Realizing high-accuracy transmission in high-rate data broadcasting networks with heterogeneous users via cooperative communication

In this paper, we propose antenna selection (AS) and user scheduling schemes in multiuser two-way relaying (TWR) networks, where a multi-antenna base station (BS) and several single-antenna users (USs) exchange information via a single-antenna relay terminal under the influence of asymmetric rate requirements and channel estimation error (CEE). The considered AS and user selection schemes maximize the overall end-to-end signal-to-noise ratio (SNR) in the presence of CEE and different data rates. For bidirectional information exchange, AS is adopted at the BS with user selection. In particular, we deduce new expression of overall outage probability (OOP) for the considered system under independent and identically distributed (i.i.d.) Rayleigh fading channels. We further derive new compact asymptotic outage expression in the high SNR regime to reveal the impact of CEE and different target rates onto the system diversity order. Based on the asymptotic expression, we formulate a concise solution for the relay location to maximize the system performance. Lastly, our analytical findings are verified through numerical and simulation results.

Cooperative virtual MIMO for wireless sleep apnoea monitoring system

The cooperative techniques have recently taken a lot of attention in the recent few years due to their efficiency in radio mobile networks specially when the direct link between the source and the destination is lost. However, their combination with the multiple input multiple output (MIMO) schemes requires further study since new strategies of cooperation are expected when the MIMO component is added to the cooperation scheme construction. In this paper, we propose a double-layer space-time (DLST) code in a distributed way between the relays in order to emulate the MIMO transmission scheme between the relays. The DLST is compared with some of the most promising codes found in the literature. We show that the proposed code is very efficient whatever the transmitted powers by the relays are. This efficiency is verified in both amplify and forward (AF) and decode and forward (DF) algorithms. We show that a gain of at least 1 dB is obtained through DLST compared to conventional MIMO schemes in cooperative communications whatever the transmission scenario is.

We consider a cellular network with multiantenna base stations (BSs) and single-antenna users, multicell cooperation, imperfect channel state information (CSI), and directional antennas, each with a vertically adjustable beam. We investigate the impact of the elevation angle of the BS antenna pattern, which is denoted as tilt, on the performance of the considered network when employing either a traditional single-cell transmission or a fully cooperative multicell transmission. Using the results of this investigation, we propose a novel hybrid multicell cooperation technique in which the intercell interference is controlled via either cooperative beamforming in the horizontal plane or coordinated beamforming in the vertical plane of the wireless channel, which is denoted as adaptive multicell 3-D beamforming . The main idea is to divide the coverage area into two disjoint vertical regions and adapt the multicell cooperation strategy at the BSs when serving each region. A fair scheduler is used to share the time slots between the vertical regions. It is shown that the proposed technique can achieve performance comparable with that of a fully cooperative transmission but with significantly lower complexity and signaling requirements. To facilitate computationally efficient simulation and design space exploration, accurate approximations of the user ergodic rate are proposed for different transmission strategies under imperfect CSI.

This paper analyzes the cell edge mobile user performance in the downlink cellular system. We develop frame-work for coverage probability and spectral efficiency. In particular, we analyzed the performance of multi-antenna mobile users under multi-antenna base stations (BSs). The expressions of coverage probability and spectral efficiency are derived for cell edge user using stochastic geometry. We investigate how much the performance of cell edge user is improved when distances connecting BSs and cell edge users are modeled with cell edge null probability distribution. The probability of coverage and spectral efficiency is studied using zero-forcing beam-forming and the performance metrics are compared between coordinated scheduling (CS) and without coordinated scheduling (w/o CS). The interesting observation from our results is that the edge user coverage and rate is closely approaching towards the inner cell typical mobile user's rate and coverage, and the performance is verified with relative probability of coverage gain analysis.

Channel estimation error and cochannel interference (CCI) problems are among the main causes of performance degradation in wireless networks. In this paper, we investigate the impact of cooperative communications on mitigating the effect of channel estimation error and CCI. Two main performance criteria, namely, the traditional outage probability and the proposed signal-to-noise ratio (SNR) gap ratio, are utilized to characterize such impact. The SNR gap ratio measures the reduction in the SNR due to channel estimation error or CCI. Taking into consideration the channel estimation error, we show that the outage probability is reduced by utilizing cooperative transmission protocols. We also show that cooperative transmission scenarios, in which each cooperating relay forwards its signal over an orthogonal channel, result in lower SNR gap ratio compared to that of the direct transmission. Thus, cooperative transmission schemes are less susceptible to the effect of channel estimation error compared to direct transmission. Moreover, increasing the number of cooperating relays reduces the effect of the channel estimation error more. Timing synchronization error arises in distributed space-time cooperative schemes, in which the cooperating relays are simultaneously transmitting their signals over the same channel. Unlike the channel estimation error, the effect of the timing synchronization error gets worse as the the number of cooperating relays increases. In this work we also study the tradeoff between the timing synchronization error and the channel estimation error, and show their net impact on the system performance. Finally, we illustrate that CCI can be modeled in a similar fashion to the channel estimation error, and hence the cooperative transmission schemes are also less susceptible to the effect of CCI.

Spectral efficiency analysis and optimal power allocation for uplink multi-user cell-free massive MIMO networks employing STBCs

In this paper, we studied the massive multiple-input multiple-output (MIMO) system performance with N antenna users and streams can be multiplexed per each user. The spectral efficiency (SE) of uplink and downlink expressions is derived for any N antenna users and these achievable using estimated channels and per user basis minimum mean squared error successive interference cancellation (MMSE-SIC) detector. This MMSE-SIC analysis shows similar to asymptotic SE of linear MMSE detectors indicating that the SE increases from having multi antenna users can be harvested using linear detectors. We also generalize the power scaling laws for massive MIMO and handling arbitrary N antenna users and show that that one can reduce the multiplication of the pilot power and payload power as 1/M where M is the number of BS antennas, and still notably increase the SE with M before reaching a non-zero asymptotic limit. Simulations show that SE increase with N antenna users, also note that the same improvement can be achieved by serving N times more single-antenna users instead. Thus the additional user antennas are particular useful for SE improvements when there is few active users in the system.

In this paper, we investigate the effect of channel estimation (CE) error on the performances for the ultra-dense small cell networks. Specifically, we examine its effect on the distribution of signal to interference and noise ratio (SINR) and trend of spectral efficiency (SE), and outage probability. From these, we found out that the SINR degradation due to the CE errors becomes larger, as the density ratio of small base station (BS) to UE increases. We derived the theoretical lower bound of the outage probability, which is the function of CE correlation coefficient. Finally, numerical results demonstrate that the CE errors diminish the increasing scaling of SE, as the small BS density increases.

We analyze the performance of massive MIMO systems with N-antenna users. The benefit is that N streams can be multiplexed per user, at the price of increasing the channel estimation overhead linearly with N. Uplink and downlink spectral efficiency (SE) expressions are derived for any N, and these are achievable using estimated channels and per-user-basis MMSE-SIC detectors. Large-system approximations of the SEs are obtained. This analysis shows that MMSE-SIC has similar asymptotic SE as linear MMSE detectors, indicating that the SE increase from having multi-antenna users can be harvested using linear detectors. We generalize the power scaling laws for massive MIMO to handle arbitrary N, and show that one can reduce the multiplication of the pilot power and payload power as 1/M where M is the number of BS antennas, and still notably increase the SE with M before reaching a non-zero asymptotic limit. Simulations testify our analysis and show that the SE increases with N. We also note that the same improvement can be achieved by serving N times more single-antenna users instead, thus the additional user antennas are particular beneficial for SE enhancement when there are few active users in the system.

In this thesis, major attention is paid to cooperative diversity as an alternative way to achieve spatial diversity when the multiple antenna structure is not an option. By adopting the cooperative relay nodes to forward information, we can mitigate the fading effects, increase the capacity, lower the bit-error rate, increase the achievable transmission range, and without sacrificing time and bandwidth efficiency. Orthogonal Frequency Division Multiplexing (OFDM) is a popular multicarrier modulation technique in the modern wireless communications, since it possesses the advantages of frequency parallel transmission, high speed communications and efficient spectrum usage. In this Ph.D. thesis OFDM transmission into the cooperative communication domain is investigated. The diversity gains from both spatial domain and frequency domain are combined and so cooperative communication can further enhance the reliable, high speed transmission, and enable the spectrum efficiency. We propose a cooperative OFDM tall Toeplitz scheme, which guarantees full cooperative diversity, taking outage diversity and multipath diversity into account; in addition it easily combats Carrier Frequency Offsets (CFOs), using Linear Equalizers (LEs) only. Compared to the conventionally used Maximum-Likelihood Equalizers (MLEs), the system complexity is reduced significantly. There are mainly two relaying protocols in cooperative relay networks: Amplify-and-Forward (AF) and Decode-and-Forward (DF). In the research on relay selection and resource allocation issues in cooperative communications, we propose a dynamic optimal combination strategy for the hybrid DF-AF cooperative OFDM communications and removing unsuitable AF relays. Subsequently, we propose relay selection and resource allocation and optimization schemes for cooperative wireless communication networks with interference based on the so called Stackelberg game approach. This approach is a proper game model for solving the relay selection and power allocation problem in a distributed manner, when pricing and power allocation of the relay both are taken into account. As an extension of our research on cooperative wideband communication, we study wideband scale-lag channels as well, present in many ultra-wideband communication applications. In our research, cooperative relaying communication network is set for multi-scale and multi-lag wideband channels. We also provide a dynamic optimal selection strategy for relay selection to take advantage of the multi-relay, multi-scale and multi-lag diversity and maximize the system Bit-Error Rate (BER) performance. Cooperative localization research is growing when larger wireless networks are deployed and more applications are developed which require accurate position information. Therefore, cooperative locationing research becomes an important part of this Ph.D. research on cooperative wideband OFDM communications. In this research, we propose a trigger relay based cooperative localization technique. Because the trigger relay only needs to be switched on by the incoming signal, and to be sent as a simple pilot to the receiver, our technique gains from the easy processing together with noise and interference immunity of the base station to relay link. Compared to AF relay and DF relay Time Difference of Arrival (TDOA) estimation cases, the trigger relay reduces the system complexity. Meanwhile, trigger relay enables the bandwidth efficient TDOA, since it significantly reduces the amount of data for transmission. Furthermore, by exploiting cooperative-multipath diversity, the improved signal detection further contributes to a better TDOA estimation.

We prove convexity of the sum-power constrained mean square error (MSE) region in case of two single-antenna users communicating with a multi-antenna base station. Due to the MSE duality this holds both for the vector broadcast channel and the dual multiple access channel. Increasing the number of users to more than two, we show by means of a simple counter-example that the resulting MSE region is not necessarily convex any longer, even under the assumption of single-antenna users. In conjunction with our former observation that the two user MSE region is not necessarily convex for two multi-antenna users, this extends and corrects the hitherto existing notion of the MSE region geometry.

We studied the massive multiple-input multiple-output system performance with N-antenna users, as the advantage of N streams can be multiplexed per user, increasing the channel estimation overhead linearly with N. Spectral efficiency (SE) of uplink and downlink expressions are derived for any N-antenna user and these are achievable using estimated channels and per-user basis minimum mean-squared error successive interference cancellation (MMSE-SIC) detectors. This analysis shows that MMSE-SIC has similar asymptotic SE as linear MMSE detectors indicating that the SE increase from having multiantenna users can be harvested using linear detectors. Also we generalize the power scaling laws for massive MIMO to handle arbitrary N and show that one can reduce the multiplication of the pilot power and payload power as 1/M where M is the number of base station antennas, and still notably increase the SE with M before reaching 114a non-zero asymptotic limit. Simulations show that SE increase with N-antenna users, also note that the same improvement can be achieved by serving N times more single-antenna users instead. Thus the additional user antennas are particular useful for SE improvements when there are few active users in the system.

Currently, the increasing energy consumption is a global issue. Information technology and telecommunication industry is one of the areas with the largest energy consumption. The growth of mobile data users is an issue and the biggest challenge for the future. The cooperative wireless communication system has been the focus of research as one of the information delivery strategy with more efficient energy consumption. Energy efficiency in the cooperative wireless communication system can be improved by using a relay between base station and user device, where the distance between base station and user can be shortened, thus, the energy transmission can be minimized. Relay mechanism can be built by utilizing the protocol in the cooperative communication system, such as amplify and forward (AF), decode and forward (DF), and quantize and forward (QF). Relay-selection is an important issue in a cooperative wireless communication system that can reduce energy consumption at the system level. This study analyzes energy efficiency of multi-relay QF cooperative communication for line-of-sight (LOS) and non-line-of-sight (NLOS) environment based on relay selection strategies: reactive and proactive relay selection. A computer simulation is conducted based on a system model and mathematical analysis. Energy efficiency is calculated based on power consumption of signal transmission and observed in the distance between the source, relay, and destination. Simulation result shows that multi-relay QF networks with relay selection consume lower energy than without relay selection, hence, the energy usage in the relay selection networks is more efficient. Moreover, the strategy of proactive relay selection provides low energy consumption and high energy efficiency compared to the reactive relay selection strategy.

Consider a MIMO multicast channel where a multi-antenna base station (BS) sends common messages to K multi-antenna users. Assuming that the channel state information (CSI) of the K users are known at the BS, we study the optimal downlink rank-one beamforming which maximizes the postprocessing SNRs of the bottleneck users. We first study the special case where the BS has two transmit antennas. Denoting γ ∊ R^k as a vector consisting of the SNRs of the K users, we derive the closed-form expression of the feasible set of γ as a two-dimension ellipsoid embedded in a K-dimension space. Based on this observation, we develop a prune and search algorithm (PASA) which guarantees to find a global optimal Tx beamforming vector and meanwhile is computationally faster than the existing suboptimal algorithms. For the general case where the BS has an arbitrary number of Tx antennas, we leverage the two-antenna result and propose an iterative 2-dimensional optimization (I2DO) method, which at each iterative step transforms the original problem into the two-antenna one and improves the solution using PASA. The simulation results show that the proposed methods outperform the existing ones in both performance and complexity.