Multicarrier CDMA for cellular overlay systems

A code-division multiple access (CDMA) cellular overlay system is investigated, employing the idea of multicarrier CDMA, which has previously received significant attention as an alternative to traditional single-carrier CDMA. Overlay is pursued here as a means of long-term transition from narrowband cellular to CDMA cellular. A major result of this paper is the demonstration that the use of multicarrier CDMA in a fading channel is particularly beneficial to the narrowband system, as the CDMA users can reduce their transmitted powers as a result of diversity. Another significant conclusion is that the use of transmitter notching in the CDMA system in order to avoid active narrowband users outperforms a strategy in which a narrowband user is avoided by simply dropping the particular carrier which overlays it. Finally, results on the use of the minimum mean-squared error (MMSE) receiver in a fading channel are extended for use in the overlay scenario     

Open-loop power control error in cellular CDMA overlay systems

Adaptive power control has widely been used in DS/CDMA systems to overcome the so-called “near-far” problem. This paper studies the adaptive open-loop power control of a cellular CDMA system, which is overlaid in the downlink by a narrowband signal. The effects of downlink power allocation schemes to power control error in the presence of narrowband interference are analyzed. In order to get a minimum power control error in the CDMA overlay situations, an optimum downlink power allocation scheme is used, which works well for a wide range of signal to narrowband interference ratio     

Adaptive LMS filters for cellular CDMA overlay situations

This paper extends and complements previous research we have performed on the performance of nonadaptive narrowband suppression filters when used in cellular code-division multiple-access (CDMA) overlay situations. An adaptive least mean square (LMS) filter is applied to a cellular CDMA overlay in order to reject narrowband interference. An accurate expression for the steady-state tap-weight covariance matrix is derived for the real LMS algorithm for arbitrary statistics of the overlaid interference. Numerical results illustrate that when the ratio of the narrowband interference bandwidth to the spread spectrum bandwidth is small, the LMS filter is very effective in rejecting the narrowband interference. Furthermore, it is seen that the performance of the LMS filter in a CDMA overlay environment is not significantly worse than the performance of an ideal Wiener filter, assuming the LMS filter has had sufficient time to converge     

Adaptive interference suppression for CDMA overlay systems

It has been proposed that CDMA systems can be assigned to spectral bands which are presently occupied by narrowband users to further increase spectral capacity. Such CDMA overlay systems could provide new options for efficient utilization of the spectrum with minimal disruption to existing narrowband users, especially if adaptive interference suppression techniques are utilized in the spread spectrum receiver. Previous studies have defined the SNR improvement ratio which can be achieved for tone interferers and for narrowband interferers for which the center frequency of the interference is at the carrier frequency of the CDMA signal. In this paper the bit-error-rate (BER) performance of the mobile-to-base link of a CDMA system for a single narrowband user which occupies a significant portion of the CDMA bandwidth is evaluated. It is shown that the narrowband model used in previous studies does not apply in this case, especially for the large, effective, bandwidths which are characteristic of the interferers in the overlay system. The dependence of the BER on the filter order, the bandwidth of the interference, and its center frequency relative to the CDMA carrier frequency are defined. Additionally the increase in BER for a digital implementation of the adaptive suppression filter relative to the optimal Wiener filter is characterized with respect to the adaptive time constant and the quantization errors due to finite wordlength. It is shown that these implementation errors can be made negligible compared to the errors which are characteristic of the optimal Wiener filter. Analytic results are validated by simulation for typical system parameters     

Inverse filtering criteria for CDMA systems

Linear blind CDMA receivers are derived using inverse filtering criteria. The receiver parameters are directly obtained without explicit estimation of the system/channel. Both synchronous and asynchronous cases are addressed, and multipath distortions are explicitly considered and compensated for. The approach is based on minimizing the receiver's output energy subject to appropriate constraints. Similar approaches have been used before in multiuser systems but without considering multipath distortions. Batch and adaptive blind algorithms are derived that are near-far resistant and do not require knowledge of the interfering users' codes. Global convergence is shown, optimality and performance issues are discussed, and some illustrative simulations are presented     

Performance of CDMA overlay system using filtering and diversity

This correspondence considers an interference suppression scheme for a direct-sequence code-division multiple-access overlay system operating in a multipath Nakagami fading environment. The receiver employs suppression filters and diversity to mitigate the effect of multipath and narrow-band jammer interference. The system performance is presented in terms of the bit error rate with different system parameters such as bit energy, jammer-to-signal power, multipath, diversity, ratio of interference bandwidth to spread-spectrum bandwidth, and number of active users. Numerical results indicate substantial improvement in system performance.     

The necessity for and use of CDMA transmitter filtering in overlaysystems

The concept of code division multiple access (CDMA) overlay has been proposed as a way to take advantage of the unused bandwidth available in sparsely populated narrowband systems. Most research up to this point has focused on the effects of the narrowband system on the CDMA system. However, because the CDMA system is likely to be overlaid on a narrowband system that is already established, it is of equal or greater importance to examine how the CDMA overlay would affect the narrowband system's performance. It is shown that even a lightly loaded CDMA system can have a detrimental effect on a narrowband system to the point where the possibility of overlay may be precluded. To solve this problem, the idea of notch filtering the CDMA signals in an effort to avoid those bands already occupied by narrowband users is examined. For certain filtering methods the effects of CDMA interference on the narrowband system are shown to be greatly reduced, thus making overlay feasible from the perspective of the narrowband system. For the CDMA system, the performance of the minimum mean-squared error (MMSE) receiver was evaluated and it was found to work quite well with CDMA signals that have been notched. With CDMA transmitter filtering, the prospects for overlay become very encouraging     

Active interference suppression in CDMA overlay systems

Communication networks involving the overlay of spread-spectrum systems an narrower band services are of increasing interest as a means of producing greater efficiencies and flexibility in the use of the radio spectrum. Although spread-spectrum systems enjoy a natural immunity to interference from narrowband sources, their performance in the presence of such interference can be significantly enhanced by active suppression techniques. The study of this problem has elicited a very rich body of methodology, which has progressed over nearly 25 years from some of the simplest signal processing paradigms to some of the most advanced. This paper provides an overview of a number of these techniques, most of which have been developed over the past decade. In particular, a progression of techniques is described, in which successively more information about the spread-spectrum signal and interference is used to make improvements on the interference suppression capability via more advanced signal processing methods. These include linear predictive methods that make use of the spectral properties of the spread-spectrum and narrowband signals, nonlinear predictive methods that make use of the spectra and first-order probability distribution of these signals, linear code-aided methods that make use of the spreading codes of the signals of interest and the second-order statistics of the narrowband interference, and finally, a maximum-likelihood code-aided technique that makes use of essentially all that is known about the useful signals and interference. Performance comparisons show that moving up this progression of improved modeling is rewarded with performance gains that can be quite significant     

Interference mitigation using transmitter filters in CDMA systems

We consider the use of transmitter filters as pre-equalizers at the base station to mitigate the interference in code-division multiple-access (CDMA) systems over a multipath fading channel. It is shown that the interference at the receiver of the mobile station can be mitigated by the transmitter filters and, thereby, the capacity of downlink channel can be improved. The minimum variance distortionless response (MVDR) beamforming technique in antenna arrays has been utilized to find the coefficients of the interference mitigation (IM) transmitter filters. To design IM transmitter filters, we assume that the channel information is available. This can be true in the time-division duplexing (TDD) mode because of the channel reciprocity. In the frequency-division duplexing mode, the downlink channel impulse responses (CIRs) shall be be transmitted to the base station. To transmit the CIRs, we consider an encoding method. In addition, to compensate inherent feedback delay, the channel prediction is also utilized     

A transmitter diversity scheme for wideband CDMA systems based onspace-time spreading

We present a transmit diversity technique for the downlink of (wideband) direct-sequence (DS) code division multiple access (CDMA) systems. The technique, called space-time spreading (STS), improves the downlink performance by using a small number of antenna elements at the base and one or more antennas at the handset, in conjunction with a novel spreading scheme that is inspired by space-time codes. It spreads each signal in a balanced way over the transmitter antenna elements to provide maximal path diversity at the receiver. In doing so, no extra spreading codes, transmit power or channel information are required at the transmitter and only minimal extra hardware complexity at both sides of the link. Both our analysis and simulation results show significant performance gains over conventional single-antenna systems and other open-loop transmit diversity techniques. Our approach is a practical way to increase the bit rate and/or improve the quality and range in the downlink of either mobile or fixed CDMA systems. A STS-based proposal for the case of two transmitter and single-receiver antennas has been accepted and will be included as an optional diversity mode in release A of the IS-2000 wideband CDMA standard     

Adaptive lattice filters for CDMA overlay

This paper presents the behavior of reflection coefficients of a stochastic gradient lattice (SGL) filter applied to a code division multiple-access overlay system. Analytic expressions for coefficients for a two-stage filter are derived in a Rayleigh fading channel with the presence of narrow-band interference and additive white Gaussian noise. It is shown that the coefficients of the lattice filter exhibit separate tracking and convergent properties, and that compared to an LMS filter, the lattice filter provides fast rate of convergence, while having good capability of narrow-hand interference suppression     

Multicarrier CDMA overlay for ultra-wideband communications

In this letter, the performance of multicarrier code-division multiple-access (CDMA) systems is studied in the presence of narrowband interference for future ultra-wideband (UWB) communications. A Nakagami fading channel is assumed, and notch filters along with diversity techniques are used in the multicarrier CDMA receiver. A complete performance analysis of error probability is given. It is shown that when the number of subcarriers jammed by narrowband interference is small, the multicarrier receiver without notch filters can work well, due to the gain of frequency diversity from nonjammed subcarriers. On the other hand, when the number of subcarriers jammed by the narrowband interference is large, using notch filters can improve the multicarrier system performance significantly.     

Decentralized dynamic power control for cellular CDMA systems

The control of transmit power has been recognized as an essential requirement in the design of cellular code-division multiple-access (CDMA) systems. Indeed, power control allows for mobile users to share radio resources equitably and efficiently in a multicell environment. Much of the work on power control for CDMA systems found in the literature assumes a quasi-static channel model, i.e., the channel gains of the users are assumed to be constant over a sufficiently long period of time for the control algorithm to converge. In this paper, the design of dynamic power control algorithms for CDMA systems is considered without the quasi-static channel restriction. The design problem is posed as a tradeoff between the desire for users to maximize their individual quality of service and the need to minimize interference to other users. The dynamic nature of the wireless channel for mobile users is incorporated in the problem definition. Based on a cost minimization framework, an optimal multiuser solution is derived. The multiuser solution is shown to decouple, and effectively converge, to a single-user solution in the large system asymptote, where the number of users and the spreading factor both go to infinity with their ratio kept constant. In a numerical study, the performance of a simple threshold policy is shown to be near that of the optimal single-user policy. This offers support to the threshold decision rules that are employed in current cellular CDMA systems.     

Asynchronous fast frequency-hopping CDMA cellular systems

We address the problem of constructing frequency-hopping codes for use in a fast frequency-hopping code-division multiple-access cellular system with a K-cell frequency-reuse scheme. Different hopping codes are used in different cochannel cells to reduce the cochannel interference. The frequency-hopping patterns possess ideal auto- and cross-correlation properties for use in the intracells and ideal autocorrelation and nearly ideal cross-correlation properties in the cochannel intercells. Additionally the cardinalities (the number of patterns) of the codes for use in the intracells meets the bound and hence are optimal. The maximum number of users which can be accommodated by using the proposed scheme is 20% more than the scheme using the same frequency-hopping code in all cells under a carrier-to-interference ratio of -15 dB     

A novel transmitter design for GLSFBC–OFDM–CDMA communication systems

Relying on mutual orthogonality between subcarriers of different users in orthogonal frequency-division multiple access (OFDMA) systems and mutual orthogonality between spreading codes in code-division multiple-access (CDMA) systems, a novel transmitter design is proposed for group layered space–frequency block code (GLSFBC)–OFDM–CDMA communication systems over frequency-selective fading channels. The proposed method is based on a three-level design of user codes: the top level (based on OFDMA) deals with group interference and intersymbol interference (ISI), the middle level (based on space–frequency block coding) results in space–frequency diversity, and the lower level (based on CDMA) handles multiuser interference. The new approach only needs one receive antenna to distinguish multiple users and suppress group interference simultaneously, so the complexity of the receiver decreases remarkably. Simulation results confirm the validity of the proposed technique.     

Downlink power allocation and adjustment for CDMA cellular systems

This letter provides a downlink power control algorithm and its convergence for adjusting as well as allocating appropriate power levels according to the need of mobiles. The adjustment process and its property provide a distinct view for understanding existing CDMA downlink power control algorithms     

The coverage-capacity tradeoff in cellular CDMA systems

In cellular CDMA systems that employ single-user detectors, in-cell interference limits the coverage of the cell. Thus for a given upper limit on transmit power, the coverage of a cell is inversely proportional to the number of users in it. This tradeoff between coverage and number of users is explicitly characterized here. Our analysis may be used in cellular planning to set hard limits on the number of users admitted into the cell in order to meet coverage requirements. Furthermore, our approach allows us to arrive at a precise definition for the pole capacity of a cell, which serves as an upper bound on the number of users a cell can support as the coverage shrinks to zero. We also present a technique to calculate cell coverage as a function of carried traffic, for any given admission policy     

Overview of radiolocation in CDMA cellular systems

Applications for the location of subscribers of wireless services continue to expand. Consequently, location techniques for wireless technologies are being investigated. With code-division multiple access (CDMA) being deployed by a variety of cellular and PCS providers, developing an approach for location in CDMA networks is imperative. This article discusses the applications of location technology, the methods available for its implementation in CDMA networks, and the problems that are encountered when using CDMA networks for positioning