Latif Ullah Khan

Improved Physical Layer Implementation of VANETs

Vehicular Ad-hoc Networks (VANETs) are comprised of wireless mobile nodes characterized by a randomly changing topology, high mobility, availability of geographic position, and fewer power constraints. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for the physical layer of VANET because of the inherent characteristics of the spectral efficiency and robustness to channel impairments. The susceptibility of OFDM to Inter-Carrier Interference (ICI) is a challenging issue. The high mobility of nodes in VANET causes higher Doppler shifts, which results in ICI in the OFDM system. In this paper, a frequency domain comb- type channel estimation was used to cancel out ICI. The channel frequency response at the pilot tones was estimated using a Least Square (LS) estimator. An efficient interpolation technique is required to estimate the channel at the data tones with low interpolation error. This paper proposes a robust interpolation technique to estimate the channel frequency response at the data subcarriers. The channel induced noise tended to degrade the Bit Error Rate (BER) performance of the system. Parallel concatenated Convolutional codes were used for error correction. At the decoding end, different decoding algorithms were considered for the component decoders of the iterative Turbo decoder. A performance and complexity comparison among the various decoding algorithms was also carried out.

Effect of network density on the performance of MANET routing protocols

A mobile Ad-hoc network consists of nodes communicating with each other without any centralized infrastructure. Mobility and concentration of nodes varies from one location to another. Mobility of nodes in populated urban areas is limited and their concentration is high as compared to rural areas where the concentration of nodes is generally low. Therefore it is necessary to study the effect of network density on the performance of MANET routing protocols. In this paper, the performance of MANET routing protocols is analyzed for scenarios of different network densities. Our analysis shows that network density significantly affect the performance of the MANET routing protocols.

LS estimator: Performance analysis for block-type and comb-type channel estimation in OFDM system

Channel Estimation in Orthogonal Frequency Division Multiplexing (OFDM) is imperative for cancellation of the impairments introduced by the fading channel. In this paper, the performance of Least Square (LS) estimator is studied for comb-type and block-type pilot insertion schemes over fast and slow fading Rayleigh channels respectively. The channel order increases proportionally with the distortions introduced from the environment. The effect of increase in channel order on the Bit Error Rate (BER) performance of LS estimator for pilot assisted channel estimation techniques is evaluated. Stability of LS estimator in terms of BER performance for block-type pilot arrangement is revealed by the Simulation results. However, increasing the channel distortion by increasing the channel order causes performance degradation of LS estimator for comb-type pilot insertion scheme.

Modified MMSE Estimator based on Non-Linearly Spaced Pilots for OFDM Systems

This paper proposes a Modified Minimum Mean Square Error (M-MMSE) estimator for an Orthogonal Frequency Division Multiplexing (OFDM) System over fast fading Rayleigh channel. The proposed M-MMSE estimator considered the effects of the efficient placement of pilots based on the channel energy distribution. The pilot symbols were placed in a non-linear manner according to the density of the channel energy. Comparative analysis of the MMSE estimator for a comb-type pilot arrangement and M-MMSE estimator for the proposed pilot insertion scheme revealed significant performance improvement of the M-MMSE estimator over the MMSE estimator.

M-AODV: Modified Ad Hoc On-demand distance vector routing scheme

The Ad Hoc On-demand Distance Vector (AODV) routing scheme is a widely used routing technique in ad hoc networks due to its low routing traffic overhead. However, the performance of the minimum hop routing used by AODV degrades significantly when the underlying system has routes that have high throughput and hop count. In this paper, we propose modifications to the routing metric and the route discovery mechanism of the AODVv2 scheme. The modified scheme is termed Modified-AODV (M-AODV). Simulation results reveal that the performance of the proposed scheme significantly improves in terms of key performance metrics when compared with AODV.

A novel channel estimation error minimizing interpolation technique for OFDM systems

The use of the efficient interpolation technique with low estimation error is imperative for estimating the channel impulse response at data subcarriers for pilot assisted channel estimation techniques in the Orthogonal Frequency Division Multiplexing (OFDM) systems. In this paper, a novel interpolation technique based on one dimensional interpolation is presented. The proposed interpolation technique is based on minimizing the interpolation error and channel noise inherent in Least Square (LS) estimation. A comparative analysis of the proposed interpolation with conventional one dimensional interpolation techniques shows its performance improvement.

On the optimal frame size of linear prediction techniques

Loss of data in Communication Systems is a major problem. The main sources of data loss include congestion, sensor faults, mismatch of measuring devices and limited memory of the buffer registers. Three linear prediction (LP) techniques i.e. Normal Equation method, Levinson Durbin Algorithm (LDA) and Leroux Gueguen Algorithm (LGA) are used to predict the missing data. The objective of this paper is to analyze the three linear prediction techniques for both internal and external prediction and the effect of frame size on different parameters i.e. computational time, prediction gain and prediction error is also being analyzed. The LP algorithms are modified and a threshold limit for prediction error is set to keep the error bounded. A sub-optimal frame size is selected for all the three LP methods and in the end the three algorithms are compared for efficient performance.

A joint error correction and ICI cancellation algorithm for OFDM systems

Orthogonal Frequency Division Multiplexing (OFDM) is a spectrally efficient and robust digital communication technique adopted by the newly developed Wireless Communication standards. Frequency offset errors due to Doppler shifts disturbs the alignment of the orthogonal subcarriers and causes them to overlap with each other which results in Inter Carrier Interference (ICI). This paper proposes the use of channel estimation for ICI cancellation. The channel frequency response at pilot subcarriers is estimated by using Least Square (LS) estimator. Along with this the (23, 12, 7) Golay Code has been employed in order to further improve the Bit Error Rate (BER) performance of the system. Simulation of the proposed OFDM system is carried out for Stanford University Interim (SUI) Channel model 6 in the existence of Additive White Gaussian Noise (AWGN).

Performance evaluation of turbo coded OFDM with channel estimation over SUI channel models

In this paper, the performance of the Least Squares (LS) estimator along with turbo codes in OFDM is evaluated for the SUI channel models in the presence of AWGN. Turbo codes are used in this paper because of its BER performance close to Shannon limit. The iterative turbo decoder exchanges soft information between the component decoders. Maximum A Posteriori (MAP) and its simplified version Max-Log-MAP algorithms are used for the component decoders. The performance and complexity comparison is also made between the decoding algorithms. Moreover the effect of memory order on the performance of the decoding algorithms is also studied.

OFDM transmission over a noisy fading channel for assessing a complete transmission acknowledgement scheme for VANETs

Various computer simulations are used to illustrate the performance of a particular protocol. However, the number of parameters and logical layers considered is the key to getting accurate results. We extend our previous work of Complete Transmission Acknowledgement Scheme (CTAS) by running the proposed model on top of the physical layer. Effect of noise and the standard Orthogonal Frequency Division Multiplexing (OFDM) scheme is implemented in the physical layer and analysed. The physical layer performance evaluation parameters such as Bit Error Rate (BER) and Signal to Noise Ratio (SNR) are selected and the results show that the proposed protocol outperforms other contemporary acknowledgement schemes.