Two similar concepts

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Abstract

The American IEEE802.11a standard and the European equivalent HIPERLAN/2 are two similar concepts for broadband wireless LANs (WLAN) in the 5 GHz band. Both standards are based on the multicarrier modulation technique OFDM (orthogonal frequency division multiplexing) combined with convolution channel coding. The baseband modulation schemes of both standards are very similar, which simplifies implementation considerably. Challenges and difficulties considered in this project regard both systems. Except for slight differences in signal mapping, most discrepancies between the standards regard the higher protocol layers. In the presented paper, the principle of frequency domain channel estimation for wireless OFDM systems will be shown. A well known noise reduction technique will be adapted to HIPERLAN/2 and IEEE802.11a standards, and its positive effects will be demonstrated by simulation results. Channel tracking has not been considered by the WLAN standards named above, although it is well known that time-variant indoor radio channels can change their characteristics within one PHY burst. This paper presents some techniques for decision directed channel tracking, applicable in wireless OFDM systems.

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Aim and Objectives

Aim:

To research on channel tracking in wireless OFDM systems

Objective:

  • To critically analyze the challenges and difficulties of both American IEEE802.11 and European equivalent HIPERLAN/2.
  • An evaluation is made on a noise reduction algorithm and its positive effects will be demonstrated by simulation results.
  • To propose some techniques for decision directed channel tracking, applicable in wireless OFDM system.

Literature Review

OFDM is a key technology for ahead of 3G communications, promising strong, high capability and high speed wireless broadband multimedia networks (Ramjee Prasad, 2004). Channel tracking in wireless OFDM refers to the format of tracking time variant channel. It consists of an apparatus and a method to estimate channel response for receiver which receives a packet of transmitted part of subcarrier values. It includes a signal-to-tone transformer to decide the subcarriers of a signal that is comparable to received packet. Due to high mobility the channel response varies considerably during the transmission of packet. For accurate channel tracking the variation trend of the channel is examined by correlation analysis (Yo-Sung Ho and Hyoung Joong Kim, 2005). The main asset of this process is its good tracking performance of channels and computational complexity. Multipath Rayleigh fading in mobile stimulates the wireless multiple input multiple output - orthogonal frequency division multiplexing (MIMO-OFDM) systems with null sub-carriers. This technique consists of frequency domain coupled with minimum mean square error (MMSE) time domain estimation and it does not require an inverse matrix calculation during each OFDM symbol used. In order to handle the time variant channel, a blind channel response interpreter is designed. This method is far better than conventional channel tracking method in time varying channel environment. Its Doppler frequency is observed to be 100Hz, bit error rate of 10-4 and signal-to-noise power ratio is 2.5dB when compared to conventional channel tracking method. At 200Hz of Doppler frequency the difference between the performance of this method and conventional is very large (Ahmad R. S. Bahai, Burton R. Saltzberg and Mustafa Ergen, 2004). This technique enables to track the time variant channel through wireless broadband technology and is more fast and good to use when compared to conventional method.

Problem Scenario

There are two main problems in designing channel tracking in wireless OFDM systems. Arrangement of pilot information is the first problem where pilot symbol refers to the reference signal used by transmitter and receiver. Design of an estimator is the second problem which does not have low complexity and good channel tracking ability. These two problems are interconnected to each other. The combination of high data rates and low bit error rates in OFDM systems tells that estimators should have low complexity and high accuracy where these two limitations work against each other and a good trade off is needed. The optimal channel estimator in based on 2D Wiener filter interpolation but 2D estimator is too complex for practical implementation (Yushi Shen and Ed Martinez, 2006). At the receiver end each packet is checked through CRC (Cyclic Redundancy Check) for the purpose of error detection. In MIMO systems each sub stream transmitted from different antennas normally experience error statistics in a small range as they experience different link conditions. Since only one CRC is used it has to retransmit the whole packet if errors are detected where there is wastage beyond necessity (Fresh Patents, 2009).

Proposed Solutions

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To accomplish the trade-off between complexity and accuracy the one-dimensional (1D) channel estimations are adopted in OFDM systems. Block-type pilot channel and Comb-type pilot channel are the two basic 1D channel estimations, where in the pilots are inserted in the direction of frequency and time. Block type estimations are based on Least Square (LS), minimum mean-square error (MMSE) and modified MMSE. Comb estimations can be founded on LS estimator with 1D interpolation, the Maximum Likelihood estimator (ML) and the parametric channel modeling-based (PCMB) estimator. The estimators are usually calculated once per block and are used until the next pilot symbol arrives in the block-type based channels (Yushi Shen and Ed Martinez, 2006). Decision feedback is proposed to improve the performance; estimators in the block are modified utilizing the decision feedback equalizer at each subcarrier. Channel conditions are estimated by the receiver using the pilots. With a model for random time and frequency selective wireless channels pulse shaping OFDM systems should be developed (Wien, im Marz, 2004). Using a simple point wise multiplication of the transmit symbols by complex valued channel coefficients, plus the addition of Gaussian noise, input-output relation of the resulting system can be approximated. Each data symbol merely experiences flat fading, apart from the noise. The receiver will request a retransmission in case there are errors in the packet content. Non-Patent document proposes a method of reducing unnecessary retransmission. Making use of encoders per antenna in each sub stream ARQ processes are performed. It is possible to eliminate the constraint of sharing single ARQ processing by multiple transmitting antennas with the help of multiple encoders per antenna, and the transmitter decides whether to retransmit the sub streams in error when multiple acknowledgement signaling are sent back to the transmitter. Significantly the throughput is increased and also the retransmission of the error-free sub streams are avoided (Fresh Patents, 2009).Hence from the above context it can be stated that two major hurdles i.e. designing of channel tracking and design of an estimator are dealt. The one-dimensional (1D) channel estimations will trade-off between complexity and accuracy. The two basic 1D channel estimations (the block type and the comb type) are inserted in the direction of frequency and time. The block-type and the comb-type are based on the methods/techniques of their own (for ex LS for block and ML for comb type respectively).Calculation and modification of the estimators are done in the block type. By applying various formulas and by avoiding the retransmission of the signal the throughput is increased.

Estimated plan is prepared for the research before conducting it as it sets the task in proper way. This plan is designed in such a way that the research is completed within the given interval of time. Gant chart helps the researcher while doing the research and always indicates the works to be done. This plan provides a clear idea to the researcher about the project and also indicates the steps that are to be taken by the person. The time duration for this plan is 2 months that is from October 23rd to December 10th 2009. In order to complete the project by on time, I prepared a Gantt chart by including 7 tasks. The first task that is considered in this research is preparing the project proposal. After preparing the project proposal, the data related to the research was gathered from various resources. This process will be done from 24th October to 5th November. After collecting the information, the data gathered was critically analyzed for 1 week during the month of November. The fourth task includes the evaluation part which was done for 1 week by taking the ideas of various people by the end of November. With the help of evaluation, final report will prepare by analyzing the whole project. The final task that is considered in this plan is reviewing the dissertation report and submitting to the tutor before the final submission date so that the document can be modified based on the feedback from tutor.

Bibliography:

  • Ahmad R. S. Bahai, Burton R. Saltzberg and Mustafa Ergen (2004) Multi-carrier digital communications: theory and applications of OFDM Information technology-transmission, processing and storage, Springer Publishers, pp.411.
  • Fresh Patents (2009) "Wireless Communication Method, Wireless transmitter and Wireless Receiver", [Internet] available at URL: , [accessed on 11th October 2009].
  • Ramjee Prasad (2004) OFDM for wireless communication systems, Artech House Publishers, pp.272.
  • Wien, im Marz (2004) "Wireless OFDM Systems: Channel Prediction and System Capacity", [Internet] available at URL: < http://publik.tuwien.ac.at/files/pub-et_8085.pdf>, [accessed on 13th October 2009].
  • Yo-Sung Ho and Hyoung Joong Kim (2005), Advances in multimedia information processing, Springer Science and Business Publishers, pp.1087.
  • Yushi Shen and Ed Martinez (2006) "Channel Estimation in OFDM Systems" [Internet] available at URL: < http://www.freescale.com/files/dsp/doc/app_note/AN3059.pdf>, [accessed on 16th October 2009]
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