Suvra Sekhar Das

Power allocation in OFDM based NOMA systems: A DC programming approach

In this work, we have considered the downlink of an Orthogonal Frequency Division Multiplexing based Non Orthogonal Multiple Access system where transmission to multiple number of users is performed on the same sub-band (time-frequency resource unit) using Superposition Coding (SC) technique. At the receiver side, the SC coded symbols are recovered with Successive Interference Cancellation (SIC). Assuming that complete channel state information is present at the base station, we propose (1) co-channel user set selection, (2) power distribution among the multiplexed users on each sub-band, and (3) power allocation across the sub-bands to maximize the weighted sum rate of the system. Since the problem is a non-convex combinatorial optimization problem, two step heuristic solution is employed. In the first step, for each of the sub-bands, a greedy user selection and iterative sub-optimal power allocation algorithm based on Difference of Convex (DC) programming is presented. In the second step, exploiting the DC structure of the modified problem, power allocation across sub-band is carried out through the same iterative power allocation algorithm. Simulation results are provided to assess and compare the performance of the proposed algorithms.

Performance Analysis of OFDM Systems with Adaptive Sub Carrier Bandwidth

Orthogonal frequency division multiplexing based wireless systems are spectrally efficient, but they are vulnerable to inter-carrier interference (ICI). In a wide area scenario, users will experience varying signal strength due to different individual path loss and also varying amount of Doppler spread because of their independent velocities. Therefore, the ICI among users will vary over a wide range. It is proposed in this work to use dynamically adaptive sub carrier bandwidth (ASB) along with adaptive bit loading to mitigate ICI in such conditions, which will keep receivers simple while maintaining maximum throughput in each situation. Results show that ASB can provide higher throughput than its fixed sub carrier bandwidth counterpart when both may use adaptive bit loading per sub carrier.

Area Spectral Efficiency of Co-Channel Deployed OFDMA Femtocell Networks

Deployment of femtocells is expected to increase the capacity of cellular networks several folds. However, since femtocells operate within the available spectrum of the macro-cellular network, the arising uncontrolled interference may limit the achievable gain. For the successful operation of femtocells, the area spectral efficiency (ASE) performance needs to be studied for useful insights. In this paper, we derive the ASE for orthogonal-frequency-division-multiple-access-based cochannel-deployed macrocell-femtocell networks considering uniformly distributed femtocell locations. The ASE is obtained from the area averaged signal to interference plus noise ratio (SINR) distributions of macrocell and femtocell networks. Since strength of the co-channel interference received by a user terminal is related to load condition in neighboring cells, the ASE is derived considering activity due to fractional load of interfering cells. We then investigate the impact of femtocell density, transmit power, and load of femtocells as well as macrocell transmit power and load on the ASE performance. We also look at the optimal femtocell radio parameters: transmit power and load that maximizes the ASE while satisfying the QoS constraints of macrocell and femtocell users at different network conditions. From the analysis, it is seen that in co-channel deployment mode, if femtocell radio parameters are suitably controlled according to varying network condition, the ASE gain can be increased several folds without affecting macrocell network performance. It is also shown that, by centrally coordinating the spectrum reuse and allocated power at each macro/femto tier, the overall performance can be improved.

Optimum pre-DFT combining with cyclic delay diversity for OFDM based WLAN systems

Cyclic delay diversity is applied in an OFDM receiver to increase the frequency-selectivity of the channel seen at the receiver for flat (or less frequency-selective) channels. Diversity combining is performed prior to the discrete Fourier transform (DFT) operation. A new method has been studied to optimize the pre-DFT diversity combining by selecting cyclic shifts and weight factors based on known channel state information. The optimum signal-to-noise ratio for maximum cyclic delay and optimum weight factors for multiple antenna receiver diversity is analyzed. Performance results in terms of bit error probability are presented. Finally, it is found that the proposed scheme drastically reduces computational complexity compared to traditional receiver diversity schemes.

Impact of Nonlinear Power Amplifier on Link Adaptation Algorithm of OFDM Systems

The impact of non linear distortion due to high power amplifier (HPA) on the performance of link adaptation (LA) - orthogonal frequency division multiplexing (OFDM) based wireless system is analyzed. The performance of both forward error control coding (FEC) encoded and uncoded system is evaluated. LA maximizes the throughput while maintaining a required block error rate (BLER). It is found that when OFDM signal, which has high PAPR, suffers non linear distortion due to non ideal HPA, the LA fails to meet the target BLER. Detailed analysis of the distortion and effects on LA are presented in this work.

Influence of PAPR on Link Adaptation Algorithms in OFDM Systems

The impact of high power amplifier (HPA) on the performance of orthogonal frequency division multiplexing (OFDM) based wireless systems using link adaptation (LA) is analyzed in this work. LA maximizes throughput while maintaining a target bit error rate (BER) at the receiver. The non linear behaviour of the HPA introduces distortion in the OFDM signal which has high peak to average power ratio (PAPR). It is found in this work that this causes LA schemes to fail to meet the target BER. It is shown in this work that to make the LA system satisfy the BER constraint, the signal to noise ratio thresholds for changing the adaptive modulation needs to be updated.

Distance Dependent Call Blocking Probability, and Area Erlang Efficiency of Cellular Networks

In this paper the capacity and coverage of a cellular network is analyzed in the perspective of Grade of Service (GoS) constraint. The cellular networks considered, unlike GSM systems, allocate variable radio resource sizes to users even for constant bit-rate (CBR) traffic. The Grade of Service is the call blocking probability. It is shown that the blocking probability (Pb) varies with user distance from the Base Station (BS). It is also shown that the effective cell-radius varies for different services (bit-rates) given a certain GoS. Furthermore the variation of useful area of a cell along with offered load is shown for different services. If maximizing energy efficiency is one of the considerations in deploying a cellular network, then it is shown that the optimum transmit power for a cell layout is different when GoS is used as the criterion as compared to minimum throughput or minimum Signal to Interference plus Noise Ratio (SINR) at cell edge.

Dynamically Adaptive Bandwidth for Sub Carriers in OFDM Based Wireless Systems

In this article the performance of orthogonal frequency division multiplexing (OFDM) systems with dynamically adaptable sub carrier bandwidth to mitigate inter carrier interference (ICI) is compared against standard OFDM systems with fixed sub carrier bandwidth. In current OFDM system, the sub carrier bandwidth or sub carrier spacing is a fixed design parameter. This is not efficient since the optimum sub carrier bandwidth depends on ICI, which is a function of several parameters such as residual frequency offset, Doppler frequency spread, received signal strength and carrier frequency. Existing algorithms to combat ICI are very complex and sometimes reduce bandwidth efficiency. As an alternative approach, it is worth verifying the benefits of using adaptive bandwidth for sub carrier (ABS). A method for implementing the above scheme is given. Results show that this type of adaptive OFDM scheme has the potential to provide a significant improvement in performance over OFDM systems which use fixed sub carrier bandwidth.

Multi rate orthogonal frequency division multiplexing

A novel multi rate orthogonal frequency division multiplexing (OFDM) system is proposed in this paper. It is expected to provide a flexible physical layer support to next generation wireless networks. It is mainly suited to inter carrier interference limited situations, when a large number of sub carriers are used within the available bandwidth constraint. The proposal is to alter the otherwise uniform performance of the sub carriers in an OFDM system by using different bandwidths for different subcarriers. This is advantageous in the situation where different users and data channels in a wireless network will have different data rates and quality of service requirements. Different sub carriers with different performance figures can be mapped to the data channels with matching requirements. This scheme can enhance the conventional OFDM systems to become flexible. It is shown that, using different sub carrier bandwidths can improve the throughput of some sub carriers at the cost of the others. Results show that overall system performance also improves with the proposed scheme even without any optimal mapping of data stream to the sub carriers for slightly higher frequency offsets.

Variable guard interval orthogonal frequency division multiplexing in presence of carrier frequency offset

The goal of this paper is to highlight the benefits of using variable guard interval (VGI) in orthogonal frequency division multiplexing (OFDM) based wireless local area network (WLAN) systems. An algorithm is derived and its performance merit for implementing VGI in the presence of carrier frequency offset is provided. OFDM system needs to maintain orthogonality among sub carriers, which may be lost due insufficient guard interval and carrier frequency offset (due to local oscillator mismatch and Doppler effects). Static guard interval is usually kept large enough to tolerate worst case channel delay spread conditions, which in turn causes loss in efficiency of the system. The use of variable guard interval is expected to improve the system throughput. Since insufficient guard interval and carrier offset together cause orthogonality loss among sub carriers, the dynamic selection of guard interval in the presence of carrier frequency offset is considered in this work. Results are presented to analyze achievable gains. It has been shown that significant improvement in throughput is achievable by this scheme