Application of Error Correction Codes for Enhancing Data Integrity in Power Line Channel

Abstract

The use of existing power lines for home/industry/substation automation has drawn the attention of many researchers in the recent years because this infrastructure is easily available everywhere. However, the Power Line channel has been primarily designed for power transfer at low frequencies. Hence, the propagation characteristics of this channel are not well suited to support high speed data transmission and ensuring reliable high speed and error free data transmission on this channel is a very challenging task. Many researchers have been attracted to this challenging field in recent years and a variety of techniques from the domain of Digital Signal Processing and Communication Engineering have been applied to solve some of the challenges posed by this application. The three critical channel parameters namely noise, impedance and attenuation are found to be highly unpredictable and variable with time, frequency and location. Further, the regulatory standards designed to prevent spurious radiation restrict the carrier power that can be used for digital modulation. In this work, we have concentrated on the use of Medium Voltage (MV) Power Line (< 30 kV) for narrow band applications. After a study of relevant literature and an understanding of mathematical models used to describe the variation of channel parameters, a suitable model has been simulated in MATLAB® platform. Simulation results presented by the channel model have been obtained for different channel conditions such as line length, noise variations and variations in transfer function (attenuation) of the channel and for different data size. The frequency band employed in narrowband power line communication is restricted to a value less than 500 kHz. An effort is made in the thesis to devise a powerful error correcting code which can eliminate the errors caused by channel impairments. Power line channel is modeled using multipath model. As noise experienced on power line channels is a mixture of Gaussian and Impulsive varieties, it is modeled by using the Middleton Class-A pdf. Taking into account the channel behavior; two channel coding strategies were deployed. In the first approach a four state Turbo code was combined with a 32-carrier OFDM modulator and the performance of this combination was studied under various channel conditions. In the second approach, a Bose-Choudhari-Hocquenghem (BCH)code was concatenated with the Alamouti Space-Time Block code and the performance of the channel was similarly evaluated. To realize the Turbo coded OFDM scheme, a four state Turbo code using Recursive Systematic Convolutional (RSC) encoder/decoder pair was designed. The output of the encoder was modulated by 32 sub carrier OFDM (designed using IFFT and FFT). The efficacy of this arrangement in ensuring data integrity over the MV power line channel was tested. To realize the second approach, a BCH code with parameters was designed and encoding/decoding processes were implemented. BCH code in concatenation with Alamouti 2x1 space time code (with PSK modulation) was tested. A partial hardware implementation was realized by employing a Digital Signal Processor TMS 320C6713 for encoding/decoding and MATLAB® for simulating the power line channel. Data input present in text form encoded and decoded after transmission through the channel. This process allows the visualization of the power of error correction algorithms. Performance evaluation of the two proposed schemes for channel code and modulation design namely Turbo coded OFDM and BCH coded space time code were carried out. The performance criteria for the evaluation include the bit error rate (BER) at a specific signal to noise ratio (SNR). The reflections at branching points (load locations) vary the attenuation profile of the link. As a result, the effect of different parameters on the channel attenuation was observed based on the number of loads and length of the link. A BER analysis was performed to compare the performance of the channel under impulsive noise conditions under three impulsive scenarios. The first scenario was specified as . The second scenario was specified as and the the third scenario was specified as . A comparison of the relative performance of uncoded and coded schemes reveals the following: Scheme 1 achieves BER of 10-5 at SNR=55 dB for (case 1 impulse noise), with channel attenuation varying between 10 dB to 50 dB. Scheme 2 achieves BER of 10-5 at SNR=50dB for (case 1 impulse noise), with channel attenuation on two paths varying between 18 dB and 6 dB.Both schemes have achieved a BER of 10-5 at SNR=66 dB for case 2 impulse noise), with 10dB to 50 dB channel attenuation for scheme 1 and with 16 dB and 34 dB channel attenuation on two paths for scheme 2. Following remark can be made with reference to the discussion on results: Both schemes 1 and 2 have given equivalent performance under similar channel conditions (attenuation and noise), when the error correcting capacity of channel code used in scheme 2 is . After a thorough study and implementation of both approaches, it was observed that both schemes exhibit equivalent performance under similar channel conditions (attenuation and noise levels). With enhanced error correction capacity with t=11, a BCH coded space time code will require lesser SNR to give the same performance as OFDM under similar channel conditions.