place in the future due to benefits of speed, power consumption, and design. Null Convention Logic (NCL) is a subcategory of asynchronous design that results in the most reliable and low-power asynchronous hardware. However, test strategies are not adopted to match the characteristics of this important asynchronous method, and the existing test methods for NCL use synchronous test hardware. In this paper asynchronous test hardware is proposed for testing NCL-based circuits to avoid problems of synchronous design as well as the need for asynchronous-synchronous interfaces and clock trees. The proposed test hardware is implemented in HDL and synthesised to define design characteristic and improvements over previous work.

Small cell network (SCN) offers, for the first time, a low-cost and scalable mechanism to meet the forecast data-traffic demand. In this paper, we propose a non-uniform SCN deployment scheme. The small cell base stations (BSs) in this scheme will not be utilized in the region within a prescribed distance away from any macrocell BSs, defined as the inner region. Based upon the analytical framework provided in this work, the downlink coverage and single user throughput are precisely characterized. Provided that the inner region size is appropriately chosen, we find that the proposed non-uniform SCN deployment scheme can maintain the same level of cellular coverage performance even with 50% less small cell BSs used than the uniform SCN deployment, which is commonly considered in the literature. Furthermore, both the coverage and the single user throughput performance will significantly benefit from the proposed scheme, if its average small cell density is kept identical to the uniform SCN deployment. This work demonstrates the benefits obtained from a simple non-uniform SCN deployment, thus highlighting the importance of deploying small cells selectively.

A Postbeamformer Interference Canceler (PIC) is a beam space processor that processes the signals derived from an array of antennas by forming a signal beam and an interference beam. The paper presents a PIC processor using the orthogonal interference beam (OIB) and studies its performance by deriving the analytical results for the output signal to interference plus noise ratio (SINR) when an array is situated in the near field of broadband directional sources. A number of examples are presented to support the analytical results.

This paper proposes a coverage-oriented femtocell network deployment scheme, in which the femtocell base stations (BSs) can decide whether to be active or inactive depending on their distances from the macrocell BSs. Specifically, as the areas close to the macrocell BSs already have satisfactory cellular coverage, the femtocell BSs located inside such areas are kept to be inactive. Thus, all the active femtocells are located in the poor macrocell coverage areas. Based on a stochastic geometric framework, the coverage probability can be analyzed with tractable results. Surprisingly, the results show that the proposed scheme, although with a lower defacto femtocell density, can achieve better coverage performance than that keeping all femtocells in the entire network to be active. The analytical results further identify the achievable optimal performance of the new scheme, which provides mobile operators a guideline for femtocell deployment and operation.

This paper studies the information-theoretic secrecy performance in large-scale cellular networks based on a stochastic geometry framework. The locations of both base stations and the mobile users are modeled as independent two-dimensional Poisson point processes. We consider a key feature of the cellular network, namely, information exchange between base stations, and characterize its impact on the achievable secrecy rate of an arbitrary downlink transmission with a certain portion of the mobile users acting as potential eavesdroppers. In particular, analytical results are presented under diverse assumptions on the availability of eavesdroppers' location information at the serving base station, which captures the benefit from the exchange of mobile users' location information between base stations.

This paper studies the information-theoretic secrecy performance in large-scale cellular networks based on a stochastic geometry framework. The locations of both base stations and mobile users are modeled as independent two-dimensional Poisson point processes. We consider two important features of cellular networks, namely, information exchange between base stations and cell association, to characterize their impact on the achievable secrecy rate of an arbitrary downlink transmission with a certain portion of the mobile users acting as potential eavesdroppers. In particular, tractable results are presented under diverse assumptions on the availability of eavesdroppers' location information at the serving base station, which captures the benefit from the exchange of the location information between base stations.

Capacity and Coverage are the two major concerns for the wireless operators to deploy cellular networks. This paper investigates the uplink capacity and coverage of relay-assisted UMTS network when multiuser detection techniques are employed at both macrocell base station (BS) and relay station (RS). A hierarchical geometric system model is used where the fixed relay stations are deployed within the macrocell. We are particularly interested in the cases where macro BSs and RSs have different capabilities in mitigating intracell and intercell interferences. Simulation results show that the capacity can be tripled and the cell range can be extended when MUD can be employed at base stations and relay stations. Furthermore, we studied the practical scenario where the users in the relay region are partially served by the relay station. This case is meaningful when there are not enough relay resources or users are restricted due to limited handover capability. We then proposed a partial relay system to enhance the cell range while maintaining a reasonable cell load.

Femtocells, despite their name, pose a potentially large disruption to the carefully planned cellular networks that now connect a majority of the planet's citizens to the Internet and with each other. Femtocells - which by the end of 2010 already outnumbered traditional base stations and at the time of publication are being deployed at a rate of about five million a year - both enhance and interfere with this network in ways that are not yet well understood. Will femtocells be crucial for offloading data and video from the creaking traditional network? Or will femtocells prove more trouble than they are worth, undermining decades of careful base station deployment with unpredictable interference while delivering only limited gains? Or possibly neither: are femtocells just a "flash in the pan"; an exciting but short-lived stage of network evolution that will be rendered obsolete by improved WiFi offloading, new backhaul regulations and/or pricing, or other unforeseen technological developments? This tutorial article overviews the history of femtocells, demystifies their key aspects, and provides a preview of the next few years, which the authors believe will see a rapid acceleration towards small cell technology. In the course of the article, we also position and introduce the articles that headline this special issue.

Femtocells are assuming an increasing important role to extend the cellular network coverage and provide high speed data service for indoor environments. To cope with the extreme interfering cases in the WCDMA uplink scenario, Home NodeB (HNB) receivers need to employ uplink attenuation at the RF frontend to suppress the interfering signals introduced by unattached co-channel users, e.g., a penetrating signal from macro-connected users in the proximity. The optimal level of uplink attenuation at the HNB receiver should be designed to balance the requirements between the noise rise limitation in HNB and maximum transmit power cap for the femto-connected users. In this paper, we introduce two optimal adaptive schemes that apply the minimum and necessary level of attenuation to the received signal and show the outage probability improvement by the method of simulation.

In this letter, we propose a successive multiuser detector (SMUD) for contention based OFDMA ranging channel compliant to the IEEE 802.16 (WiMAX) standard. A ranging channel consists of a set of subcarriers in specific time slots shared by multiple users, so the multiple access interference (MAI) limits the performance of ranging detectors. Different from existing methods that treat the MAI as noise, the proposed SMUD successively detects the channel paths of active ranging signals and cancels their interference for further detection. This approach significantly suppresses the MAI and improves both user detection and parameter estimation performance.

In orthogonal frequency-division multiplexing (OFDM) systems, the integer part of carrier frequency offset (CFO) as a multiple of subcarrier spacing needs to be estimated from the frequency-domain signal. The performance of the autocorrelation based method is quite poor, while the channel knowledge based method is too complex for hardware real-time implementation. To provide improved tradeoffs between the performance and complexity, we propose a new hybrid algorithm that filters away unlikely integer CFO candidates with the autocorrelation method and determines the most likely estimate with channel knowledge. The analytical performance of the proposed hybrid method agrees with the simulation results and both demonstrate the cost-effectiveness of the method even in challenging wireless channel conditions.

This paper studies fractional carrier frequency offset (CFO) estimation for orthogonal frequency-division multiplexing (OFDM) systems. In the IEEE 802.16 (WiMAX) standard, a training symbol structure with three highly correlated but not identical segments is specified. The existing CFO estimation methods require the segments in the training symbol to be identical, so cannot work for the generalized training structure. In this paper, we proposed a universal fractional CFO estimator to solve the problem. Both the analytical and numerical results are presented to confirm the performance of the proposed method.

In orthogonal frequency division multiplexing (OFDM) systems, coarse frame timing can be acquired from one or more training symbols preceding every OFDM burst. The existing literature studied the case where there was only one training symbol consisting of identical segments. We generalize the timing synchronization methods to take advantage of multiple training symbols and only require the segments to be highly correlated but not necessarily identical. We construct a series of component timing metrics, one for each pair of the highly correlated segments, and combine them linearly to minimize the false alarm probability while keeping the asymptotic missed detection probability to the same level as other techniques. The OFDM data symbols in a downlink burst can have different power levels to reach the users at different distances. We take that into account and yield more realistic results than those in existing literature which only considered the equal power case. The performance of the proposed method is analyzed in three scenarios generalized from the IEEE 802.11 and IEEE 802.16 standards. Numerical results are presented to confirm the robustness of the proposed method in various channel conditions.

Acquisition of the timing in an IDMA system must take place before signal detection and decoding are performed. Acquisition in the presence of severe multiple-access interference with time varying codes makes the task even more difficult. Inefficient designs cause a large number of false alarms and/or missed detections. This paper applies a powerful acquisition technique to IDMA systems in the uplink. Under high multiple access interference conditions conventional acquisition techniques used for DS-CDMA systems simply fail. The proposed method utilises soft data from the iterative IDMA receiver to effectively cancel interference from the co-channel users. Analytical performance in terms of the number of users, spreading gain, performance of the IDMA receiver, and noise variance is developed, together with simulation results.

Extrinsic information transfer (EXIT) charts are utilized to optimize the iterative multiuser detector receiver in a multiuser turbo-coded CDMA system. The (receive) power levels are optimized for the system load using a constrained nonlinear optimization approach. The optimal decoding schedule is derived dynamically using the power optimized EXIT chart and a Viterbi search algorithm. Dynamic scheduling is shown to be a more flexible approach which results in a more stable QoS for a typical system configuration than one-shot scheduling, and large complexity savings over a receiver without scheduling. We verify through simulations that complexity savings of over 50% and power savings of over 8dB can be achieved. We show that the optimized power levels combined with adaptive scheduling allows for efficient utilization of receiver resources for heavily loaded systems.

Coherent OFDM detection requires accurate channel state information (CSI). Mobile radio channels are both time and frequency dispersive, especially at high vehicular speeds, which makes channel estimation a challenging problem in system design. Conventional preamble-based and pilot-aided channel estimation require numerous reference signals, which significantly compromises the system throughput. This paper proposes a novel low complexity iterative turbo channel estimation technique which makes use of preamble, pilots and soft decoded data information in an iterative fashion to improve the system performance over the time and frequency selective fading channel while maintaining the system throughput. The numerical and analytical results show that the proposed technique can approach the performance of systems with perfect CSI with much fewer preamble and pilots symbols compared to existing channel estimation methods.

A joint iterative channel estimation and data detection receiver is proposed in this paper for multiple-input- multiple-output orthogonal frequency deivision multiplexing (MIMO-OFDM) system, which employs two mandatory MIMO profiles as in IEEE 802.16e MIMO mobile WiMAX system, namely Alamouti space time coding (STC) and spatial multiplexing (SM), over time and frequency dispersive fading channel. A novel channel estimator with time-domain and frequency-domain combining of channel estimates from preamble/pilot/soft decoded data information is proposed and integrated with maximum ratio combining (MRC) data detector and co-antenna interference canceler (IC) for STC system and SM system respectively. Numerical results show that the proposed receiver has 2dB gain compared to conventional non-iterative receivers in pedestrian low mobility condition and more than 6dB gain in vehicular high Doppler environment.

In this paper we derive a novel extrinsic information transfer (EXIT) function for a channel estimator that uses both pilot symbols and soft decoding information. We consider a CDMA system with forward error correction that employs iterative multiuser detection based on soft interference cancellation and single user decoding. The J function is useful in convergence analysis as it provides a mapping between mutual information and variance. J functions are derived for both the flat-fading Rayleigh channel and channel estimate, allowing the a priori inputs to the channel estimator, interference canceller and decoder to be modeled. Algorithms for obtaining the EXIT functions for the three receiver blocks are proposed. We then combine the EXIT functions to create an EXIT chart that can be used to predict the behaviour of the overall receiver. Simulations are run for a 3GPP-like receiver in a mobile environment with a vehicle speed of 100 km/h. It is found that under these conditions, the detector trajectory closely matches our proposed EXIT chart.

In this paper, we develop low-complexity iterative channel estimation techniques for emerging IDMA systems. The channel estimators make use of pilots as well as soft decoded data information. We derive a lower bound for channel estimation error that reflects the reliability of soft decoded data. We show that the estimators perform close to a minimum variance unbiased estimator as the mean square error (MSE) approaches the lower bound. Numerical results on the MSE and BER performance also show that the proposed channel estimators are able to track the time-varying channel states.

Current OFDM systems assume the channel is not time varying within one OFDM frame, and use channel estimates obtained from preamble or pilots for data symbols within the same frame. This performs poorly in rapid dispersive fading channel with high mobility. This paper proposes a novel iterative turbo channel estimation to improve the performance of OFDM system over fading channel with both time and frequency selectivity. Unlike the conventional channel estimation techniques only using preamble or pilots, the proposed channel estimation technique makes use of preamble, pilots and soft coded data information in a turbo iterative approach. Mean square error analysis and simulation results show that the proposed channel estimation technique can approach the performance of perfect channel state information (CSI) with fewer pilots insertion.

In this paper the system model of an uncoded and fully synchronised interleave division multiple access (IDMA) scheme is transformed into an equivalent matrix model. This new model allows for additional alternative insights into the principles of IDMA and it is shown that the arising matrices are highly structured and very sparse. Furthermore the optimal detector for IDMA is derived and compared to the standard iterative decoder. The authors use exit charts and an empirical rank analysis to highlight that systems without an alternating signature mask always have a low rank and that parallel interference cancellation then fails.

This paper proposes a high-performance and relatively low-complexity approximated maximum likelihood estimator (MLE) for carrier frequency offset (CFO) in practical orthogonal frequency multiplexing (OFDM) systems, where the preamble has repetition structure in time domain, null subcarriers are allocated at two ends of the frequency spectrum, and the channel impulse response is assumably not longer than the cyclic prefix. We show the optimality of conventional MLEs do not hold in such systems, and derive a universal MLE and its modified Cramer-Rao lower bound in the general case. To reduce the complexity, we propose to cascade it with a low-complexity rough CFO estimator so that only a moderate number of maxima searches are needed to achieve satisfactory performance. Simulation with the IEEE 802.16 preamble symbol confirms the excellent performance of the proposed method.

In this paper, we investigate the performance of DS-CDMA systems in dispersive fading channels. We develop a low-complexity receiver that jointly performs channel estimation and multiuser decoding in an iterative manner. Pilot symbols, together with the soft decoding information on the transmitted data are harnessed to estimate the time-varying channel states, which in turn leads to improved detection and decoding results. We show through simulation and mean square error analysis that the proposed system can sustain a high-throughput WCDMA transmission.

This paper presents a novel design method for a spatially interpolated beamformer. By separating the design of the physical antenna array and the digital signal processors, we remove the restriction on the half-wavelength spacing requirement of a uniform linear array to achieve higher efficiency. To be more realistic, however, we fix the element positions regardless of the steering angles. The maximum energy concentration properties of discrete prolate spheroidal sequences are exploited to optimize the weighting coefficients and filtering window. Two examples are given to demonstrate the high flexibility, low computational complexity, and non compromised performance of the proposed beamformer.

In this paper, we propose a low-complexity multistage filter for the partitioned-spreading CDMA (PS-CDMA). The proposed method embodies a MMSE filter that operates on the soft output of interference cancellation. Extrinsic information on the partitioned symbol is exchanged between the MMSE filter and the partitioned-spreading APP detector to improve the system performance. It is shown via variance evolution the signal-to-noise ratio of this novel technique achieves that of the single-user channel even for heavily loaded systems. It also outperforms IDMA by a fraction of dB to a few dBs, for a variety of system loads. The complexity of the proposed system is much lower that of its counterparts for conventional CDMA, which makes it a viable alternative for future 3G or beyond 3G systems

Multiple-access interference (MAI) has been considered as a major performance-limiting factor in the next-generation CDMA systems. Multiuser detection (MUD) methods have been proposed to mitigate the MAI from the co-channel users by incorporating the cross-correlation properties between users. Recently, two classes of emerging techniques, probabilistic data association (PDA) and Markov Chain Monte Carlo (MCMC) methods, have been applied to the multiuser detection. In this paper, we present a new method, named Monte Carlo PDA (MC-PDA), that incorporates the concepts of both to give a more reliable inference of the CDMA symbols by appropriately modelling and updating the MAI. The methodology is general and can be applied to other communication channels

This paper investigates a multiple-access communication receiver system that receives coded data modulated using either direct-sequence code division multiple access or narrowband binary phase shift keying, with an antenna array in a multipath propagation environment. We describe an iterative receiver that improves the initial estimates from the antenna array, and therefore reduces the multiple access interference. Simulation results show that the bit error rate performance approaches that obtained when only one user's signal is incident on the array. This occurs even with a large number of users in comparison to the product of the spreading gain and array size.

We view the asynchronous random code division multiple-access (CDMA) channel as a time-varying convolutional code. We study the case where the users encode their data, and, therefore, the single user transmitters and the CDMA channel appear as the concatenation of two coding systems. At the receiver we employ serial turbo decoding strategies. Unlike conventional turbo codes where both the inner and outer code may be selected, in our case, the inner code is due to the CDMA channel which we assume to be random. Nevertheless, the decoding system resembles the decoder of a serial turbo code and single-user performance is obtained even for numbers of users approaching the spreading code length.

This paper introduces an iterative multiuser receiver for direct sequence code-division multiple access (DS-CDMA) with forward error control (FEC) coding. The receiver is derived from the maximum a posteriori (MAP) criterion for the joint received signal, but uses only single-user decoders. Iterations of the system are used to improve performance, with dramatic effects. Single-user turbo code decoders are utilized as the FEC system and a complexity study is presented. Simulation results show that the performance approaches single-user performance even for moderate signal-to-noise ratios.

This paper discusses a code-division multiple access (CDMA) iterative multi-user receiver with forward error control (FEC) decoding. The maximum a-posteriori probability (MAP) criteria is used to derive the receiver. The decoding is done using a turbo-code decoder with modifications which are discussed. Iterations of the system are used to attain large performance improvements over conventional systems.

A coding scheme, termed turbo-codes was proposed, which achieves results very close to the Shannon-limit. The encoding and decoding of turbo-codes is reviewed. In particular, a comparison of the current methods used for terminating the trellis of the turbo-code are described. A novel approach to termination of the code is introduced which removes the need to transmit tail bits across the channel. A reduction in transmitted symbols by 2 percent is achieved for PCS CDMA type applications. Simulations confirm that the novel approach is better in terms of BER than other methods of termination.