International Conference and Workshop on Emerging Trends in Technology 2013, TCET, Mumbai, India 63 Enhancement of Quality of Service of MIMO-OFDM B U Rindhe 1 EXTC Department, SIGCOE, Navi Mumbai, India (Research Scholar, SGBAU) 91-022-2753608 Ext.39 burindhe@yahoo.com Jyothi Digge 2 EXTC Department TEC, Navi Mumbai, India (Research Scholar, SGBAU) 91-022-27718134 jyothijayarajdigge@rediffmail.com S K Narayankhedkar 2 EXTC Department MGM COET, Navi Mumbai, India 91-022-27423403 skniitb@yahoo.com ABSTRACT Orthogonal frequency division multiplexing is a promising technology for high data rate transmission in wide band wireless system for achieving high downlink capabilities in a future cellular system. This paper demonstrate, how to improve the capacity of the system and transmission quality of orthogonal frequency-division multiplexed (OFDM) along with multi-input multi-output (MIMO) used with adaptive modulation can effectively eliminate fading in wireless channels. To minimize the overall transmit power, greedy algorithm approach was proposed for the optimal bit and power allocation strategy, the performance of adaptive bit and power allocation MIMO-OFDM system based on greedy algorithm is completely studied and the performance comparison among greedy algorithm, chow algorithm and average algorithm is represented. The analysis and the simulation are considered in two stages. First stage involves single-input single-output (SISO) OFDM system. This is compared with the performance of fixed OFDM transmission where a constant rate is applied to each subcarrier. Second stage involves MIMO-OFDM and we compared the performance of MIMO-OFDM system under different antennas numbers. General Terms Measurement, Performance, Design, Experimentation, Theory, Verification. Keywords Greedy Algorithm, MIMO-OFDM, Adaptive Modulation, Dynamic Allocation. 1. INTRODUCTION In poor wireless channels, the continuously changing channel gain makes the wireless channel capacity change accordingly. In the traditional communication systems, the radio transmission system scheme is dead against the worst conditions. But the channel is not always stay in the bad state, this conservative idea greatly reduce the spectral efficiency of system [1]. Up to now, many adaptive bit and power allocation algorithm have been put forward. In this paper, a modified adaptive bit and power allocation algorithm based on greedy algorithm is put forward [2-3], [7-9]. The target of this algorithm is to minimize the transmission power under a given fixed system performance. In the algorithm, first the bit and corresponding power are allocated to each sub-carrier according to SNR channel condition in the receiver, and then the residual power is allocated to the sub- carriers, which can use them most efficiently. In this way, this algorithm is efficiency while calculation is less complex. We introduced the model of adaptive bit and power allocation strategy for MIMO-OFDM system also adaptive bit and power allocation based on greedy algorithm for MIMO- OFDM system as well as margin adaptive quadrature modulation (MAQM) schemes. We derive the spectral efficiency for average allocation and adaptive allocation using greedy algorithm in the algorithm first the bit and then corresponding power are allocated to each subcarrier according to signal to noise ratio (SNR) channel condition in the receiver and then the residual power is allocated to the subcarriers. In this way this algorithm is efficiency while calculation is less complex. 2. ORTHOGONAL FREQUENCY DIVIS- ION MULTIPLEXING Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique used in applications catering to both wired and wireless communications [4-6]. However, in the wired case, the usage of the term Discrete Multi-Tone is more appropriate. The OFDM technique divides the frequency spectrum available into many closely spaced carriers, which are individually modulated by low-rate data streams. In this sense, OFDM is similar to frequency division multiplexing access (FDMA). The bandwidth is divided into many channels, so that, in a multi-user environment, each channel is allocated to a user. However, the difference lies in the fact that the carriers chosen in OFDM are much more closely spaced than in FDMA (1 kHz in OFDM as opposed to about 30 kHz in FDMA), thereby increasing its spectral usage efficiency. The orthogonality between the carriers is what facilitates the close spacing of carriers. The orthogonality principle essentially implies that each carrier has a null at the center frequency of each of the other carriers in the system while also maintaining an integer number of cycles over a symbol period. The motivation for using OFDM techniques over time division multiplexing access (TDMA) techniques is twofold. First, TDMA limits the total number of users that can be sent efficiently over a channel [10-12]. In addition, since the symbol rate of each channel is high, problems with multipath delay spread invariably occur. In stark contrast, each carrier in an orthogonal frequency division multiplexing (OFDM) signal has a very narrow bandwidth (i.e. 1 KHz); thus the resulting symbol rate is low. This results in the signal having a high degree of tolerance to multipath delay spread, as the delay spread must be very log to cause significant inter-symbol interference (e.g. > 500usec).