FDMA Systems Research Articles

Statistical distributions of instantaneous power and peak-to-average power ratio for single-carrier FDMA systems

This paper analyzes the distributions of instantaneous power and peak-to-average power ratio (PAPR) for single-carrier frequency-division multiple-access (SC-FDMA) signals, where localized FDMA (LFDMA) and interleaved FDMA (IFDMA), as well as a block-interleaved FDMA (B-IFDMA) are considered as a part of the discrete Fourier transform (DFT)-precoded orthogonal frequency-division multiple access (OFDMA) systems adopted in the recent mobile communications standard. Comparisons of analytical and simulation results show a good match in terms of both the measures, which confirms the validity of the analytical framework developed in this work.

Resource allocation with multicell processing, interference cancelation and backhaul rate constraint in single carrier FDMA systems

We consider a cellular system where base stations (BSs) cooperate to receive data from mobile terminals (MTs) using single carrier frequency division multiple access (SC-FDMA). This yields a distributed multiantenna system, with corresponding multicell processing gain. Moreover, the exchange of information among BSs is limited due to the rate constraint on the backhaul network. A first contribution of the paper is a scheduler of the signals shared among BSs on the backhaul with the objective of maximizing the SC-FDMA system sum rate. A greedy algorithm is proposed as a viable solution of the problem. A fundamental feature of the scheduler is that BSs may share received signals only within a subset of the subcarriers of each SC-FDMA block. As a further contribution of the paper, to increase the system sum rate, we also consider interference cancelation, where BSs detect some messages without cooperation, and transmit on the backhaul a suitable linear combination of received and detected signals. The scheduling problem is suitably modified to take into account interference cancelation, thus selecting the MTs for which detection occurs before sharing the signal on the backhaul. Numerical results for typical cellular configurations are presented.

Cooperative diversity schemes for uplink single-carrier FDMA systems

Single-carrier frequency division multiple access (SC-FDMA) is considered as a promising technique that is adopted in the uplink for the upcoming 3GPP long term evolution (LTE). This is because it keeps most of the advantages of OFDMA in addition to low peak to average power ratio (PAPR). Designing diversity achieving schemes for SC-FDMA is a challenging task. The code should not affect the PAPR among other constraints. In this paper, we propose cooperative diversity schemes for SC-FDMA systems. Specifically, distributed space-time/frequency codes are proposed. The proposed distributed space-frequency code (DSFC) takes into account the low PAPR which is the feature of the system. Both decode-and-forward (DAF) and amplify-and-forward (AAF) protocols are considered. Erroneous decoding at the relay node is taken into account. Moreover, we propose a bandwidth efficient distributed space-time code (DSTC) for slow fading relay channels with DAF. Analytical and simulation results indicate that the proposed schemes achieve full diversity at the expense of small performance loss compared to the proposed codes in the literature.

A DS-CDMA Cellular System Using Band Division and Channel Segregation Distributed Channel Allocation

SUMMARY In DS-CDMA cellular communications systems, the single frequency reuse can be utilized. Since large other-cell interference is produced, the well known soft handover or site diversity must be used. If the single frequency reuse is not utilized to avoid the other-cell interference, we will face the frequency allocation problem, similar to FDMA systems. In this paper, a DS-CDMA cellular system using band division is proposed. The available wide frequency band is divided into several narrow frequency bands and the different frequency bands are allocated to adjacent cells so as to avoid the large other-cell interference. For the frequency allocation, the channel segregation distributed channel allocation (CS-DCA) algorithm is applied. The link capacity is evaluated by computer simulation.

Seamless Symbol Rate Switching Scheme for Multi-Rate FDMA Modem

A new seamless symbol rate switchable modem for multi-rate FDMA systems is proposed in this paper. In the new modem, a novel clock phase compensation algorithm makes it possible to switch the symbol rate synchronously between the transmitter and the receiver, with no degradation in BER when the symbol rate is changed. In addition, by matching the interpolation filter to the symbol rate, this modem is capable of operating at lower clock speeds, which greatly reduces the consumption power. Computer simulations confirm its fundamental performance. Simulation results show that the proposed power-efficient symbol rate switchable modem can change the symbol rate without degrading BER performance.

A memetic algorithm for channel assignment in wireless FDMA systems

A new problem encoding is devised for the minimum span frequency assignment problem in wireless communications networks which is compact and general. Using the new encoding, which reduces search space dramatically over previous problem encodings, an optimization algorithm is developed which combines a genetic algorithm global search with a computationally efficient local search method from the literature. This memetic algorithm is shown to be more effective than six previous approaches in the literature on a suite of established test problems. Further, it shown that the integration of the global search with the local search is important; neither component by itself is nearly as effective.

Adaptive coding in MC‐CDMA/FDMA systems with adaptive sub‐band allocation

AbstractThe MC‐CDMA/FDMA scheme is a candidate for the air‐interface of beyond 3G mobile communications. An efficient adaptive sub‐band allocation (ASBA) approach has been recently shown to provide a considerable gain in the uncoded system performance. In this paper, adaptive coding is proposed for application in conjunction with ASBA. This is proved to yield a significant performance improvement, especially if a user‐service prioritisation is considered in the ASBA. Copyright © 2004 AEI

Effects of nonlinear distortion on switched multibeam FDMA systems

The effects of using a multicarrier amplifier (MCPA) in the transmit chain of a cellular frequency-division multiple-access system utilizing switched multibeam base-station antennas is investigated. By combining several carriers prior to amplification, the signal envelope will be time varying and the MCPA will introduce nonlinear distortion of the amplified waveforms. It is shown how the main beam direction and frequency of any intermodulation product of any order can be calculated. A frequency allocation scheme is presented that reduces the intermodulation distortion for the mobile users. By Monte Carlo simulations, the probability distribution function of the received intermodulation distortion power is estimated, assuming a GSM system, as a function of the number of antenna elements and the number of active users. Comparisons with a one-element reference antenna are made, and it is shown that the received intermodulation distortion power for the users in the system is substantially reduced when the number of beams is increased or the user activity is reduced.

Memory truncation and crosstalk cancellation for efficient Viterbi detection in FDMA systems

Memory truncation and crosstalk cancellation for efficient Viterbi detection in FDMA systems

Orthogonal frequency‐division multiple access and its application to CATV networks

AbstractWe introduce the concept of orthogonal frequency‐division multiple access (OFDMA) and investigate its application to send upstream information from subscriber premises to the cable head‐end in cable TV (CATV) networks. The carrier and timing synchronizations needed to ensure orthogonality between different subscriber signals are achieved by sending common pilot signals from the head‐end. Whereas N demodulators are required to accommodate N simultaneous users in conventional FDMA systems, OFDMA only requires a single demodulator followed by an N‐point discrete Fourier transform (DFT). We analyze the performance of this technique and compare it to time‐division multiple access (TDMA) which appears in several international standards. Our results indicate that OFDMA is an attractive multiple access technique which offers increased robustness to narrowband interference, impulse noise, and other signal impairments which characterize typical CATV networks.

Self-organizing channel assignment for wireless systems

CDMA has many benefits compared to TDMA and FDMA systems mainly because it is inherently self organizing. This article shows how the benefits of self organization can be extended to TDMA and FDMA systems also, and describes the results of research conducted by the authors. Self-organizing channel assignment not only increases the capacity and improves the call quality, but also obviates the need for frequency planning, which opens the door for innovative new services and technologies to be introduced into the operators' systems. This makes self-organization useful even in areas where operators are not yet facing capacity problems.

Increased capacity using CDMA for mobile satellite communication

The performance of a spread-spectrum CDMA (code-division multiple access) system in a mobile satellite environment is analyzed. Comparisons to single-channel-per-carrier FDMA systems are presented which show that the CDMA approach provides greater capacity. Results from computer simulations, laboratory tests, and field tests of a prototype modem are also presented. The tests results show excellent performance of the modem in the mobile environment and also the feasibility of the spread-spectrum approach to satellite mobile communications. >

Up‐link power control of satellite earth‐stations as a fade countermeasure of 20/30 GHz communications systems

AbstractAll 20/30 GHz satellite communications systems suffer from atmospheric attenuation in cloudy or rainy weather, particularly if high link availabilities are required. In FDMA systems the power transfer characteristic of each carrier is linear, so any up‐link fade degrades the overall performance on a dB‐by‐dB basis. This, together with the fact that the 30 GHz up‐link attenuation is much greater than the 20 GHz down‐link attenuation, makes adaptive countermeasures of up‐link fades most important. This paper shows how the overall link performance can be improved by applying up‐link power control. It is explained how this improvement depends on the up‐link/down‐link balance, the dynamic range of the up‐link power control, and the up‐link and down‐link atmospheric attenuation.If a small number of carriers can be allocated in an intermodulation‐free frequency plan, the satellite transponder can be operated near saturation in order to optimize the down‐link performance. Computer simulations of up‐link power control for such an FDMA scheme have been performed based on measured three‐carrier characteristics of a transponder TWT. It is shown that the overall link performance in faded conditions can be significantly improved with up‐link power control. This may be utilized to increase the availability of a satellite link or to relax the earth‐station performance in terms of G/T or EIRP if the link margin provided by an earth‐station with fixed EIRP is satisfactory.

Selection of gold and kasami code sets for spread spectrum CDMA systems of limited number of users

AbstractSpread spectrum code‐division multiple‐access (CDMA) systems are currently considered as very attractive alternatives to the more familiar FDMA and TDMA systems, especially in the presence of multipath fading and external interference. The problem of code selection in a CDMA system with a finite number of users is addressed in this paper. A simple and efficient method for the selection of finite code sets with relatively high processing gain from Gold and Kasami code families of relatively large sizes is described in detail. Selected code sets of period 63 are presented along with their overall average performance parameters for different number of users.