

Articles
by
V.C. Ravichandran 
Total Records (
5 ) for
V.C. Ravichandran 





K. Chitra
and
V.C. Ravichandran


The effects of turbocoding on the coherence multiplexed optical PPM CDMA systems
are The effects of turbocoding on the coherence multiplexed optical PPM CDMA
systems are shown by evaluating bit error rate (BER) with respect to system parameters such as interleaver length(K), number of simultaneous users (N), code length (f), initial pulsewidth (t0) and normalized threshold(θ) by including the effects of thermal noise and shot noise. Turbocodes increase the number ofsimultaneous users besides reducing the BER for a decreasing normalized threshold. As the initialpulsewidth reduces from 7ps to 5ps the BER reduces from 1.6392x10^{17} to 7.0167x10^{18}. For a fixednumber of simultaneous users (N=20), an initial pulsewidth, (t0 = 5ps) and a normalized threshold (θ =0.84), it is seen that the turbocodes of interleaver length 10 reduces BER in the order 10^{10}, 10^{18} and10^{32} for the code length varying as 63, 127 and 255 respectively. On comparison with the directdetection it is found that the balanced detection could accommodate an additional number of 8 users ata reduced BER of 10^{16}. 





G.K.D. Prasanna Venkatesan
and
V.C. Ravichandran


Fourth Generation wireless communication demands a better multiple access technique for reducing the Multiple Access Interference (MAI) and Inter Symbol Interference (ISI) and to improve the bit error rate performance. MCCDMA is the best candidate that would satisfy the demands of 4G wireless systems. MCCDMA does not require more rake figers and it shows better performance than OFDM at low signal power in Ultra Wide Band (UWB) channel. UWB uses bandwidth above 20% of center frequency which severely suffers from frequency selective fading. This study analyses the performance of Orthogonal Frequency Division Multiplexing (OFDM) and Direct Sequence (DS) CDMA systems. The simulation results shows that the MCCDMA performs well interms of BER comparatively with other techniques in wide band channel. UWB SalehValenzulea channel model proposed by intel is used in the simulation for analyzing the performance of MCCDMA systems. 




B. Jothi Mohan
and
V.C. Ravichandran


In this study resource allocation in ad hoc network is formulated, as a utility maximization problem the constraint considered is rate constraint. Rate constraint is defined such that the traffic flow from source node to the destination should be less than the net channel capacity between them but at the optimum value. In cross layer design optimization of rate constraint can be done using three layer constraints namely congestion constraint of transport layer, routing constraint of network layer, schedulable constraint of link layer. These three components form the core components of rate optimization. We formulate a congestion price to interact with congestion constraint, routing constraint and scheduling constraint in a fixed wireless channels. This principle can be extended to time varying channel and multirate devices. Here in this study we establish a relationship between congestion window and local congestion price. Resource allocation namely wireless links of fixed bandwidth is done to optimize these constraints. We are considering this problem as utility maximization with the constraints of rate, routing and scheduling. The resources considered are inter nodal paths of fixed channel bandwidths, for optimal utilization. Let us look at the resource allocation problem using congestion price as the flow variable. We prove the convergence of the algorithm. In this analysis we have considered time invariable and fixed rate devices. The stability of the system is established. Its performance is characterized with respect to an ideal reference system which has the optimum rate region at link layer. The congestion window is adjusted by values of local source rate constraint. 





A. Sivasubramanian
and
V.C. Ravichandran


In long haul networks, the random birefringence induced in the optical fiber leads to a considerable Polarization Mode Dispersion (PMD). Polarization Dependent Loss (PDL) mainly occurs in optical components and depends on the state of polarization of optical signals. The presence of PMD and PDL causes pulsewidth narrowing and the pulsewidth reduction depends on states of polarization at which the input light launched and also the input pulsewidth. A system comprising of a PDL element sandwiched between two PMD elements was considered. This system was characterized using neural network approach. Back propagation algorithm was applied to train the network with four input vectors namely PMD, PDL, input pulsewidth and the angle describing the input states of polarization and one output vector indicating effective squared pulsewidth difference. On analysis, it was found that the pulsewidth reduction was higher for a PMD of 30ps, a PDL of 3.5 and input pulsewidth of 100ps at various (Linear and Circular) input states of polarization with the angle describing the input state of polarization to be π/4. Similarly, for a given value of PMD, PDL, input pulsewidth and a specific pulsewidth reduction, the input state of polarization at which the light was to be launched can also be determined using neural network approach. 




S. Malarkkan
and
V.C. Ravichandran


This study presents performance of call admission control and resource reservation schemes based on the mobility of the users in WCDMA cellular systems. In order to guarantee the handoff dropping probability, the mobility of the user is predicted based on a realistic mobility model. The mobility prediction scheme used in this study is used to estimate the set of candidate cells into which the mobile may move in the near future and calculates the likeliness value for each candidate cell. It also estimates the time slot at which the mobile may enter into the candidate cells based on the distance between the current location of the mobile and the candidate cell center distance. Based on the mobility prediction, resource is reserved in terms of Interference Guard margin (IGM) to guarantee some target handoff dropping probability. The admission threshold is adaptively controlled to achieve a better balance between guaranteeing handoffdropping probability and maximizing resource utilization. Simulation results show that the call admission control scheme with mobility based resource reservation scheme outperforms well when compared with the fixed reservation scheme. 





