a simulation and analysis of ofdm system for 4 g communications

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A Simulation and Analysis of OFDM System for 4G Communications

International Journal of Advanced Research in Computer Engineering & Technology

Abstract:

The increase in the number of wireless devices and the requirement for higher data rates places an increasing demand on bandwidth. This necessitates the need for communication systems with increased throughput and capacity. Multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) is one way to meet this need. OFDM is used in many wireless communication devices and offers high spectral efficiency and resilience to multipath channel effects. Though OFDM is sensitive to synchronization errors, it makes the task of channel equalization simple. MIMO makes use of multiple antennas to increase throughput without increasing transmitter power or bandwidth. This thesis presents an introduction to the multipath fading channel and describes an appropriate channel model. Several modulation schemes are presented that are often used in conjunction with OFDM. Mathematical definitions and analysis of OFDM are given along with a discrete implementation common to modern communication systems. Synchronization errors are described mathematically and simulated, as well as techniques to estimate and correct those errors at the receiver. Lastly, space time coding, spatial multiplexing, and beam forming are presented as techniques used in (MIMO).

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(a) Block diagram of System Implemented

(b) Frequency Domain Model used in the System

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(c) Receiver Block Diagram for M-QAM

(d) Block Diagram of Single Carrier System

(e) Block Diagram for Multiple Carrier System

(f) Synchronization Block in OFDM Receiver

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(g) Frequency Estimation Algorithm

(h) Scrambler used to Generate Pseudo-Binary Pilot Sequence

(i) Residual Frequency Tracker

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(j) Four State Convolution Encoder used in project

(k) State Diagram for Encoder in Fig. (j)

(l) Trellis Diagram for Convolution Encoder in Fig. (j)

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(m) Alamouti for Two Transmit Antennas

(n) Alamouti for One Receive Antenna

(o) Channels used for Two Transmit Antennas and One Receive Antenna in this project

(p) Viterbi Decoding Algorithm

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(q) MIMO Channel

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Experimental Results and Simulations: (Note: Results may vary depending on signals to be transmitted or being received) A: - Amplitude Modulation (AM): Simulations of double side band AM and double side band suppressed carrier AM transmission

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B: One channel (Plots of the received energy as a function of distance i.e. , describing the effects of path loss)

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C: - Simulation of preamble for OFDM as used in IEEE 802.11a. The signal is being passed through a multi-path channel. Both the short and long training symbols are hard coded and baseband modulated using the IFFT.

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D: - Simulation of the preamble for OFDM as used in IEEE 802.11a. Both the short and long training symbols are hard coded and baseband modulated using the IFFT.

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E: - Simulation of Superposition of Waveforms

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F: - Simulation of Channels

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G: - Simulation for Demonstrating the use of the FFT/IFFT for OFDM Modulation

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H: - Simulation of Preamble for OFDM as used in IEEE 802.11a

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I: - Simulation of Pulse Amplitude Modulation

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J: - Simulation of preamble for OFDM as used in IEEE 802.11a. Both the short and long training symbols are hard coded and baseband modulated using the IFFT.

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K: - Simulation of Binary Phase Shift Keying (BPSK)

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L: - Plots of Constellation for Rectangular M-QAM

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M: - Simulation showing the effects of Sampling Clock Offset

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N: - Simulation showing the Effect of Frame or Symbol Timing Offset

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Conclusion: This thesis presented an introduction to MIMO-OFDM, specifically, the topics of the channel, OFDM, synchronization, and MIMO. Mathematical descriptions of the channel, OFDM, and synchronization, were given as well as MATLAB simulations to verify, illustrate concepts, or present a practical implementation. This thesis also provided in detail the effects present in a wireless multipath channel and the importance of the channel model. To that end, a frequency domain channel model was given and simulated for the wireless multipath fading channel, briefly reviewed some of the modulation schemes used in digital communications and used throughout this thesis. Thesis also gave a comparison of single carrier systems and multicarrier systems as the motivation for OFDM. Mathematical definitions were presented on modulation and demodulation in the presence of a channel. This thesis gave a basis for understanding the synchronization errors common to OFDM; a detailed discussion of synchronization errors was presented. Mathematical descriptions of the errors and simulations of the errors were presented to illustrate the effect that timing and frequency offsets have on the uncoded constellations. also showed how the synchronization errors could be estimated at the receiver and corrected. For each type of synchronization error, algorithms were presented and implemented in a MATLAB simulation. As an extension to OFDM, presented MIMO as a way to increase performance in a communication system. Three topics were showcased along with examples of each. Space time coding techniques such as convolutional codes and the Alamouti algorithm were shown to increase the reliability by transmitting multiple copies. Both had coding gains but the Alamouti scheme also introduced spatial gain. A maximum likelihood detector, the Viterbi decoder, was also presented as a way to decode the convolutional encoder. Two types of spatial multiplexing, D-BLAST and V-BLAST, were shown to be an effective way to increase the overall data rate by transmitting multiple independent streams. Lastly, beam forming was introduced as a way to direct the coverage area. In this way, more users can be serviced by the reduction of interference from multiple users. It is hoped that after reading this thesis the reader has a better understanding of channel models, MIMO-OFDM, the effects of the channel on OFDM, and the ways in which errors are estimated and corrected. This thesis aimed to be an introductory look into OFDM. Future systems will include MIMO-OFDM and, as such, there is a lot of research being done in this area. One of the assumptions made about the channel was that it was static. While this assumption works well for standards such as IEEE802.11, it does not work well for cell phones and other wireless devices. In the future, we may see many devices that must be able to operate at speeds were Doppler shifts affect data transmission. Another consideration is the techniques used in space time coding. Turbo codes and low density parity-check codes are now replacing convolutional codes. Also, only the 2x2 MIMO case was considered for STBCs.