精品论文推荐An Adaptive Bit Loading Algorithm for MIMO-OFDMA Systems with Fixed RateCui QingDepartment of Communication and Information System, Beijing University of Posts andTelecommunications, Beijing (100044)E-mail: cuiqing122yahoo.cnAbstractThis paper considers the optimal resource allocation for MIMO-OFDM systems in multi-user downlinkscenario. By using zero-forcing based SDMA scheme, the multi-user MIMO channels are converted to several SISO channels, which facilitate the proposed bit loading algorithm with the objective of minimizing the system bit error rate (BER). We provide the general mathematical formulation of the optimization problem which includes multi-user interference cancellation, multi-user diversity, bits and power allocation. We consider only the case that the total power and rate are defined by a system. Thus, our proposed algorithm can be expressed through a closed form equation, and it has very low complexity comparing with the iterative ones.Keywords: MIMO; OFDM; multi-user; Zero-forcing; adaptive bit and power allocation1. Intr oduct i on Multiple-input multiple-output (MIMO) systems have been recognized as a promising candidate for next-generation wireless communications due to their potential for dramatic gains in channel capacity 12. In a single-user context, the diversity gain over the single-input single output (SISO) is roughly the smaller number of antennas between the base station and the user terminal.Because of the size and cost constraints the number of the receive antennas are always the smaller one. It is obvious that significant capacity benefit cannot be obtained from the multiple transmit antennas at the base station. The solution to this problem is to serve multiple users simultaneously which can exploit multi-user diversity gain by taking advantage of the independence of the channel conditions of different users.On the other hand, there is growing interest in orthogonal frequency division modulation (OFDM), which has become a mature technique for providing high speed data services and is adopted in many existing systems, such as IEEE802.11a/g Wireless Local Area Networks, IEEE802.16 (known as WiMax), and also a strong candidate in ultra-wideband (UWB) standard. Since OFDM converts a frequency-selective fading channel into a set of parallel flat-fading channels, SDMA-related algorithms can be implemented on each sub-carrier. It is also found capable of providing further system capacity gain by exploiting multi-user diversity in OFDM systems 34, deducing a new multiple access technique so called orthogonal frequency division multiple access (OFDMA). By combining SDMA and OFDMA, multi-user diversity can be exploited both in the spatial domain and the frequency domain.For the above reasons, MIMO-OFDMA-SDMA system has considerable potential to increase degrees of freedom of the rich scattered channels and facilitates adaptive resource allocation which can enhance system throughput further more. A set of parallel, independent and spectrally flat sub-channels must be shaped before resource allocation scheme is applied. For downlink, zero-forcing (ZF) technique or block diagonalization (BD) has been proposed for SDMA to cancel the co-channel multi-user interference 56. It is feasible and desirable to move the most of the signal processing complexity from the user terminals to the base station by adopting preprocessing the inter-user interference at the base station, while subsequently a simple receive method which neglects the other users interference can be utilized at the user terminal.This paper considers the optimal resource allocation in ZF based MIMO-OFDMA-SDMAdownlink system for the purpose of minimizing the average BER. The MIMO-OFDMA-SDMA-1-system accommodates the users in both frequency domain and spatial domain. So the resource allocation not only refers to the power and bits allocation among the sub-carriers, but also user selection and optimal beam-former on each sub-carrier.The rest of this paper is organized as follows. In section 2, we describe the system model andpresent the ZF based optimal beam-forming which results in a set of independent parallels to accommodate multiple users data and power. And then the optimal resource allocation algorithm with discrete bit loading scheme are demonstrated. Section 3 presents the simulation results comparison. Finally, Section 4 concludes this paper.2. System Model an d Optim al Sch e me 2.1 Syste m Model In this paper, we consider a downlink adaptive multi-user MIMO-OFDM system equippedwith N sub-carriers,NT transmit antennas at a base station and K mobile users, each withNR receive antennas. And the MIMO channel on sub-carrier n between the base station and userk can be characterized by aNR