CHAPTER 1 1.1 Introduction The future wireless communication systems beyond the 3G are expected to have high data-rate in order to support a large number of users with fairness in the service. With the increasing numbers of subscribers, multipath fading and path loss become more significant. So, improved and flexible multiple access methods are needed to cope with these impairments . Among various multiple access techniques orthogonal frequency division multiple access (OFDMA) is one of the favoured schemes for use in broadband transmission system design as it is capable of overcoming intersymbol interference (ISI) and utilizing the given radio spectrum efficiently by dividing it into a large number of small sub channels to different users, provided that each subchannel is allocated to at most one user at a time  . In addition to OFDMA, multi-input multiple-output (MIMO) transmission technique has also been intensively investigated. Multiple antennas installed at transmitter and receiver can considerably increase link capacity as well as link reliability compared to single-input single-output (SISO) systems [4 6]. In a MIMO OFDMA system, a wideband channel is divided into multiple narrowband subchannels using orthogonal subcarriers. Data are transmitted in parallel through subchannels, and the total transmit power should be distributed to these subchannels. MIMO systems achieve spatial multi-user access using singular value decomposition (SVD) .
The demand for spectrum and the total transmit power increased with the increasing numbers of wireless devices and applications. A base station needs to communicate to multiple users, with limited resources, e.g. total transmit power and available frequency bandwidth. As frequency allocated for each sub channel is limited, the one of the parameter that can combat with this problem is power allocation scheme. The transmitted power is adjusted according to the channel condition so as to maintain the overall system performance and ensure received signal quality. The spectral efficiency of MIMO transmission can be dramatically increased if perfect CSI is available   which allow the system to take advantage of the available spectrum and the radio channel.
Recently, the problem of how to dynamically allocate the radio resources to improve the performance of OFDMA wireless systems has been the subject of intensive research. Broadly speaking, two classes of resource allocation schemes namely fixed resource allocation and dynamic resource allocation   are available. Fixed resource allocation schemes uses time division multiple access (TDMA) and frequency division multiple access (FDMA) to assign an each user. A fixed resource allocation scheme is not optimal since the scheme is fixed regardless of the current channel condition. On the other hand, dynamic resource allocation allocates a dimension adaptively to the users based on their channel gains. Due to the time-varying nature of the wireless channel, dynamic resource allocation makes full use of multiuser diversity to achieve higher performance. Two classes of optimization techniques have been proposed in the dynamic multiuser OFDMA literature: rate adaptive (RA)   and margin adaptive (MA)  . The rate adaptive objective is to maximize each user's error-free capacity with a total transmit power constraint. The margin adaptive objective is to achieve the minimum overall transmit power given the constraints on the users' data rate or bit error rate (BER).
Apart from multiantenna OFDMA, the used of cooperative communication further add a new communication paradigm promising significant capacity and coverage increase as well as enhanced performance in current and future wireless networks by introducing cooperative relay channels . The relay channel can be thought of as an auxiliary channel to direct channel between the source and destination. This communication reduces the shortcomings...
Please join StudyMode to read the full document