In this paper, we develop a multiple access (MA) mechanism where users transmit upsampled and circularly shifted orthogonal frequency division multiplexing (OFDM) signals in the uplink. These signals pass through MA doubly-dispersive channel and get combined at the base station (BS). We show that the composite signal received at the BS forms an equivalent single-user orthogonal time frequency space (OTFS) system. We show that such low-complexity transmission of OFDM symbols in the uplink from low-capability devices yields diversity gain as the OTFS. We extend this MA scheme for orthogonal time sequency multiplexing (OTSM) and block based single carrier (block SC) transmissions, where the composite received signal at the BS forms an equivalent single-user OTSM or block SC received signal depending on the transmission. We also develop successive interference cancellation (SIC) and turbo decoding principles-based receivers for multi-user OTFS, OTSM, and block SC transmissions, and a frequency domain SIC receiver for OFDM. We analyze their complexity with both analytical and simulation methods. Further, we compare the multi-user uncoded and coded performance of OTFS, OTSM, and block SC with multi-user OFDM. We also evaluate their performance under the effects of nonlinear high-power amplifiers in multi-user scenarios.

Orthogonal time frequency space (OTFS) modulation has established itself as a dependable protocol for high-speed vehicular communication. This pioneering technique operates within a novel 2-D delay-Doppler domain waveform. When compared with conventional modulation methods like orthogonal frequency-division multiplexing (OFDM), OTFS demonstrates superior performance enhancements in scenarios involving rapidly moving wireless channels. This article begins by initially unveiling the input–output association of the OTFS signal within the delay-time domain. A comprehensive comparison with the established OFDM waveform highlights the potential of OTFS for achieving a notably lower bit error rate (BER) under various conditions, which has been obtained by using the minimum mean square equalizer (MMSE) equalization technique. Finally, we have proposed a novel and low-complexity VLSI architecture for the OTFS transmitter and the receiver by using the lower–upper (LU) decomposition technique for the first time in the literature. We have compared the performance metrics of our proposed transmitter architecture with the existing work, where our design works 7.394% faster than others, utilizing 89.354% less in the number of lookup tables (LUTs) and 79.984% less in the number of flip-flops (FFs), which shows that our design is more optimized in latency and resource utilization. There is no architecture design of the OTFS receiver part in the existing literature to compare; we have shown the resource utilization of our proposed receiver architecture for the first time in the literature, followed by timing analysis and functionality testing of the proposed architecture.

The evolution of fifth-generation (5G) networks needs to support the latest use cases, which demand robust network connectivity for the collaborative performance of the network agents, such as multirobot systems and vehicle-to-anything (V2X) communication. Unfortunately, the user device’s limited communication range and battery constraint confirm the unfitness of known robustness metrics suggested for fixed networks, when applied to time-switching communication graphs. Furthermore, the calculation of most of the existing robustness metrics involves nondeterministic polynomial (NP)-time complexity, and hence are best fitted only for small networks. Despite a large volume of works, the complete analysis of a low-complexity temporal robustness metric for a communication network is absent in the literature, and the present work aims to fill this gap. More in detail, our work provides a stochastic analysis of network robustness for a massive machine-type communication (mMTC) network. The numerical investigation corroborates the exactness of the proposed analytical framework for the temporal robustness metric. Along with studying the impact on network robustness of various system parameters, such as cluster head (CH) probability, power threshold value, network size, and node failure probability, we justify the observed trend of numerical results probabilistically.

Cellular network planning, which includes coverage planning and network dimensioning is essential for designing and deploying mobile networks. The Key Performance Indicators (KPI)s, such as average Cell Spectral Efficiency (CSE), user throughput, etc., are used to evaluate the design of cellular systems. These metrics are primarily dependent on area-averaged Signal-to-Interference-plus-Noise- Ratio (SINR) distribution of the users of the cell. In this work, we have developed a general semi-analytical framework for area-averaged SINR distribution and evaluated system level KPIs such as average CSE for Orthogonal Frequency Division Multiple Access (OFDMA)-Frequency Division Duplex (FDD) system based single frequency network. In our model, we consider International Telecommunication Union-Radiocommunication (ITU-R) specified channel path-loss models for Non-Line- of-Sight (NLoS) and Line-of-Sight (LoS) scenarios, directional antennas for base stations (BSs) with ITU-R specified antenna gains, shadowing correlation between desired and interfering signals, Nakagami-$\shapeparam$ fading channels to characterize NLoS and LoS conditions and, Bernoulli random variables to represent load conditions of interfering BSs, to model area- averaged SINR distribution. We have demonstrated that our model is in close agreement with Monte- Carlo simulations. Using our model, the average CSE is evaluated for different channel conditions and verified with simulations.

Link adaptation (LA) schemes maximize average spectral efficiency (ASE) using accurate channel state information (CSI) and knowledge of signal to noise ratio (SNR) distribution. In practical systems, partial CSI is available at the transmitter through feedback of modulation and coding scheme index, often known as channel quality indicator (CQI). However, parameters of SNR distribution (${\Theta }$) are not known or only assumed to be known. We propose two LA schemes, for execution at the receiver and transmitter, respectively, each without prior knowledge about${\Theta }$. In the receiver centric schemes, parametric estimation of SNR distribution is performed using unquantized SNR samples. In the transmitter centric scheme, we estimate${\Theta }$from CQI samples using an iterative joint quantization-estimation algorithm. These parametric estimates form long-term SNR distribution, which is used to compute SNR switching thresholds. Using these thresholds, rate and power adaptation decisions are made based on instantaneous SNR. We have derived the maximum-likelihood estimator and Cramer–Rao lower bound for the proposed estimator using CQI feedback for Nakagami-$m$fading channels and demonstrate that near optimal ASE can be achieved while using the proposed schemes as in an ideal scenario, where perfect knowledge about CSI and SNR distribution are availablea priori.

In this work we present a complex systems science analysis of the Self Organized Time Division Multiple Access (SOTDMA) algorithm. We translate the interaction among member nodes into a functional topology graph in order to measure the effect of each individual node’s adaptability on the global performance. The functional complexity metric corresponding to the functional topology is shown to have substantial correlation with important Key Performance Indicators (KPIs), namely probability of collision and probability of correct packet detection. We further use the functional complexity metric to analyze the trade-off between the two aforementioned KPIs in terms of system parameters. We finally show that the results obtained using this approach satisfy the predefined KPI constraints imposed on the algorithm and thus is successful in capturing the system behavior.

Generalized frequency division multiplexing (GFDM) is one of the new waveforms with several attractive features such as lower out of band radiation, higher spectral efficiency, and better immunity to carrier frequency offset errors as compared with orthogonal frequency division multiplexing. However, it requires receivers with high computational complexity. Among different linear receiver structures, the joint minimum mean square error (joint-MMSE) receiver for GFDM achieves the best bit-error-rate (BER) performance, but it is the most computationally complex linear receiver. In this paper, we present a novel low complexity joint-MMSE receiver. We derive the closed-form expression for the joint-MMSE receiver filter by harnessing circulant properties of the matrices involved in the demodulation process. Amidst different operations involved in the receiver processing, inversion of a positive definite circulant-plus-diagonal matrix is found to be the costliest. Two low-complexity algorithms for the inversion of a positive definite circulant-plus-diagonal matrix are developed to design the low complexity receiver. We also propose a low complexity bias correction to improve BER performance. Our proposed receiver achieves a log-linear order of complexity without any noticeable loss in BER performance.

Flexible frequency reuse schemes namely fractional frequency reuse (FFR) and soft frequency reuse (SFR) are suggested in the literature to improve the cell edge throughput over unity frequency reuse in orthogonal frequency division multiple access networks. In this study, the authors focus on real time (RT) traffic for which blocking probability and Erlang load are the important performance metrics. Both FFR and SFR have signal‐tointerference‐plus noise‐ratio (SINR) threshold and power ratio as important design parameters, whereas FFR has a bandwidth partitioning ratio as an additional parameter. They investigate the cell capacity and the cell edge performance in terms of Erlang load with blocking probability constraint brought about by the suitable choice of these parameters. They also analyze the fairness aspect which is a critical indicator of user satisfaction. They give a complete comparison between FFR and SFR for the cell edge as well as total cell for both RT and BE traffics. For RT traffic, with proper choice of the parameters, the cell edge and the total cell performance in FFR and SFR is improved by a notable gain over the reference scheme. For BE traffic, both schemes provide significant gains at cell edge over reuse one and reuse three schemes.

The expected operating scenarios of fifth‐generation (5G) pose a great challenge to orthogonal frequency division multiplexing which has poor out of band spectral properties, stringent synchronisation requirements and large symbol duration. Generalised frequency division multiplexing (GFDM) which is the focus of this work has been suggested in the literature as one of the possible solutions to meet 5G requirements. In this study, the analytical performance evaluation of minimum mean square error (MMSE) receiver for GFDM is presented. The authors also proposed precoding techniques to enhance the performance of GFDM. A simplified expression of signal‐to‐interference and noise ratio (SINR) for MMSE receiver of GFDM is derived using special properties related to the modulation matrix of GFDM, which are described in this study. This SINR is used to evaluate the bit error rate performance. Precoding schemes are proposed to reduce complexity of GFDM–MMSE receiver without compromising on the performance. Block inverse discrete Fourier transform (BIDFT) and discrete Fourier transform (DFT)‐based precoding schemes are found to outperform GFDM–MMSE receiver due to frequency diversity gain while having complexity similar to zero‐forcing receiver of GFDM. It is shown that both BIDFT‐ and DFT‐based precoding schemes reduce peak‐to‐average power ratio significantly. Computational complexities of different transmitters and receivers of precoded and uncoded GFDM are also presented.

In this work, we develop a multi-objective optimization framework for reducing network wide energy consumption through base station (BS) sleep mode (SLM) in an OFDMA based cellular systems. While putting BSs in SLM, we take into account the effects such as coverage and overlap. After putting BSs in SLM, in order to ensure sufficient coverage, overlap (for hand-overs), and to achieve maximum network throughput of given set of active BSs, we consider the following objectives: power consumption minimization, area spectral efficiency maximization, coverage maximization, and overlap minimization; to find an optimal set of active BSs that are needed to support the required traffic demand. The system model incorporates accurate modeling of inter-cell interference considering pathloss, fading effects (shadowing and small scale fading), and activity of neighbouring cells. We provide analytical models to estimate cell capacity, network coverage, and blocking probability etc. from the distribution of signal-to-interference-plus-noise-ratio. Since, finding the optimal set of active BSs for SLM operation is a challenging combinatorial optimization problem, we use Genetic Algorithm to obtain the solutions of considered multi-objective optimization problem with fast convergence rate. We also study the trade-offs between the objectives. The results show that using the proposed mechanism significant amount of energy saving can be achieved while maintaining trade-offs between the objectives.

In Orthogonal Frequency Division Multiple Access (OFDMA) networks there is variation of Grade of Service (GoS) experienced by the users within a cell. Hence a comprehensive performance analysis for such networks must include fairness of GoS considerations. Methods to enhance fairness of GoS include call admission control (CAC) schemes. A simple dynamic CAC scheme that responds to the bandwidth occupancy level is considered here. It leverages the fact that several regions within the cell offer a GoS that is much higher than acceptable. Users from such regions may be blocked even when sufficient resources are available, a technique which we refer to as forced blocking. Such forced blocking of GoS rich users, if performed at times of high bandwidth occupancy, may enable to improve the GoS of the remaining users. However, the rate of application of forced blocking to any class of users must be chosen such that the GoS of that class does not degrade beyond an acceptable limit. The performance of such an algorithm is studied in terms of parameters such as satisfied service area of the cell, min-max metric of fairness, variance of GoS and cell-average as well as cell-edge GoS. Both discrete event simulations (DES) and analytical models are employed in this performance study, for which First Come First Served (FCFS) performance serves as the reference.

In this paper, we derive the outage probability (OP) expressions for co-channel OFDMA macrocell- femtocell networks as a function of femtocell radio parameters and load conditions of neighboring macrocells. From the derived OPs, for a given macrocell network load, we analyze the minimum and maximum transmit power for femtocells that maintains OPs of both macrocell and femtocell users within the acceptable level. The results show that the OP performance in co-channel deployment mode is highly sensitive to macrocell load. It is seen that by dynamically adjusting transmit power of femtocells along with the activity status of suchannels according to varying network load condition, the co-channel interference can be effectively controlled while meeting QoS requirements of macrocell and femtocell users.

In this paper, we develop a cell load coupling model for load balancing and interference coordination in OFDMA based cellular networks. An inter-cell interference (ICI) model which captures the coupling between resource utilization (or cell load) of different cells and dynamic ICI in the presence of shadowing and small scale fading is presented. Further, a method to estimate the cell load from the distribution of Signal to Interference plus Noise Ratio is proposed. A user association policy which minimizes the network wide resource utilization is described considering both streaming and elastic traffic parameters. We also show the impact of arrival rate of streaming and elastic traffic on the system performance.

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.

To achieve high capacity in cellular networks, frequency reuse factor of unity is used. However, it suffers from heavy co-channel interference at cell edge regions. This leads to poor Signal to Interference plus Noise Ratio (SINR) and hence poor performance. Fractional Frequency Reuse (FFR) is one of the methods being considered to improve cell edge performance. In this work we present the impact of SINR threshold and bandwidth partitioning on the successful deployment of FFR scheme. We have considered both Real Time and Best Effort traffic.

Deployment of femtocells is expected to overcome indoor coverage problems and enhance the Area Spectral Efficiency (ASE) in Orthogonal Frequency Division Multiple Access (OFDMA) cellular systems such as Long Term Evolution and Worldwide Interoperability for Microwave Access. However, unplanned deployment of co-channel femtocells in existing macro-cell networks results in severe interference in the system which limits the possible gain achievable in ASE. In this paper, we derive the ASE of OFDMA based co-channel macrocell-femtocell networks as a function of femtocell radio parameters such as transmit power, load and femtocell density. The ASE has been derived considering activity of interfering base stations under fractional load conditions in macrocell and femtocell networks. We investigate on the optimal femtocell radio parameters that maximizes the ASE for a given network condition while maintaining Quality of Service (QoS) requirements of both macrocell and femtocell users. It is seen that for uncontrolled deployment of femtocells in co-channel deployment mode, the maximum ASE is achieved at the cost of QoS of macrocell users. However, it is shown that if radio parameters are suitably controlled, the ASE can be maximized without affecting macrocell user performance.

Orthogonal Frequency Division Multiple Access (OFDMA) based mobile networks are expected to provide increased system capacity by offloading traffic from existing macro cells. However, the gain may be limited due to random and uncontrolled interference caused by dense deployment of femtocells particularly in co-channel deployment mode. In this paper, we present an analytical framework to evaluate the capacity enhancement of the OFDMA-based co-channel macrocell-femtocell networks. We study the throughput performance and investigate the achievable offloading gain in terms of number of supportable users in the system. We also study the impact of macrocell power configurations on the performance of the network. It is seen that the macrocell offloading gain increases with femtocell density and the performance of the network is heavily influenced by macrocell power configuration and load condition. The results show that, in co-channel deployment mode, offloading gain can be improved by choosing appropriate femto load and femtocell transmit power according to the network traffic condition.

Using (BS) sleep mode to save energy has received major attention in recent years. In this work, a centralized approach to identify the combination of active base stations and RAN parameter, namely vertical antenna tilt angle, required to meet a certain traffic demand is developed. The method uses Genetic Algorithm(GA) to reduce the otherwise exponentially increasing search space. We consider Orthogonal Frequency Division Multiple Access based single frequency networks with fractional load modeling, to capture the effect of varying co- channel interference due to lower than peak traffic load condition to evaluate the performance of the proposed scheme. Results on percentage energy savings obtained under different traffic load conditions are presented for dense deployment of base station where highest amount of energy saving is possible. Impact of large and small search space on the energy saving is also shown.

Voice over Internet Protocol (VoIP) being highly sensitive to errors is usually transmitted using diversity mode (Space Frequency Block Coding (SFBC)) of Multiple Input Multiple Output (MIMO) in Orthogonal Frequency Division Multiple Access networks. In Long Term Evolution, the Closed Loop Rank - 1 (CLR-1) being more error resilient than higher order MIMO modes is also one of the attractive options that can be used for VoIP transmission. This work presents an algorithm for Semi-Persistent VoIP scheduling using CLR-1 MIMO mode. The performance is evaluated following ITU specified test methodologies. Since it requires frequency reuse-unity to be used, it is expected that cell edge users will experience heavy co- channel interference. As MIMO communication is affected differently by different interfering MIMO modes, comparison of several combinations of using SFBC and CLR-1 in desired and interfering link is presented for the proposed algorithm. Finally, predication of PRB usage and VoIP capacity for CLR- 1 are also shown.

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.

This work presents a method to estimate the amount of radio resource required to support a given number of Voice Over IP (VoIP) users in Orthogonal Frequency Division Multiple Access (OFDMA) based cellular systems. The proposed method is useful for the Call Admission Control (CAC) module, which needs to estimate the radio resource utilization before admitting new calls in the system. It is also useful in predicting the maximum number of VoIP users that can be supported for different cell scenarios and spatial user distributions. The proposed method is evaluated for Dynamic as well as Semi-Persistent (SP) scheduling algorithms. Results are presented for 1×2 Maximum Ratio Combining (MRC) and 4×2 Space Frequency Block Code (SFBC)- Frequency Switched Time Diversity (FSTD) modes. The 3rdGeneration Partnership Project (3GPP)-Long Term Evolution (LTE) system parameters are used for performance analysis in this work.

In wireless communication systems, the spatial multiplexing mode of Multiple Input Multiple Output (MIMO) is known to provide significant gain in spectral efficiency in the high Signal to Noise Ratio (SNR) region. Between the two methods, Singular Value Decomposition (SVD) and Bell labs Layered Space Time (BLAST), for implementing spatial multiplexing, the SVD scheme provides much higher spectral efficiency but it is much more complex to implement than the BLAST scheme. The aim of this work is to present an algorithm for antenna selection, which works together with BLAST method and compare the performance with that of SVD method. The comparison is done considering linear receiver architecture, and that the receiver has noisy channel estimates. The channel model used for performance evaluation is as per International Telecommunication Union (ITU)-specifications. It is shown in this work that, the proposed algorithm used with BLAST is very simple to implement and performs at par with SVD.

Fourth Generation and beyond broadband wireless mobile networks, specifically 3rd Generation Partnership Project (3GPP)-Long Term Evolution (LTE) and beyond are considering issues related to the deployment of femtocells. Femtocells have the potential to improve Area Spectral Efficiency (ASE) several folds. However, interference is added to the system due to large number of uncontrolled, user-deployed femtocells. Hence improvement of overall system throughput becomes limited. In this paper, downlink coexistence between macro/micro cells and cochannel femtocells in a single frequency Orthogonal Frequency Division Multiple Access (OFDMA)/Frequency Division Duplex (FDD) network is investigated. The impact of density, load and transmit power variation of the femtocells on the performance of macro/micro cell and femtocell users as well as the effect of macro/micro cell load on femtocell performance is presented. Guidelines for femtocell deployment with the objectives of maximizing sum-cell-throughput and maximizing number of active femtocells are derived. Methods of centralized control as well mixed centralized and distributed control of femtocell radio parameters to meet the above objectives are presented. The analysis is done with International Telecommunication Union (ITU) specified channel models using system level simulator.

The large bandwidth available in Ka-band of the satellite channel is one of the prime motivations for broadband communication via satellite. In tropical regions where there are heavy rains, the link availability due to rain fade is a major concern in the Ka-band. Rain fade mitigation techniques are required to address the problem. Methods for Adaptive Modulation and Coding (AMC) controlled from the application layer, which can be applied to any typical modem which supports multiple data rates, in order to maintain link availability is presented in this work. Algorithms for estimating the link quality, from the application layer, which is essential for successful implementation of AMC is also detailed in this work.

In a pre-coded Multi User Multiple Input Multiple Output (MU-MIMO) system, the channelization can be done either by using any of the two basic access techniques, namely Orthogonal Frequency Division Multiple Access (OFDMA) and Space Division Multiple Access (SDMA), or by combining them. From resource allocation point of view, choice of any technique will require different fairness conditions among users. In this paper, we have studied these different fairness conditions when combined with basic or joint access schemes mentioned above, while applied in a MU-MIMO based UTRA-LTE system. We have evaluated the resource allocation fairness issue when two well-known linear MU-MIMO precoding is used on a UTRA-LTE system. User grouping issue is dealt with when SDMA component is considered in the system. The results in this work provides an indicative analysis of the usability of different channelization techniques for considered system.

In future time division duplex (TDD)-based broadband wireless systems, it will be possible to exploit the channel reciprocity to implement channel state information (CSI)-based multi user multiple input multiple output (MU-MIMO) techniques, which will ensure highly efficient spectrum usage. To increase the cell coverage while ensuring the quality of service (QoS) for all UEs across the cell area, fairness should be maximized as much as possible. This paper presents a novel way to help improving fairness performance in the physical layer, via fair power allocation together with resource allocation, in MU-MIMO precoding scenarios where the common approach of guaranteeing fairness at MAC layer is not feasible. The results presented in this paper show that the proposed algorithm is able to reduce the system outage event to a large extent, thus increases fairness.

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.

Orthogonal frequency division multiplexing (OFDM) is widely used in wireless systems because of its high spectral efficiency, though it is known that it is highly vulnerable to frequency synchronization errors, which severely impede its performance. Such errors arise due to carrier frequency offset and Doppler frequency spread. They destroy the orthogonality among sub carriers and introduce inter carrier interference (ICI). The interference power is proportional to the received signal strength and amount of frequency offset. Several proposal to reduce the impact of ICI in OFDM use very complex signal processing and some have associated bandwidth efficiency loss. Since ICI is inversely proportional to the sub carrier bandwidth, it is interesting to investigate the gains in a system with adaptable sub carrier bandwidth. A system which can support variable sub carrier bandwidths simultaneously meeting the needs of different ICI conditions is compared against a normal OFDM system which uses fixed carrier bandwidth (FSB). Results show that the VSB schemes can be preferred over FSB OFDM system when users with different receive signal strength and Doppler condition have to be served simultaneously.

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.

In orthogonal frequency division multiplexing (OFDM) systems, there are several degrees of freedom in time and frequency domain, such as, sub-band size, forward error control coding (FEC) rate, modulation order, power level, modulation adaptation interval, coding rate adaptation interval and power adaptation interval, which can be exploited for link adaptation to achieve high spectral efficiency. The variable channel parameters such as delay spread, Doppler frequency spread, average signal to noise ratio and instantaneous channel gains influence the link adaptation parameters. Optimal adaptation of the link parameters based on the channel conditions would lead to highly complex systems with high overhead. Hybrid strategies to vary the adaptation rates to tradeoff achievable efficiency and complexity are presented in this work.

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.

Cyclic delay diversity can be used to increase the frequency selectivity of the wireless channel seen from the receiver. In this work, we have used this simple technique on top of a spatial multiplexing based orthogonal frequency division multiplexing system. In our analysis, it is found that nearly 2 b/s/Hz of 10% outage capacity improvement can be achieved on a basic 2times2 system. This is achieved with little changes at any standard spatial multiplexing system. Also, the number of transmit antennas can be increased to increase the link capacity without making any changes to the receiver. Thus, the proposed technique can be used with any existing MIMO-OFDM system. We have shown significant improvement in FER by using this scheme compared to a simple spatial multiplexing scheme.

In this work, we have studied bit and power allocation strategies for multi-antenna assisted Orthogonal Frequency Division Multiplexing (OFDM) systems and investigated the impact of different rates of bit and power allocations on various multi-antenna diversity schemes. It is observed that, if we cannot find the exact Signal to Noise Ratio (SNR) thresholds due to different reasons, such as reduced Link Adaptation (LA) rate, Channel State Information (CSI) error, feedback delay etc., it is better to fix the transmit power across all sub-channels to guarantee the target Frame Error Rate (FER). Otherwise, it is possible to use adaptive power distribution to save power, which can be used for other purposes, or to increase the throughput of the system by transmitting higher number of bits. We also observed that in some scenarios and in some system conditions, some form of simultaneous bit and power allocations across OFDM sub-channels are required together for efficient exploitation of wireless channel.

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.

An algorithm for implementing variable guard interval (VGI) in Orthogonal Frequency Division Multiplexing (OFDM) based wireless networks is presented in this article. The required guard interval is derived as a function of the root mean square (rms) delay spread of the channel, time duration of discrete fourier transform used in OFDM and the required signal to noise ration (SNR). In this work the algorithm has been verified in a highly dynamic channel condition and results show that the algorithm can reduce guard interval significantly and is also found to be robust against time variation of rms delay spread and mean excess delay of the channel.

Several multi-carrier spread spectrum (MC SS) access techniques which are based on Orthogonal Frequency Division Multiplexing (OFDM) have been developed till date. Different schemes are expected to perform optimally in different channel conditions. Their performance is also influenced by receiver impairments, such as synchronization error and channel estimation errors. This work is targeted towards unified verification of performance of the schemes for different spreading gains, load conditions and sub carrier grouping schemes in quasi static realistic indoor channel environment in the presence of carrier frequency offset and channel estimation error for sub carrier hopping and non hopping systems. The results presented in this article will prove very important in deciding on a robust access scheme and its relevant parameters for down link indoor wireless local area networks.

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.

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.

The dynamic selection of guard interval in orthogonal frequency division multiplexing (OFDM) based wireless local area network (WLAN) systems is described in this work. It derives an algorithm and provides its performance merit for implementing variable guard interval (VGI). The proposed system improves mean, outage and maximum throughput significantly as compared to standard OFDM system using fixed guard interval without sacrificing bit error rate performance.