Self-organizing OFDMA systems by random frequency hopping

In this paper, an adaptive and self-organizing spectrum allocation policy for ad hoc femtocells in two-tier networks based on usage of random frequency hopping is presented. As femto- and macrocells normally share the same frequency resources, the proposed approach is able to minimize InterCell Interference (ICI) in an adaptive manner. Each femto- and macrocell user chooses random carrier frequencies for transmission corresponding to a given probability density function (pdf). If the level of interference reaches a certain limit, which is typically the case for femtocell edge users, the probability density function of the carrier frequencies is changed in a way that it is quasi-orthogonal to the one of the macrocell users nearby. The changeover point between different probability density functions is realized in a self-organizing manner regarding the detected level of interference. Hence, the approach is able to adjust the trade-off between spectral efficiency and introduced interference during operation. We present analytical models for the Signal-to-Interference-and-Noise-Ratio (SINR), the Bit Error Ratio (BER) and the required transmission power of Orthogonal Frequency Division Multiple Access (OFDMA) femtocells applying adaptive random frequency hopping. The mobile radio channel is characterized by non-frequency selective Rayleigh fading and all results are verified by simulations implemented in MATLAB. In order to evaluate the improvements, the results are compared to systems using centralized orthogonal planning, systems using neither centralized nor distributed planning, and systems using static random frequency hopping. The comparison shows that adaptive random frequency hopping is capable of reducing the BER by a factor of 15 and the transmission power of femtocells by 17 dBm compared to systems where the macrocell user transmits in the same subbands as the femtocell user.

This paper study the performance of an orthogonal frequency division multiple access (OFDMA) system under fractional load based in long term evolution (LTE) downlink. Fractional load is defined by a situation where only a fraction of the bandwidth is used for transmission due to lack of traffic. This type of situation should result in a global increase of signal to interference and noise ratio (SINR) conditions in the network. We propose different methods integrating the transmission pattern selection to the packet scheduling functionality of the enode-B depending only on the channel quality indicator (CQI) reports from the user equipments. Fractional load handling is operated without inter eNode-B coordination. We conclude that the system can operate under fractional load conditions if the CQI is calculated with wideband interference. Further gain can be obtained if the CQI is calculated with local interference, however, in that case, it is necessary to inject a certain time correlation in the transmission patterns due to reporting delay.

A frequency-hopping (FH) space-frequency (SF)-coded clustered orthogonal frequency division multiple access (OFDMA) system is considered. It is shown that SF coding reduces the number of collisions in OFDMA with FH provided that the subcarrier clusters are formed randomly at each hopping instant. Analytical expressions for the collision rate, which is defined as the expected number of collisions per symbol, are derived. Bit error rate (BER) computations via simulations in a frequency-selective fading channel corroborate the theoretical findings for SF-coded FH-OFDMA.

In orthogonal frequency division multiple access (OFDMA) systems, efficient resource allocation is very important to improve the performance of the OFDMA system. The general principle of resource allocation in an OFDMA system is to assign each subcarrier to the user with the best channel condition for that subcarrier. However, in delay tolerant traffic, current work did not consider that packets on retransmissions may have different allocation criteria from those on their first transmissions. In this paper, the resource allocation scheme is investigated by considering different criterions to packets on first transmissions and on retransmissions. Under a bit error rate (BER) constraint, the objective is to maximize the throughput of packets on first transmissions, meanwhile decreasing the longest packet delay caused by retransmitted packets and guaranteeing all packets on retransmissions to be sent out. A low-complexity suboptimal algorithm that separates allocation for packets on retransmissions and on first transmissions is proposed. Adaptive modulation scheme is adopted to guarantee the BER constraint, except the modulation for each retransmitted packet remains the same. In order to improve retransmission liability, the retransmitted packets are combined with its failed packets in previous transmission.

Deployment of relay nodes (RNs) in cellular orthogonal frequency division multiple access (OFDMA) systems provides an effective solution to increase high data rate coverage and improve cell throughput. However, a challenging issue is that additional interferences caused by RNs may substantially compromise the performance gain if no measure is taken. In this study, the authors address the problem of interference coordination in relay-aided cellular OFDMA systems, aiming at exploiting the benefits of RNs while minimising the negative effects of interferences introduced. The authors first analyse the possible interference scenarios in multi-cell systems. Based on the insights into their analysis, the authors propose resource allocation with interference coordination (RAIC) scheme for cellular OFDMA systems. RAIC selectively perform one of three resource allocation algorithms according to the offered traffic load in the system, to mitigate interferences and thus enhance system throughput. The authors conduct intensive simulation experiments based on the model with realistic broadband channel propagation conditions. Numerical results demonstrate that their proposed RAIC can effectively improve system throughput compared with the resource allocation schemes without appropriate interference coordination.

The multiple access scheme for the long term evolution (LTE) physical layer is based on Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink. The main drawback of OFDMA is the peak-to-average power ratio (PAPR) problem. Dummy sequence insertion (DSI) concept can be an efficient PAPR reduction method for the LTE downlink system because it is very simple to implement and side information is not needed. However, the DSI has to sacrifice a decrease in the transmitted data rate for a large number of dummy subcarriers. In this paper, a novel approach to the DSI concept is proposed for complete application to an LTE system. For effective utilization of physical resources, the unused space of the LTE system is calculated, and dummy subcarriers are inserted into the unused space. Therefore, there is no degradation in transmitted data rate. Simulation results show that the proposed method can reduce PAPR considerably without degradation of bit error rate (BER) performance as a LTE downlink system.

Long Term Evolution (LTE) has adopted Single Carrier Frequency Division Multiple Access (SCFDMA) technique for uplink and Orthogonal Frequency Division Multiple Access (OFDMA) for downlink. SCFDMA is often called as Discrete Fourier Transform (DFT) spread OFDMA. In this paper, Wavelet based SCFDMA is proposed for analyzing Bit Error Rate (BER) performance. Analysis is carried out using different wavelets and different modulation schemes under AWGN channel. This analysis will show that the reduction in BER takes place by using wavelet transform in SCFDMA. Thus wavelet based SCFDMA provides better BER performance than that of DFT based SCFDMA.

In this paper, we propose a joint channel estimation and symbol detection (JCESD) algorithm relying on message-passing algorithms (MPA) for orthogonal frequency division multiple access (OFDMA) systems. The channel estimation and symbol detection leverage the framework of expectation propagation (EP) and belief propagation (BP) with the aid of Gaussian approximation, respectively. Furthermore, to reduce the computation complexity involved in channel estimation, the matrix inversion is transformed into a series of diagonal matrix inversions through the Sherman-Morrison formula. Simulation experiments show that the proposed algorithm can reduce the pilot overhead by about 50%, compared with the traditional linear minimum mean square error (LMMSE) algorithm, and can approach to the bit error rate (BER) performance bound of perfectly known channel state information within 0.1 dB.

In the uplink of Orthogonal Frequency-Division Multiple Access (OFDMA) systems, the user carrier frequency offsets result in inter-carrier-interference (ICI) and multipleuser-interference (MUI), leading to a degradation of the bit error rate (BER). This paper treats this uplink scenario and derives the average uplink capacity and the BER using the signal-to-interference-and-noise ratio (SINR) analysis. Adaptive power allocation is suggested to increase the capacity. When the frequency offsets are modeled as zero-mean Gaussian or Uniform random variables, the BER is derived as a closed-form infiniteseries. The series requires at least 50 terms to ensure sufficient accuracy.

A new concept-traffic diversity is proposed for resource allocation of orthogonal frequency division multiple access system. With traffic diversity, the transmitter designates different bit error rate (BER) level for each kind of traffic. The transmission data rate corresponds to the BER level. At the receiver, this separated traffic, which belong to the same user, will be aggregated. Simulation results show that traffic diversity can increase the throughput by 27%.

The overall performance of Orthogonal Frequency Division Multiple Access (OFDMA) system is greatly affected by the peak-to-average power ratio(PAPR). To address the PAPR problem, a tone reservation(TR) method based on FFT/IFFT structure is discussed, then we propose to replace FFT by the fast discrete cosine transform(DCT) which could reduce the computational complexity. Moreover, the experiments prove that the system bit error rate(BER) will not be affected and higher PAPR reduction can be achieved.

In this letter, a novel resource allocation method is proposed for Discrete Fourier Transform Spread Orthogonal Frequency Division Multiple Access (DFT-s-OFDMA) systems in Long Term Evolution (LTE). The proposed method is developed based on a minimal metric loss criterion and performs better than the commonly used Recursive Maximum Expansion (RME) method.

Orthogonal Frequency-Division Multiple Access (OFDMA) systems have attracted considerable attention through technologies such as 3GPP Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX). OFDMA is a flexible multiple-access technique that can accommodate many users with widely varying applications, data rates, and Quality of Service (QoS) requirements. OFDMA has the advantages of handling lower data rates and bursty traffic at a reduced power compared to single-user OFDM or its Time Division Multiple Access (TDMA) or Carrier Sense Multiple Access (CSMA) counterparts. In our work, we propose a Particle Swarm Optimization based resource allocation and scheduling scheme (PSORAS) with improved quality of service for OFDMA Systems. Simulation results indicate a clear reduction in delay compared to the Frequency Division Multiple Access (FDMA) scheme for resource allocation, at almost the same throughput and fairness. This makes our scheme absolutely suitable for handling real time traffic such real time video-on demand.

This paper deals with our investigations into improving the performance of orthogonal frequency division multiple access (OFDMA) system with low computational complexity. In the proposed scheme, by taking attribute of a user channel like channel capacity decrease rate, subcarriers are allocated depending on the individual users' priority. This scheme is high reliable technique by ordering the capacity decrease rate and allocating subcarriers to users depending on the proposed user' priority. Also we apply error-based bit allocation scheme according to channel condition. And to reduce the computational complexity and iterations until reaching a final allocation, simplified adaptation based on bit error rate (BER) is suggested and compared with original algorithm. As the channel evaluation parameter, signal to noise ratio (SNR) estimation method is used. Our proposed scheme is simulated in comparison with a conventional resource allocation scheme. It is shown that proposed method demonstrates better results than the simulated schemes.

Co-Channel Interference (CCI) caused by different Multiple Input Multiple Output (MIMO) signalling schemes impact the same or other MIMO schemes in different ways. Due to the symbol-by- symbol processing in a desired Single Input Multiple Output (SIMO) link, an interfering Space-Time Block Code (STBC) link may become equivalent to an interfering Spatial Multiplexing (SM) link. Using this knowledge and understanding, we propose an interference cancellation receiver robust to different types of MIMO interferers at cell edge for the Downlink (DL) of cellular systems. The receiver systematically performs a multiple symbol processing: this is the appropriate processing when the signal of interest or the signal of interferer is correlated across symbols, which is the case for STBC transmission. We evaluated different link combinations in terms of Signal to Interference and Noise Ratio (SINR) statistics and Bit Error Rate (BER) performance in cellular Orthogonal Frequency Division Multiple Access (OFDMA) . We have found that the proposed multiple-symbol linear interference cancellation receiver performs satisfactorily when any kind of single 'logical' stream MIMO scheme is present as interferer.