Showing papers on "Co-channel interference published in 1995"

Multiple access cellular communication with dynamic slot allocation and reduced co-channel interference

Abstract: An apparatus and method reduces co-channel interference in multiple-access cellular communication systems in which frame time or frequency slots are allocated between uplink and downlink. An omnidirectional antenna or a set of directional antennas are used in each cell base station to communicate with users. The frame slots (710,715,720) in which the antennas communicate uplink and downlink information are arranged in accordance with a predetermined frame organization to reduce mixed co-channel interference (CCI). Mixed CCI occurs when a downlink transmission from one base station antenna in a given cell interferes with uplink reception in another base station antenna in a frequency reuse (FR) cell. A potentially-interfering antenna in a given cell is therefore directed to transmit downlink information in a different portion of the frame than that in which a potentially-interfered-with antenna in the frequency reuse cell receives uplink information. The frame slots may be allocated such that only a portion (720) of the available slots are dynamically allocated in accordance with user demand, while the remaining portions are assigned to either uplink (710) or downlink (715) communication.

Co-channel interference cancelling receiver for TDMA mobile systems

Abstract: A method for co-channel interference cancellation in TDMA mobile systems exploiting the characteristics of mobile channels is proposed. The independently fading multipath channels provide a distinct waveform coding on each of the co-channel signals, which is a basis for signal separation in receivers. This enables the use of joint detection methods in receivers provided that accurate channel estimates can be obtained for all the cochannels. A joint detection and channel estimation algorithm for multiple co-channel signals are derived. The performance of the receiver is verified by simulations with the GSM system assuming that the strongest interferer is cancelled.

Wireless radio modem with minimal interdevice RF interference

Abstract: A wireless radio modem for incorporation into a host system includes radio frequency modulation/demodulation circuitry employing electronic device elements that operate in a frequency range that minimize the RF interference between the radio modem and the host system. Radio modem power conservation is further maximized by 1) simplifying signal modulation processing by use of a single sideband suppressed carrier waveform transition table, thereby reducing processing requirements; and 2) incorporating a "sleep mode" feature in which all non-timer circuitry is powered-down when not in use.

Bandwidth efficient QPSK in cochannel interference and fading

Abstract: The performances of QPSK in the presence of cochannel interference in both nonfading and fading environments are analyzed. Three approaches for representing the cochannel interference are investigated. These are a precise error probability method, a sum of sinusoids (sinusoidal) model, and a Gaussian interference model. In addition to determining precise results for the performance of QPSK in cochannel interference, we examine the validity of these two interference models in both additive white Gaussian noise (AWGN) environments and in different flat fading environments; Rayleigh, Ricean, and Nakagami. Nyquist pulse shaping is considered and the effects of cross channel ISI produced by the cochannel interference are accounted for in the precise interference model. Also accounted for are the random symbol and carrier timing offsets of the interfering signals. Two performance criteria are considered. These are the average bit error rate and the interference penalty. The latter is defined as the increase in signal-to-noise power ratio (SNR) required by a system with cochannel interference in order to maintain the same BER as a system without interference. Attention is given, in particular, to the outdoor microcellular fading environment. In this environment, the fading experienced by the interfering signals may be represented by a Rayleigh-fading model while the fading experienced by the desired signal may be represented by a Ricean or a Nakagami-fading model. >

Interference diversity gain in frequency hopping GSM

Abstract: Wideband random frequency hopping is one of the key properties of GSM in an evolution towards PCS (personal communication services). Random frequency hopping introduces diversity on the transmission link, which improves the performance. In this paper, the impact of interference diversity in GSM is analyzed by means of bit level simulations of the GSM full rate speech channel. The analysis shows that the gain of interference diversity is highly system load dependent. The interference diversity gain is most striking in a system with low channel utilization. These results, together with system level simulations of frequency hopping GSM, show that random frequency hopping has implications for frequency planning. They indicate that GSM systems with simple frequency planning can be designed without sacrificing the overall system quality.

Adaptive equalization for co-channel interference in a multipath fading environment

Abstract: The paper concerns the feasibility and achievable performance of adaptive filtering in an interference-limited multipath fading environment as encountered in indoor wireless communications. In a typical cellular radio application, the performance-limiting impairment is interference due to synchronous data streams from other co-channel and adjacent channel users (CCI and ACI). The receiver under consideration employs an adaptive fractionally spaced decision feedback equalizer (DFE) Which exploits the correlation of the cyclostationary interference to achieve superior performance relative to the worst case when the interference is stationary noise. The paper presents ideal calculations which confirm that significant DFE performance gains are potentially achievable by explicitly accounting for the cyclostationary CCI. Two adaptive DFE strategies are considered. One approach is to adapt the DFE directly using iterative algorithms such as least mean square (LMS) or recursive least squares (RLS). Another approach is to compute the minimum mean square error DFE using an RLS channel impulse response (CIR) estimate and a sample estimate of the CCI autocorrelation obtained from the CIR estimation error during training. The best approach for adaptive equalization, in terms of adaptation speed and system performance, is to employ an RLS DFE which does not explicitly estimate the CIR or the CCI autocorrelation. >

Channel allocation and downlink beamforming in an SDMA mobile radio system

Abstract: On the downlink of an SDMA mobile radio system, co-channel interference has to be kept down by beamforming. In this paper two DOA-based beamforming approaches are presented minimizing array signal power while maintaining given signal-to-noise-and-interference ratios for all users. The linear approach is computationally cheap, so that it is suited for channel allocation as well, quickly evaluating the spatial separability of a specific mobile radio scenario. The nonlinear algorithm yields optimum results but only converges quickly, if provided with a good starting point. Therefore, its basic application is burst-to-burst updating of the beamformer.

System aspects of cellular radio

Abstract: The greatest single factor in enhancing spectral efficiency of a network is the mass deployment of microcells. By this simple technique we can repeatedly and efficiently reuse the precious spectrum. The number of users a network can support is fundamentally dependent on the common air interface (CAI) over which users communicate. User capacity is dependent on many factors, but the cardinal ones are the amount of spectrum the regulators allocate, the size of the radio coverage area from a base station (BS), and the amount of interference a particular radio link can tolerate. In this article we are primarily concerned with the system aspects associated with the CAI. We focus on the critical importance of BS siting. Starting with existing large cells, we deliberate on the problems that might arise in siting BSs in three dimensional microcells, in order to consider suitable multiple access methods for future cellular environments. >

Method of allocating frequency bands to different cells, and TDMA cellular radio system

Abstract: The invention relates to a TDMA cellular radio system, and a method of allocating available frequency bands to different cells in a TDMA cellular radio system wherein at least some of the receivers within the system employ interference cancellation methods for cancelling the effects of co-channel interference in a desired signal. To enable an efficient utilization of interference cancellation methods and the resulting capacity increase in the system, the frequency bands available for the cellular radio system are, according to the method of the invention, so allocated to different cells that groups (50-55) of adjacent service areas are formed in the system, each group employing the same frequency band within the service areas, whereby at least one co-channel signal interfering with the desired signal is significantly stronger than other co-channel interfering signals.

TDMA cellular network application of an interference cancellation technique

Abstract: This paper studies the deployment of a co-channel interference cancellation technique in narrowband TDMA cellular networks. Particularly the case in which the strongest co-channel interferer is canceled in the receiver is investigated by network simulations. The potential gain can either be realized as higher system capacity or lower network outage rate. The receiver algorithm for interference cancellation is presented and its performance is simulated for the use of the capacity simulations. The capacity gain is shown in comparison with the network without interference cancellation.

Downlink relative co-channel interference powers in cellular radio systems

Abstract: One method to achieve high capacity in cellular radio systems involves interference cancellation techniques at the receivers. To design and evaluate the performance of such interference cancellation receivers, it is essential to use realistic models of the co-channel interference. In the paper we study the ordered statistic of the relative co-channel interference, i.e. the ratio between the power level of i:th strongest interferer and the total co-channel interference power. Results from Monte-Carlo simulations show that for base stations on a symmetric grid, hexagonal cells or half-square Manhattan like street cells, the probability density functions (pdf) of the interference ratios are almost independent of the cluster size and of the base station activities. The results demonstrate the dominance of the strongest interference and show that this dominance is even more pronounced in those situations when the signal to interference ratio is low. The presented results provide a base for more realistic models for performance evaluation in cellular systems than the commonly used Gaussian interference model.

Energy-efficient time-division radio that reduces the induction of baseband interference

Abstract: A method and apparatus are disclosed that mitigate the induction of baseband interference that can from time-division radios (e.g., radios that radiate an RF signal with a time-varying power envelope). A radio comprising an embodiment of the present invention radiates, in transmit mode, an information-bearing signal. In some or all idle modes, the radio radiates at least one energy-reducing pulse, but in an idle-mode channel. These energy-reducing pulses reduce the energy in the fundamental frequency component of the average RF power envelope of the radio and also reduce the energy in the harmonics of the fundamental frequency.

Method and apparatus for mitigation of self interference using array processing

Abstract: A diversity receiver for use in radio communication systems is described. Using array processing techniques, intersymbol interference can be mitigated from a desired signal. Channel tap estimates of the desired signal are used explicitly to cancel intersymbol interference as opposed to conventional techniques which use equalizers.

Blind cancellation of co-channel interference

Abstract: A co-channel system can be viewed as a multiple-input, multiple-output communication system. By extending the scalar constant-modulus algorithm to vector-valued signals, we develop several methods for blind cancellation of co-channel interference. In particular, to prevent the receiver from locking on to the same signal more than once, we propose the determinant constant-modulus algorithm. For the special case of constant-modulus signals and a memoryless channel, this method can outperform other known methods, as demonstrated by simulation.

Spatio-temporal equalization for multipath environments in mobile radio applications

Abstract: Combined spatial and temporal equalization using an antenna array combined with a decision feedback equalization scheme is investigated. In particular a TDMA type system with a relatively short training sequence is considered. Three algorithms are introduced. The first two algorithms are based on indirect schemes, where the channels to each receiver antenna element are identified. The identified channels and the correlations of the residuals are then used for the tuning of the beamformer/equalizer coefficients. The spatio-temporal correlations of the residuals are used in the first algorithm while in the second algorithm only the spatial correlations of the residuals are considered. The third algorithm forms a number of beams by using mixtures of different delayed versions of the training sequence as reference signals. It then performs temporal equalization by combining the outputs from the different beamformers, with appropriate delays. This latter algorithm requires less computations for the tuning of the equalizer, at the expense of a performance degradation in general. The algorithms are evaluated with simulations of multipath scenarios involving co-channel interference.

NTSC rejection filter with switched tomlinson precoder for reducing NTSC co-channel interference in ATV receivers

Abstract: A digital advanced television (ATV) receiver includes a multi-tap feedforward filter for reducing NTSC co-channel interference in a received ATV signal. A complementary Tomlinson precoder is used to compensate the transmitted ATV signal for inter-symbol interference (ISI) created in the receiver by the feedforward NTSC rejection filter. Data components of the transmitted signal are non-linearly precoded while synchronization components are linearly precoded to improve sync recovery S/N performance in the receiver.

Polynomial perceptrons and their applications to fading channel equalization and co-channel interference suppression

Abstract: The paper investigates the behaviours of polynomial perceptrons (PP) introduces a fractionally spaced recursive polynomial perceptron (FSRPP) with low complexity and fast convergence rate. The nonlinear mapping abilities of PP and FSRPP are analyzed. Applications of PP for fading channel equalization and co-channel interference (CCI) suppression in a 16-level quadrature amplitude modulation receiver system are considered. Computer simulations are used to evaluate and compare the performance of PP and fractionally spaced bilinear perceptron (FSBLP) with that of the synchronous decision feedback multilayer perceptron (SDFMLP), fractionally spaced decision feedback multilayer perceptron (FSDFMLP), and the conventional decision feedback equalizer (DFE). The results show that the performance of FSBLP is clearly superior to that of the other structures.

DS-CDMA reverse link channel assignment based on interference measurements

Abstract: The authors propose a DS-CDMA reverse link channel assignment algorithm based on received interference. A new channel is assigned if the required power is less than the ‘current interference margin’. The proposed algorithm is adaptable to dynamically changing environments with the capability of assigning more channels than conventional channel assignment methods in nonhomogeneous traffic loads and path loss changes.

A coded 16 QAM scheme for fast fading mobile radio channels

Abstract: An adaptive Viterbi algorithm, derived from a dynamic estimate of the fading channel is used for the decoding of a convolutional coded 16 QAM system in a mobile environment. The estimates are obtained by a sequence of known pilot symbols embedded in the data stream, and perform compensation for Rayleigh fading. The likelihood criterion in the Viterbi decoder is also modified by these channel estimates through a metric weighting function. We demonstrate through computer simulations, that our new technique achieves a BER improvement of 7-10 dB at P/sub e/=10/sup -3/ in a fast flat Rayleigh fading environment compared to an uncoded system. The BER performance of our new technique in a co-channel interference (CCI) controlled environment is also studied, and the results show that it may achieve a 40% to 85% improvement in capacity over the standard modem scheme for the new US digital cellular system, /spl pi//4-QPSK. >

Near-least-squares radio frequency interference suppression

Abstract: We present an algorithm for the removal of narrow-band interference from wideband signals. We apply the algorithm to suppress radio frequency interference encountered by ultra- wideband synthetic aperture radar systems used for foliage- and ground-penetrating imaging. For this application, we seek maximal reduction of interference energy, minimal loss and distortion of wideband target responses, and real-time implementation. To balance these competing objectives, we exploit prior information concerning the interference environment in designing an estimate-and-subtract-estimation algorithm. The use of prior knowledge allows fast, near-least-squares estimation of the interference and permits iterative target signature excision in the interference estimation procedure to decrease estimation bias. The results is greater interference suppression, less target signature loss and distortion, and faster computation than is provided by existing techniques.

Reduction of mixed co-channel interference in microcellular STDD systems

Abstract: STDD (shared time division duplexing) is an attractive candidate for high quality, low delay multiple access in microcellular systems. We show by means of theoretical analysis that STDD is superior to conventional TDMA as well as TDMA with slow speech activity detection (SAD) in terms of capacity. We demonstrate how a type of mixed co-channel interference particular to STDD can be eliminated by means of directional antennas at the base stations for both reception and transmission, combined with appropriate organization of the time burst. Numerical results are given for a frame length of 2 ms. Capacity gains of the order of 100% over TDMA are obtained with 40 speech slots per frame and a packet dropping rate of 0.1%.

Channel estimation in a CDMA receiver using multiple access interference cancellation

Abstract: A method for estimating a channel in a receiver of a cellular radio system, and a receiver in a cellular radio system, the receiver comprising means (22) for subjecting the received signal to elimination of multiple access interference. To obtain good channel estimates, the receiver of the invention comprises means (23) for computing channel estimates from a signal that has undergone elimination of multiple access interference.

Capacity and service extension for future wireless networks using adaptive antennas

Abstract: The spatial filtering properties of adaptive antenna arrays in wireless networks make it possible to confine the radio energy associated with a given user to a small addressed volume. This reduces the overall level of co-channel interference within the service area and ultimately leads to an increase in capacity for interference limited systems. In addition, by using the antenna array to spatially separate and reject multipath energy, the resulting delay spread can be significantly reduced offering the possibility of service in areas which would otherwise be restricted by time dispersion. The article show that adaptive antenna arrays, when applied to a DECT system, can increase service provision to areas which would otherwise be limited by multipath propagation. Furthermore, the use of spatial filtering allows the possibility of multiple users on the same frequency and time slot by minimising co-channel interference from other users. The capacity increase achievable can be as much as seven times.

OFDM for broadcasting in presence of analogue co-channel interference

Abstract: This paper treats the problems of digital TV terrestrial broadcasting in presence of co-channel interference from analogue TV services. The channel capacity is calculated for the optimum distribution of the transmitted power. An OFDM scheme is presented which is designed to be robust to the analogue TV interference. Power and number of bits per symbol are assigned to each OFDM channel to give the maximum data rate for a given signal to interference ratio. Simulations of the proposed scheme, for 8 MHz bandwidth and interference from a system G PAL signal, show that acceptable error rate can be achieved for e.g. 27 Mbit/s and 39 Mbit/s at signal to interference ratios of -6 dB and 0 dB, respectively. The proposed scheme is suitable for the next generation terrestrial TV networks which are going to coexist with the current analogue systems. >

Analysis of performance and capacity of coherent DS-CDMA reverse link communications

Abstract: The performance and achievable capacity of a coherent reverse channel of an IS-95-like DS-CDMA system previously described by Ling (see IEEE Proc. VTC'93, p.400-403, May 19993) are evaluated. A general analysis of the receiver performance that extends the previous results and provides insight into the system performance and design considerations is given. A similar analysis is then performed for evaluation of the pilot-channel-based coherent communication described by Brismark et al. (see IEEE Proc. VTC'94, Stockholm Sweden, June 1994). A comparison between these two schemes is given. It is also shown that the system capacity of the reference symbol-based coherent reverse channel can be further improved by adopting orthogonal signaling to reduce the intracell interference. Thus, it is possible to significantly increase the reverse channel capacity of an IS-95-like DS-CDMA system at a lower implementation complexity by employing such a coherent communication scheme.

Low-complexity co-channel interference cancellation and macroscopic diversity for high capacity PCS

Abstract: More than a factor of 3 increase in overall PCS capacity can be achieved in small, hand-held communication devices with limited battery power or in small, low-complexity base stations when incorporating simple interference cancellation techniques without training sequences in the channel access algorithm that realizes macroscopic selection diversity based on signal quality measurement. The comparison is done with respect to microscopic selection diversity and macroscopic selection diversity based on channel power measurement. Results were obtained by computer simulations.

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.

Analysis of interference cancellation for a multicellular CDMA environment

Abstract: This paper considers the performance of adaptive interference cancellation applied to a CDMA microcellular environment, by employing a circular geometry and a closed form expression for the bit error rate of a CDMA system with interference cancellation to analyze the effect of out-of-cell interference. Results indicate that out-of-cell interference will severely limit the benefits of interference cancellation in a multicellular system. Attempts to cancel all out-of-cell interference will further degrade the performance. However, selective interference cancellation in which only the strongest out-of-cell interferers are cancelled may result in significant performance enhancement.

A method and means for frequency hopping in a radio communication system

Abstract: Frequency hopping and hops between time-divided channels (K1, K2, ..) is carried out in a mobile telecommunications system. Hop sequences are established for individual connections for hops between the channels, thereby improving utilization of the channels in the system. Interference (I) is measured on the channels (K1, K2, ..) for individual connections between a base station (BS1) and a plurality of mobile stations. The connections are bidirectional, having an uplink and a downlink. The measured interference values of the uplink and the downlink respectively are stored in an interference list (IL) for each of the connections. The drawing shows three interference lists (IL1up-IL3up) in the uplink, including channel numbers (K1, K2, ..) and the measured interference values (I1, I2, ..). The interference values are converted to weight values (0.1; 0.3; ..) in a conversion circuit (I/Wup), and the weight values are then stored in weight lists (WL1up-WL3up). The weight values are analyzed in an allocation circuit (W/Hup), which allocates channels to individual hop sequence lists (HL1up-HL3up). Each connection is allocated a respective hop sequence list. The hop sequence lists are sent to respective mobile stations, thereby providing the mobile stations with information as to which hop sequence list shall be used.

Channel coding for co-channel interference suppression in wireless communications

Abstract: Co-channel interference is a major impairment in wireless systems with channel re-use. In practice the performance of time division multiple access (TDMA) and frequency division multiple access (FDMA) systems is limited by a few dominant co-channel interferers. We present channel codes that are matched to an adaptive linear receiver, so that the combination provides interference suppression. It is shown that a simple one symbol parity check code is capable of suppressing one interferer, a repetition code of length N is capable of suppressing N-1 interferers, and a code of K information symbols and N channel symbols is capable of suppressing N/K interferers. With simple parity check codes and repetition codes, the tap settings of an adaptive combiner can be configured using a 20-40 symbol training sequence. Simulations for multiple cells reveal significant capacity improvement is possible.