Academic literature on the topic 'Relayed communication systems'

Relays can be used in cellular systems to increase coverage as well as reduce the total power consumed by mobiles in a cell. This latter benefit is particularly useful for mobiles operating on a depleted battery. The relay can be a mobile, a car or any other device with the appropriate communication capabilities. In thesis we analyze the impact of using relays under different situations. We first consider the problem of reducing total power consumed in the system by employing relays intelligently. We find that in a simulated, fully random, mobile cellular network for CDMA (Code Division Multiple Access), significant energy savings are possible ranging from 1.76 dB to 8.45 dB. In addition to reducing power needs, relays can increase the coverage area of a cell by enabling mobiles located in dead spots to place relayed calls. We note that use of relays can increase the useful service area by about 10% with real life scenarios. We observe that in heavy building density areas there is more need of relays as compared to low building density areas. However, the chance of finding relays is greater in low building density areas. Indeed, having more available idle nodes helps in choosing relays, so we conclude that unlike present day implementations of cellular networks, the base station should admit more mobiles (beyond the capacity of the cell) even if they are not placing calls since they can be used as relays. One constraint of using relays is the potential to add interference in the same cell and in neighboring cells. This is particularly true if the relays are not under power control. Based on our analysis, we conclude that in interference limited systems like CDMA the relays have to be under power control otherwise we will reduce the total capacity by creating more dead spots. Thus, we believe that either the base station should be responsible to allocate relays or relays should be provided with enough intelligence to do power control of the downlink. Finally, we show how utility of data services can be increased by use of relays.

Recently, the application of SWIPT in the energy-limited wireless system has attracted much attention from researchers due to the SWIPT technology provide the capability to prolong the limited lifetime of the wireless system. For a wireless communication system, a relay node helps to extend the network coverage, while the MIMO relay technique helps in enhancing the system performance. In this thesis, the investigation concerning the application of SWIPT in the MIMO relay system is investigated.

The relay based cooperative network is one of the promising techniques for next generation wireless communications, which can help extend the cell coverage and enhance the diversity. To deploy relays efficiently with limited power and bandwidth under certain performance requirements, resource allocation (RA) plays an increasingly important role in the system design. In recent years, with the fast growth of the number of mobile phone users, great portion of CO2 emission is contributed by wireless communication systems. The combination of relay techniques and RA schemes reveals the solution to green communications, which aims to provide high data rate with low power consumption. In this thesis, RA is investigated for next generation relay based green wireless systems, including the long-range cellular systems, and the short-range point-to-point (P2P) systems. In the first contribution, an optimal asymmetric resource allocation (ARA) scheme is proposed for the decode-and-forward (DF) dual-hop multi-relay OFDMA cellular systems in the downlink. With this scheme, the time slots for the two hops via each of the relays are designed to be asymmetric, i.e., with K relays in a cell, a total of 2K time slots may be of different durations, which enhances the degree of freedom over the previous work. Also, a destination may be served by multiple relays at the same time to enhance the transmission diversity. Moreover, closed-form results for optimal resource allocation are derived, which require only limited amount of feedback information. Numerical results show that, due to the multi-time and multi-relay diversities, the proposed ARA scheme can provide a much better performance than the scheme with symmetric time allocation, as well as the scheme with asymmetric time allocation for a cell composed of independent single-relay sub-systems, especially when the relays are relatively close to the source. As a result, with the optimal relay location, the system can achieve high throughput in downlink with limited transmit power. In the second contribution, the power consumption in relay based 60 GHz cooperative networks is studied, which is based on three-terminal diversity amplify-and-forward (DAF) and diversity DF (DDF) relaying strategies. A total power consumption model including drive power, decoding power, and power consumption of power amplifier (PA) is proposed, excluding the transmit power, as it is relatively small compared to decoding power and PA power in the indoor environment. This model is formulated as a function of drive power, which gives an easy access to the system level power allocation. To minimise the system total power consumption, the optimal drive power can be allocated to the source node by numerical searching method while satisfying the data rate requirement. The impact of relay locations on the total power consumption is also investigated. It is shown that, with the same data rate requirement, in the small source-relay separation case, DAF consumes slightly less power than DDF; while with larger source-relay separation, DAF consumes much more power than DDF. In the future work, multiuser relay-based short-range communication systems will be considered for the 60 GHz communication in the fading channel scenario, which extends the proposed power consumption model in a more practical way. The power consumption model of other components, such as analog-to-digital converter, data buffer, modulation/demodulation could also be considered to provide more details about green P2P communications.

The thesis investigates robust linear and non-linear transceiver design problems for wireless MIMO relay communication systems with the assumption that the partial information of the channel is available at the relay node. The joint source and relay optimization problems for MIMO relay systems are highly nonconvex, in general. We transform the problems into suitable forms which can be efficiently solved using standard convex optimization techniques. The proposed design schemes outperform the existing techniques.

This thesis focuses on transceivers design for interference MIMO relay systems. Based on the MMSE criterion, several iterative algorithms are proposed to jointly optimize the source, relay, and receiver matrices subjecting to the individual power constraints at the source and the relay nodes. These algorithms have better performance than some existing algorithms and provide a better performance-complexity trade-off which is interesting in practical interference MIMO relay communication systems.

The increasing demand for mobile applications such as streaming media, software updates, and location-based services involving group communications has prompted the need for wireless communication technologies that can support reliable high data rates. However, wireless channel is subject to signal fading that severely degrades the system spectral efficiency. By exploiting the spatial diversity, multiple-input multiple-output (MIMO) techniques can provide both theoretically attractive and technically practical solutions to combat channel fading. Moreover, in the case of long source-destination distance, single or multiple MIMO relay node(s) is necessary to combat the pathloss and/or shadowing effects of wireless channel and relay signals from the source to the destination.In this thesis, we focus on multiuser MIMO relay systems. We first present joint source, relay, and receiver optimization algorithms for the uplink system based on the minimum mean-squared error (MMSE) criterion subjecting to individual power constraints at the source and the relay nodes. The proposed algorithms outperform the existing techniques in terms of both MSE and bit-error-rate (BER). Next, in the downlink system, we consider multicasting multiple data streams among a group of users with the aid of a relay node, where all the nodes are equipped with multiple antennas. The downlink system performance is optimized subjecting to both power constraints at the source and the relay nodes and quality-of-service (QoS) constraints at the receivers.Then we present the duality between the uplink and the downlink of a multi-hop MIMO relay system. Based on this duality, we propose an optimal design of the source precoding matrix and relay amplifying matrices for multi-hop MIMO relay system with a nonlinear dirty paper coding (DPC)- based transmitter at the source node. The proposed nonlinear transmitter algorithm outperforms the existing decision feedback equalizer (DFE)-based receiver schemes.Multiuser MIMO relaying is then considered in an interference system where a group of transmitters communicate simultaneously with their desired destination nodes with the aid of multiple relay nodes, all equipped with multiple antennas. Transmit-relay-receive beamforming technique is exploited to minimize the total source and relay transmit power in conjunction with transmit power control such that a minimum QoS threshold is maintained at each receiver. The proposed scheme generalizes the existing single-hop MIMO interference systems and the single-antenna, dual-hop interference relay systems to the dual-hop interference MIMO relay systems with any number of source, relay, and destination nodes, all equipped with multiple antennas.The above algorithms are developed assuming that the instantaneous channel state information (CSI) knowledge of both the source-relay link and the relay-destination link is available at the scheduler. However, in practical relay communication systems, the instantaneous CSI is unknown, and therefore, has to be estimated. Hence, we finally propose a bandwidth efficient MIMO channel estimation algorithm that provides the destination node with full knowledge of all channel matrices involved in a dual-hop MIMO communication. The proposed approach attains smaller channel estimation error and is applicable for both one-way and two-way MIMO relay systems.