INTERNATIONAL JOURNAL FOR TRENDS IN ENGINEERING & TECHNOLOGY VOLUME 5 ISSUE 1 – MAY 2015 - ISSN: 2349 - 9303
Improving The Performance Of Multiuser OFDM Wireless System With UWB Channel Jayanthi k2
Bhuvaneshwari P1
Associate Professor SNS College of Technology,Department of ECE, jayanthiecesnsct06@gmail.com
1
SNS College of Technology, ME Communication Systems, Department of ECE, bhuvaneshwari2531@gmail.com
Abstract— The multi user OFDM system can be used to produce a highly flexible and effective communication system. In the existing multi user OFDM system, resource allocation to each user is the major problem. Also the existing multi user OFDM system uses frequency diversity technique that requires sparse bandwidth and multiple receivers improving the complexity. In order to increase the system performance, the proposed system allows multiple user data to be multiplexed together to form a single OFDM symbol where the resources are allocated equally to all users. The proposed multiuser OFDM system achieves double the capacity and better performance in terms of Signal-to-Noise Ratio and Bit Error Rate of a single user OFDM system and the UWB channel is used to securely transmit the data through the channel. Also, the proposed system reduces the receiver complexity by using polarization diversity to receive the multipath components for multiple users. Index Terms— BER, Cyclic Prefix, Diversity, Inter Symbol Interference, Rayleigh channel, SNR. —————————— ——————————
1. INTRODUCTION independent fading. The optimal combining of independently faded copies give a guide for decision at the receiver. The optimal combiner used is the maximum ratio combiner.
1.1 OFDM In OFDM, a large number of closely spaced orthogonal subcarriers are used to carry data on many simultaneous parallel data streams or channels. The sub-carriers are modulated with Quadrature Amplitude Modulation (QAM) or Phase-Shift Keying (PSK) which maintains total data rates similar to existing single-carrier modulation schemes in the same bandwidth at a low symbol rate. The key advantage of OFDM over single-carrier schemes is its ability to deal with with severe channel conditions without complex equalization filters by using orthogonality property. Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrow band signals rather than one quickly modulated wideband signal. Inter Symbol Interference (ISI) can be eliminated by the use of guard intervals between symbols and exploit echoes and time-spreading to achieve a diversity gain, i.e. a signal-to-noise ratio up gradation. This also facilitates the design of Single Frequency Networks (SFNs), in which several adjoining transmitters send the same signal concurrently at the same frequency.
1.3 FREE SPACE CHANNELS A channel has a certain capacity for transmitting the information signal from the transmitter to the receiver. There are three commonly used wireless channels such as AWGN channel, Rayleigh channel, Rician fading channel.
1.3.1 AWGN Channel Additive white Gaussian noise (AWGN) channel is a basic or frequently used channel model for analyzing modulation schemes. In this model, a white Gaussian noise is added to the signal that passes through the AWGN channel. This implies that the channel’s amplitude, frequency response is flat with unlimited or boundless bandwidth and phase frequency response is linear for all frequencies without any amplitude loss and phase distortion and the modulated signals pass through the channel. Fading does not exist for this type of channel[7]. The transmitted signal gets distorted only by AWGN process.
1.2 DIVERSITY
1.3.2 Rayleigh Channel
Diversity is a technique that is used to give back for fading channel impairments. Diversity technique can be implemented by using two or more receiving antennas that receives multipath components. To counteract the effect of ISI, equalization technique is used whereas Diversity is usually employed to reduce the intensity and extent of the fades experienced by a receiver in the fading channel[2]. The information signal is modulated through M different carriers in frequency diversity system. Here each carrier should be isolated from the other subcarrier by at least the coherence bandwidth, so that different copies of the signal go through
Constructive and destructive interference, and phase shifting of the signal occurs due to the effect of multipath transmission. This causes Rayleigh fading. There is no line of sight (NLOS) path in which no direct path between transmitter and receiver exists in Rayleigh fading Channel[6]. If the signal bandwidth of the channel is smaller than coherence bandwidth, the channel is called flat; or else it is frequency-selective fading channel.
1.3.3 Rician Channel Rician distribution can be obtained when Rayleigh fading have a strong line of sight (LOS)[7]. In environments where there is a dominant Line-of-Sight (LOS) path between the transmitter and the receiver, the complex Gaussian distributed fading coefficient
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