Thokozani Shongwe

Selected subcarriers QPSK-OFDM transmission schemes to combat frequency disturbances

To combat the impairments caused by frequency disturbances in the power line communications (PLC), a modified conventional QPSK-OFDM transmission scheme is presented. The idea of this scheme is to first group the N OFDM subcarriers into groups of M and then transmit data by selecting a subset of the subcarriers in the group. Real and imaginary parts of QPSK symbols are independently assigned to the selected subcarriers in a group, such that the minimum squared Euclidean distance is maximised. With this kind of symbol assignment to subcarriers our scheme has no net loss in terms of SNR requirements, in AWGN, in comparison to the conventional QPSK-OFDM, even though it has half the data rate of the conventional QPSK-OFDM. We refer to the conventional QPSK-OFDM as Scheme A. Our scheme displays a superior performance over Scheme A and another scheme (Scheme B), in the presence of frequency disturbances and also frequency selective fading noise. We further modify Scheme B and come up with additional two new QPSK-OFDM schemes that have better performance than Scheme B in AWGN and impulse noise. To encode, we apply a (n, k) RS code and a simple permutation code on the conventional QPSK-OFDM scheme, which significantly improves the decoders performance in the presence of frequency disturbances. A simple narrow band noise model is developed and presented.

Interleaving and nulling to combat narrow-band interference in PLC standard technologies PLC G3 and PRIME

Three functional blocks of the PRIME and PLC G3 technologies (encoder/decoder, interleaver, and modulator) are studied in detail, for a PLC channel with narrow-band interference (NBI). The study reveals that these three blocks can be used together effectively so as to improve the performance of the overall system in the presence of NBI. We therefore present effective methods for combating NBI in PRIME and PLC G3, based on these three functional blocks.

Cascaded PLC-VLC Channel Using OFDM and CSK Techniques

This paper puts in Cascade the power line communications (PLC) channel and the visible light communications (VLC) channel, in order to use the PLC channel as backbone for the VLC channel. This combination is suitable for applications in which hybrid PLC-VLC systems are needed. We investigate the behavior of the cascaded channels for a full link transmission. Quadrature phase shift keying combined with orthogonal frequency division multiplexing (QPSK-OFDM) is used over the PLC channel and color shift keying (CSK) is deployed over the VLC channel to convey the information. Cascaded channel variances are analyzed. Complete simulated bit error rate (BER) is analyzed and presented for multiple scenarii that could occur in the two channels.

The effects of periodic impulsive noise on OFDM

The effect of periodic impulsive (short duration) noise on OFDM is investigated. We present results on the nature of periodic impulsive noise, showing that the PDF of periodic impulsive noise is not exactly Gaussian. We also present results showing that periodic impulsive noise can be more devastating to OFDM compared to random impulsive noise. This is because periodic impulsive noise energy is not spread by the FFT on the receiver side of the OFDM, instead it appears periodic in the frequency domain. Results showing the effect of nulling to mitigate periodic impulsive noise are presented. We suggest a simple short block code (as opposed to long block codes) that can effectively combat the effects of periodic impulsive noise.

Narrow-band interference model for OFDM systems for powerline communications

A narrow-band interference (NBI) model for the powerline communications channel is presented. We give frequency domain details and analysis of the NBI model specifically for OFDM systems; it can easily be adapted to model NBI for other communications systems. We also show that by making the same assumptions as in the Middleton class A model, our NBI model becomes the Middleton Class A noise model.

PSK to CSK mapping for hybrid systems involving the radio frequency and the visible spectrum

This paper presents an efficient technique to map phase shift keying (PSK) signalling to colour shift keying (CSK) constellation, to establish a full link in hybrid systems involving the radio frequency (RF) and the visible spectrum. It fits in systems combining (first link) wireless communication technologies such as the wireless fidelity (WiFi) or wired communication technologies such as power line communications (PLC) to visible light communications (VLC) technology (second link). On the first link, PSK technique is used to convey the information, while, on the second link, a technique based on colour variation is deployed. WiFi standards targeted are those that employ PSK as sub-carrier modulation techniques (IEEE 802.11a/11g/11n). The PSK complex constellation observed at the output of the first link is converted into colours using the hue-saturation-value/intensity (HSV/I) colour models. The constant lighting required in VLC corresponds with the coordinate I of the HSI and the colour constraint is met by assigning adequate current intensities to the red-green-blue LEDs (RGB-LEDs) used. The design meets the requirements of CSK constellation design outlined in IEEE 802.15.7. The performance of the system is analysed through bit error rate curves obtained by simulations, for binary PSK (BPSK) and quadrature PSK (QPSK), 8PSK and 16PSK constellations. The results show that as the constellation size increases, the performance of the system decreases.

Good Synchronization Sequences for Permutation Codes

For communication schemes employing Frequency Hopping/Multiple Frequency Shift Keying modulation, we present an algorithm for finding good non-binary synchronization sequences, which are permutations, to be used with permutation codes to synchronize/resynchronize data in channels with background noise and interference(frequency jamming/fading). For the synchronization sequences, new analytical expressions for the probability of false acquisition are also given. Using simulation results, we show that our synchronization sequences perform better than some conventional non-binary synchronization sequences, in the presence of background noise and interference.

Inter-building PLC-VLC integration based on PSK and CSK techniques

This paper presents the implementation of an integration technique used to combine power line communications (PLC) and visible light communications (VLC) channels in inter-building scenarios. It considers the narrow band PLC (NBPLC). Phase-shift keying (PSK) is used to convey the information over the PLC link and colour shift keying (CSK) technique is used on the VLC link. The colour wheel method is exploited to map PSK symbols to colours. We define a parameter x, which represents different magnitudes of the PSK complex symbol observed over the PLC channel. x is adjusted to an optimal value to meet the requirement of the CSK design. The bit error rate (BER) is analysed and the performance of the system is presented for multiple values of the modulus of the detected PSK symbol. A practical implementation is performed to verify the PSK-CSK mapping method. The impact of the suns rays on the red-green-blue (RGB) symbols is analysed and the interference source is highlighted.

Reducing the probability of sync-word false acquisition with Reed-Solomon codes

In our previous work we introduced a method for avoiding/excluding some symbols in Reed-Solomon (RS) codes, called symbol avoidance. In this paper, we apply the symbol avoidance method in sync-word based synchronization of RS encoded data. With the symbol avoidance method we reduce the probability of the RS encoded data being mistaken for the sync-word used to delimit the start/end of the data. The symbols in the RS code are avoided according to the sync-word used, such that the sync-word has very low probability of being found in the RS codewords, where it was not inserted. Therefore, for different sync-words, different symbols need to be avoided in the RS code. The goal here is to reduce the probability of false acquisition of the sync-word in the RS encoded framed data.

The effect of hamming distances on permutation codes for multiuser communication in the power line communications channel

We partition permutation sequences into groups to form permutation codes for multiuser communication. Each group becomes a codebook for each user in a multiuser communication system. We present simulation results for the performance of different partitions of permutation codes for multiuser communication, where the codes are to be used in channels with background noise and jamming, such as the Power Line Communications (PLC) channel. With the help of the simulation results, we show that by partitioning codebooks according to distance properties we can affect the performance of the codes. The permutation codes have codewords of lengthM with symbols taken from an alphabet whose cardinality is M, where M is any integer. Each symbol may be seen as representing one out of the M frequencies in an M-ary Frequency Shift keying modulation scheme, for example. Each user has a codebook of cardinality greater or equal to M and there can be a maximum of M − 1 users communicating at the same time through a multiple access OR channel.