Showing papers on "Co-channel interference published in 1994"

A signal design without co-channel interference for approximately synchronized CDMA systems

Abstract: A signal design is proposed for a CDMA system that is approximately synchronized. The signals are called pseudoperiodic sequences. The set of pseudoperiodic polyphase sequences are introduced from a set of periodic polyphase sequences with ideal autocorrelation for small shifts and zero cross correlation for all shifts. The system works without cochannel interference, even when the signals are modulated by data. >

Periodic interference avoidance in a wireless radio frequency communication system

Abstract: A hierarchical communication system is described whereby periodic interference can be detected and avoided utilizing constituents' computer controllers and sync signals generated in response to the interference. The use of a predictive sync signal allows radio frequency communication to be optimally timed to efficiently make use of the interference-free time within the periodic interference signal.

Transmitter Power Control for Co-channel Interference Management in Cellular Radio Systems

Abstract: Transmitter power control is a necessity to combat “near-far” problems in radio systems using receivers with limited “dynamic range” (such as conventional DSCDMA systems). Transmitter power control, however, can also be used to control cochannel interference, i.e. interference from other users using the same channel (code or time slot). For this purpose, it has been shown that the optimum transmitter power configuration is found by solving an eigenvalue problem. The paper reviews some recent results in this area. The basic models as well as the concepts of achievable C/I, up/down-link equivalence and C/Ibalancing are introduced. Both the interference limited (noise-less) case as well as models including thermal noise are treated. Results show that substantial improvements in system capacity can be achieved, particulary in conjunction with Dynamic Channel Allocation. The optimum power control schemes are shown to be robust against implementational shortcomings.

Comment on "Performance of optimum transmitter power control in cellular radio systems"

Abstract: In the original paper, it was conjectured that the system performance in the up-link and down-link in a cellular radio system under optimum transmitter power control should be statistically similar. In this comment we show that the achievable signal-to-interference ratios in the up- and down-links are, in fact, identical at every instant. >

Radio frequency sharing personal communications system

Abstract: A radio communications system which is channelized and constituted in cellular array shares the spectrum with other fixed radio communications systems but complies with FCC rules as to interference to these fixed systems. It still maintains however good quality in its signals. Interference is determined by real-time field measurements of interference received from fixed stations on both forward and reverse offset channels by base stations and a mobile terminals. Such real-time measurements besides sensing the quality of a signal can be used to determine interference to the nearby microwave fixed users. In addition compensation is made for multiple users and thus no worst case compromises need be adopted as in prior systems.

Method for supervising base station radio channels

Abstract: This invention relates to a method for supervising base station radio channels to detect channels with interference and/or poor quality. This is achieved according to the invention by measuring and analysing connection separately for each radio channel. For instance radio channels with poor quality and/or interference in the frequency hopping sequence can be detected.

Narrow-band interference rejection in orthogonal multi-carrier spread-spectrum communications

Abstract: In order to combat strong narrow band interference, different measures called notch-filtering in the time domain (based on LMS algorithm) and in the transform domain (based on FFT) with spread-spectrum (SS) have been analyzed. Another interesting approach is based on orthogonal multi-carrier transmission called the orthogonal-frequency-division-multiple-access (OFDM) technique. This technique permits to achieve a high spectral efficiency and to combat the frequency selective fading and the interference in the channel by switching-off some sub-carriers. The aim of the article is to study the performance of the combination of a spread-spectrum system with OFDM in the presence of narrow-band interference. This combination allows one to use the spectrum in an efficient way, to exploit frequency diversity, to increase the robustness against fading and interference and to retain many advantages of a spread-spectrum system. The interference is considered as narrow-band multi-tone sine interference. A method for evaluating both interference and fading process based on frequency domain analysis is studied. The estimated interference and fading process is used for weighting each received chip before despreading. The results showed that the combination of spread-spectrum/OFDM in the presence of multi-tone narrow-band interference in a frequency/time selective fading channel is a promising approach. >

Some Reflections On The Interference Channel

Abstract: This paper provides a short overview of the advances on the interference channel, which is a well-known channel in multiuser information theory, that have appeared in the literature since 1976. Several open problems are defined.

Interference canceling equalizer (ICE) for mobile radio communications

Abstract: This paper proposes an adaptive interference canceling equalizer (ICE) that not only equalizes intersymbol interference (ISI), but also cancels co-channel interference (CCI) in the received signal in Rayleigh fading environments. ICE employs a novel detection scheme, the recursive least squares maximum likelihood sequence estimation (RLS-MLSE), which simultaneously estimates time varying channel parameters. Diversity reception is used to enhance the signal detection performance of ICE. A computer simulation of 40 kb/s QPSK mobile radio transmission demonstrates the possibility of system capacity improvement by ICE. Simulations of ICE with diversity are carried out under the conditions of a two-path Rayleigh frequency selective fading channel with 50 /spl mu/sec delay, 40 Hz maximum Doppler frequency, and 15 dB average E/sub b//N/sub 0/. ICE for single CCI cancellation attains an average BER of 10/sup -2/ under a carrier-to-interference ratio (CIR) of -14 dB. ICE with two independent CCI signals attains less than 1.5/spl times/10/sup -2/ average BER for average CIR/spl ges/-10 dB. >

On distributed power control for radio networks

Abstract: Due to co-channel interference, the carrier to interference ratios (C/I) of some mobiles in a wireless network may drop below a desired quality threshold /spl gamma/, either upon admission of a new mobile or if channel conditions vary. By using dynamic, local measurement of the power gains between base stations and mobiles, we develop a new distributed algorithm for power control which operates by adjusting the transmitted powers from the base stations so as to maintain the C/I of every link above the desired threshold. >

Spatial and temporal filtering for co-channel interference in CDMA

Abstract: This paper provides a review of the challenges and opportunities that arise for improving the user capacity of spread spectrum multiple access (SSMA) or code division multiple access (CDMA) based on spread spectrum techniques. Co-channel interference (CCI) in CDMA is the most dominant factor in the limitation of user capacity, while the capacities of FDMA and TDMA capacities are primarily bandwidth limited. From an information theoretical viewpoint, however, an optimum multi-user receiver for CDMA can be derived by utilizing CCI as redundant information distributed among multiple accessing users. In practical channels such as personal, indoor, mobile radio communication channels, more feasible receivers are required to combat the multipath fading that causes estimation errors of the channel characteristics due to time variance. An overview of several CCI cancellation schemes for sub-optimum but practical multi-user receivers for CDMA is provided by classifying them in the temporal domain, the spatial domain, or in both domains. >

Multicarrier DS CDMA systems in the presence of partial band interference

Abstract: Applies a multicarrier signalling technique to a direct sequence CDMA system. This type of signalling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. The authors use bandlimited spreading waveforms to prevent self-interference, and evaluate the system performance over a frequency selective Rayleigh channel in the presence of partial band interference. >

Co-channel interference cancellation in optical synchronous CDMA communication systems

Abstract: Three co-channel interference cancellation techniques are proposed for synchronous optical CDMA communication systems. Modified prime sequence optical codes that exhibit a grouping characteristic are employed. In the first technique the desired user collects primary decisions from receivers of all interfering users and subtracts them, after properly weighting, from its received signal. In the second and third techniques the desired user collects photodetector outputs from users in its same group and subtracts them from its received signal after a proper scaling. >

Radio telephone system for minimizing co-channel interference utilizing geographic separation of signals and transmission power control

Abstract: A system for minimizing co-channel interference between transmission sectors at their common periphery. The system exploits the geographic separation of potentially interfering mobile transmitters and utilizes transmission power control. The system employs sectored antennas in a combined frequency hopping, TDMA system wherein each subscriber in a particular sector is assigned a time slot in the system based upon that subscriber's geographic location within that sector. The transmission power in the sector is reduced during certain predetermined time slots and subscribers who are remote from the fringe of the sector are assigned to these power reduced time slots. During up-link the transmission power for subscribers located proximal to the fringe are reduced to cause less co-channel interference between their signals and the signals from other subscribers in different sectors having the same time slot.

Applying the intelligent cell concept to PCS

Abstract: We have concluded that the spectrum efficiency can be achieved in wireless PCS systems by applying the intelligent cell concept. The intelligent cell can either reduce interference by intelligently delivering the signal to the mobile unit, or can tolerate a great deal of interference while receiving with it. The former isolates the signal with multiple zones. In the latter, the signal resides with the interference but retains its processing gain. This paper presents the philosophy of achieving spectrum efficiency by applying the intelligent cell concept in wireless PCS systems. >

Signal design for approximately synchronized CDMA system without co-channel interference using pseudo-periodic sequences

Abstract: A signal design is proposed for a CDMA system that is approximately synchronized. The signals are called pseudo-periodic sequences. The set of pseudo-periodic polyphase sequences are introduced from a set of periodic polyphase sequences with ideal autocorrelation for small shifts and zero crosscorrelation for all shifts. The system works without co-channel interference, even when the signals are modulated by data. The number of sequences can be infinite, so the security of the channel is very good, when the sequences are assigned secretly. Data assigning methods for signals are also proposed by designing codewords using "coded addition of sequences" or modulated orthogonal sequences. >

A new architecture for mobile radio with macroscopic diversity and overlapping cells

Abstract: A new mobile radio cell architecture is presented that reduces interferences, increases system capacity, improves transmission quality and allows uncritical handoffs. Two main features characterize our proposal: macroscopic diversity supply of mobiles by means of three base stations which leads to high carrier-to-interference ratios and link quality improvement, and overlapping cells, thereby introducing large handoff zones. This system is ideally suited for existing GSM-style digital cellular networks and allows a traffic capacity increase due to a frequency re-use factor of 3.

Analysis of Electromagnetic Compatibility Between Radar Stations and 4 GHz Fixed-Satellite Earth Stations

Abstract: The susceptibility of 3700– to 4200–MHz fixed–satellite service earth stations to interference from radar signals, and the mechanisms by which such interference can occur, are examined. It is shown that interference can occur even if all currently applicable NTIA and FCC spectrum engineering requirements for radar emissions and earth station receiver systems are satisfied. It is further shown that while most interference problems can be resolved by installing appropriate radio frequency (RF) filtering on either the radar transmitter RF output or the earth station RF input, determination of the system that requires filtering depends critically upon the interference coupling mechanism. Methods for determining the interference coupling mechanism are presented.

Cascaded combination of cancelling co-channel interference and decoding of error-correcting codes for CDMA

Abstract: In this paper, we propose and analyze a direct sequence spread spectrum multiaccess (DS/CDMA) receiver that employs an integrated filtering and decoding technique for cancellation of cochannel interference (CCI) in cascaded form. We consider channels that experience multipath returns which is one of the factors responsible for signal degradation in mobile communication systems. The idea behind the integrated filtering is that CCI can be more reliably estimated by exploiting coding. We investigate the BER performance of the proposed scheme by theoretical analysis. Both numerical and simulation results exhibit the significant improvement that can be attained by the proposed scheme compared to the conventional scheme. >

Performance of trellis-coded co-channel interference canceller for cellular radio

Abstract: Co-channel interference is a major factor limiting the spectral efficiency of a cellular radio system. A new co-channel interference canceller leading to a significant increase of the traffic capacity of a cellular system is proposed. In this scheme, maximum-likelihood sequence estimation is extended to cancel co-channel interference and is combined with trellis-coded modulation to enhance the co-channel interference cancelling capability, and thus the name trellis-coded co-channel interference canceller (TCC) is given. The performance of the TCC is compared with that of a conventional canceller without trellis coding based on computer simulations assuming static, Rayleigh fading and Nakagami-Rice fading channels. It is shown that the TCC significantly improves the bit error rate performance over a conventional co-channel interference canceller especially for Nakagami-Rice fading channels, typically encountered in a microcellular system, at the cost of a moderately increased computational complexity.

Simulation of co-channel interference in coexisting cellular TDMA networks

Abstract: Uplink co-channel interference (CCI) between minimum-shift-keyed (MSK) time-division multiple access (TDMA) data packets generated by transmitters distributed uniformly over a circular area has been simulated in software, taking distance-dependent attenuation, log-normal shadowing, and multipath fading into account. The radio-link model and analytical derivation of bit error probability versus signal-to-interference ratio of a link with a single interferer are presented. The computer model is used to extend the bit error rate analysis to the case of multiple cochannel interferers. Numerical bit error rate (BER) results obtained from simulations of mutually interfering TDMA packet transmissions are compared with known theoretical results for the MSK channel under additive white Gaussian noise (AWGN) only. Possible improvements and future applications of the model to the problem of resource sharing between coexisting networks are suggested. >

Bounds and estimates of the uplink capacity of cellular systems

Abstract: We present bounds and estimates of the uplink spectral efficiency of cellular systems employing various multiple access schemes, in particular TDMA and CDMA in conjunction with FEC coding. We determine the cumulative distribution function of co-channel interference using a Monte Carlo technique and use this in conjunction with Shannon's (1949) bound on channel capacity to obtain upper bounds. Estimates are also obtained using simulated performance of practical codes. We show that coded TDMA with 100% re-use gives a greater capacity than with cluster size 3, and that CDMA with voice activation has approximately twice the capacity of either. Adaptive TDMA is identified as promising scheme for further capacity improvement. >

Two stage co-channel interference cancellation in orthogonal multicarrier CDMA in a frequency selective fading channel

Abstract: We have proposed a multi-carrier DS-SS CDMA system that eliminates the effect of multipath fading. Instead of multipath diversity, frequency diversity is obtained. This paper presents a co-channel interference cancellation technique and applies it to the multi-carrier CDMA system. Any bit of the desired user gets interference from two bits of any other user. The interference cancellation technique first removes the interference contributed by the first interfering bit then removes all interference.

Interference analysis in the GSM-system

Abstract: In the planning phase of a cellular radio network information on the expected radio interference between cells is needed many times. A new approach to the prediction of radio interference in GSM is introduced. Instead of concerning the carrier to interferer ratio C/I, often used in planning analog systems, an adequate algorithm for planning digital systems is presented. Including the fading statistics of the mobile radio channel, the algorithm calculates the probability that a digital quality threshold in terms of bit error rate (BER) is exceeded. The benefits of discontinuous transmission (DTX) in GSM to interference reduction is studied. Furthermore the impact of a partly traffic loaded interferer cell on interference is analysed. As a result of the analysis the use of DTX and a traffic load factor of e.g. 75% can reduce interference down to 63%. >

RBF growing algorithm applied to the equalization and co-channel interference rejection problem

Abstract: Recently the RBF network has been successfully applied to a number of communication signal processing problems, including equalization and co-channel interference rejection. Determining the number and locations of the centers is an important but usually neglected issue. A growing RBF algorithm for determining the number and location of the centers is presented and the performance is compared to a RBF with prescribed centers. The growing algorithm's advantages are its limited need for prior information and its ability to better utilize the allocated centers. As an example, the growing algorithm reduced the number of centers by more than a factor of 2/sup 31/ over the prescribed method to achieve a desired performance for an equalization and interference rejection application. >

Neural-net based receiver structures for single- and multi-amplitude signals in interference channels

Abstract: This paper presents analysis and performance evaluation results for several neural-net based receiver structures which effectively combat additive channel interference, such as co-channel interference (CCI) and adjacent channel interference (ACI). Although the idea of employing neural net based receivers for interference channels is not new, the novel technical contributions of the authors' paper can be summarized as follows. (i) Propose, analyze and evaluate a training algorithm for Nyquist filtered single- and multi-amplitude signals which is based upon a novel non-uniform signal sampling technique. (ii) Propose and evaluate neural net structures employing a novel non-linear activation function for the detection of multi-amplitude signals. (iii) Present novel bit error rate (BER) performance evaluation results for coherent and noncoherent single- and multi-amplitude signals, including binary phase shift keying (BPSK), quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), operated in generalized CCI and ACI channels. The authors' research has demonstrated that, as compared to more conventional detection techniques, the proposed neural net receivers provide significant performance improvements in CCI and/or ACI channels. Their tolerance for inaccuracies in symbol timing synchronization also makes them good candidates for practical modem implementation. >

Co-channel interference cancellation technique using orthogonal convolutional codes

Abstract: This paper proposes a new co-channel interference cancellation technique which utilizes orthogonal convolutional codes. For spread spectrum multiple access, co-channel interference (CCI) limits the performance of a communication link. To remove this interference, several CCI cancellation techniques have been proposed. Especially techniques which do not require the receiver to have knowledge of the cross-correlation between user sequences have been presented. However, these methods leave residual interference after the cancellation caused by errors on the initial decision. To reduce the residual interference and improve the initial decision, the proposed scheme utilizes the error correcting capability of the orthogonal convolutional code. This paper evaluates the performance of this scheme. It is shown that up to about 40 users, the proposed CCI canceller has a performance which is near the optimum performance when multiuser interference is completely canceled. It is also shown that the proposed CCI canceller offers an improvement in capacity by a factor of 1.5/spl sim/3 as compared with a conventional canceller. >

Spatial processing of broadband underwater acoustic communication signals in the presence of co-channel interference

Abstract: High throughput multiple-access communication networks are being considered for use in underwater acoustic channels. The bandwidth limitations of underwater acoustic channels require the receivers to process broadband signals in the presence of several active users. To deal with the resulting multiple-access interference in addition to the high intersymbol interference, the spatial variability of ocean multipath is exploited in a multichannel, multiuser receiver. Two configurations of such a receiver, a centralized and a decentralized one, are presented in fully adaptive modes of operation. The results of their performance in an experimental shallow water channel demonstrate superior performance of spatial signal combining. The use of multiple input channels is shown to provide a high level of tolerance for the near-far effect in both centralized and decentralized receivers. >