Experiments on pilot symbol-assisted coherent multistageinterference canceller for DS-CDMA mobile radio

A three-stage coherent multistage interference canceller (COMSIC) employing pilot symbol-assisted (PSA) channel estimation for replica generation of multiple access interference (MAI) is implemented and its performance in the presence of frequency selective multipath fading is experimentally evaluated by a multipath fading simulator. A fast transmission power control (TPC) method suitable for COMSIC is also proposed, in which the signal-to-interference plus background noise power ratio (SIR) at the matched filter (MF) based RAKE receiver is measured to achieve a short TPC delay and the target signal-to-interference ratio value is compensated by an outer loop so that the measured block error rate (BLER) is equal to the prescribed target value. The experimental results show that as expected the COMSIC satisfactorily reduces the MAI even when the number of active users is equal to the spreading factor in a multipath fading environment, and thus, improves the bit error rate (BER) performance in a multiuser environment. The results also show that the proposed fast TPC method with a two-slot delay associated with COMSIC works satisfactorily and the combination of COMSIC and fast TPC significantly decreases the transmission power of a mobile station (required transmission power of a mobile station with COMSIC at the average BER of 10^-3 is decreased by approximately 2.0 (3.0) dB compared with the MF-based RAKE receiver with (without) antenna diversity reception). This extends the cell coverage, battery life, and increases the system capacity in the reverse link     

Analysis and optimization of pilot symbol-assisted Rake receivers for DS-CDMA systems

The effect of imperfect channel estimation (CE) on the performance of pilot-symbol-assisted modulation (PSAM) and MRC Rake reception over time- or frequency-selective fading channels with either a uniform power delay profile (UPDP) or a nonuniform power delay profile (NPDP) is investigated. For time-selective channels, a Wiener filter or linear minimum mean square error (LMMSE) filter for CE is considered, and a closed-form asymptotic expression for the mean square error (MSE) when the number of pilots used for CE approaches infinity is derived. In high signal-to-noise ratio (SNR), the MSE becomes independent of the channel Doppler spectrum. A characteristic function method is used to derive new closed-form expressions for the bit error rate (BER) of Rake receivers in UPDP and NPDP channels. The results are extended to two-dimensional (2-D) Rake receivers. The pilot-symbol spacing and pilot-to-data power ratio are optimized by minimizing the BER. For UPDP channels, elegant results are obtained in the asymptotic case. Furthermore, robust spacing design criteria are derived for the maximum Doppler frequency.     

Comparison of pilot symbol-assisted and differentially detectedBPSK for DS-CDMA systems employing RAKE receivers in Rayleigh fadingchannels

The capacity of a code-division multiple-access (CDMA) system is a function of the bit error rate (BER) performance of individual users. Therefore, it is important to optimize the individual links before proceeding to system level analysis. This is particularly true for operating in a fading channel where the performance without diversity reception is rather poor. This paper compares the BER performance of differential detection and pilot symbol-assisted coherent detection of a direct-sequence (DS) spread-spectrum (SS) signal on a frequency-selective Rayleigh fading channel using RAKE reception. Both equal gain and maximal ratio combining are considered, and the effect of convolutional coding with interleaving is studied. It is shown that in the particular cases considered in this paper, rate 1/8 convolutionally encoded pilot symbol-assisted BPSK performs better than coded differential detection, thus providing a higher system capacity     

Pilot symbol-assisted coherent multistage interference cancellerusing recursive channel estimation for DS-CDMA mobile radio

A pilot symbol-assisted coherent multistage interference canceller using recursive channel estimation is proposed for DS-CDMA mobile radio cellular system. Since the channel variation caused by fading is recursively estimated at each interference cancelling stage, the accuracy of channel estimation is improved successively. Computer simulation results show that the required E_b/N₀ at the average BER of 3×10^-2 is improved by ~3.5 dB compared to the matched filter receiver for 10 users and two paths with equal power, and where f_dT=10^-3 (f_d: fading maximum Doppler frequency, T: data symbol duration)     

Chip-delay locked matched filter for DS-CDMA systems using longsequence spreading

This paper considers an improved single-user detection technique for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems using long sequence spreading (random-CDMA) Most of the known detection schemes for DS-CDMA suffer from either poor performance under power-imbalance (near-far like) conditions, excessive complexity, or incompatibility with systems employing long sequence spreading. To address these problems, this paper considers a signal-to-noise ratio maximizing linear time-invariant filter for one-shot bit symbol detection exploiting some information about the interferers. This filter, referred to as the chip-delay locked matched filter (CLMF), exploits the cyclostationarity in multiple-access interference, and it can offer good near-far resistance while remaining suitable for systems with long sequence spreading. The CLMF requires knowledge of interferers chip delays and signal powers; however, knowledge of their pseudonoise sequences is unnecessary. This paper also demonstrates the improvement in performances offered by the CLMF over other single-user receivers such as the conventional matched filter and noise-whitening matched filter performance is evaluated in terms of probability of outage for single-rate and dual-rate DS-CDMA systems using bandwidth-efficient chip pulses, over a single-path additive white Gaussian noise channel. Errors in the interferer chip delay estimates degrade the CLMF performance. However, if the root-mean-square value of these errors is less than 5% of the chip interval, then this degradation is small     

Channel coding scheme for orthogonal multicarrier DS-CDMA system

The scheme does not reduce the transmission rate of information nor increase the transmitted signal bandwidth of the uncoded multicarrier system. Simulation results for the suggested scheme in a multipath fading channel show considerable coding gain over the uncoded case     

Two-stage Wiener filter based cancellation receivers for DS-CDMA

The authors present the two-stage Wiener filter based cancellation receiver for DS-CDMA, i.e. a receiver using Wiener filters to regenerate the interferers and to make the final despreading. Theoretical calculation as well as simulation results show that the two-stage Wiener filter performs considerably better than any other two-stage receiver     

Coherent optical transversal filter using silica-based waveguidesfor high-speed signal processing

A coherent optical transversal filter with a tapped delay-line structure is described. Utilizing phase information this filter can express arbitrary tap coefficients (including general complex numbers) by coherently combining tapped signals. Experimental filters using silica-based single-mode optical waveguides, monolithically integrated on silicon substrates were developed for stable and high-speed operation. Experimental results demonstrate the operating capabilities of the filters for high-speed signal processing. The operating bandwidth was experimentally estimated. Degradation of the frequency characteristics due to both deviation of fiber fabricating parameters and errors in setting tap-coefficients has been estimated theoretically     

Coherent optical orthogonal frequency division multiplexing

Coherent optical orthogonal frequency division multiplexing is proposed to combat dispersion in optical media. It is shown that optical-signal-to-noise ratio penalty at 10 Gbit/s is maintained below 2 dB for 3000 km transmission of standard-singlemode fibre without dispersion compensation.     

Design of IIR orthogonal wavelet filter banks using lifting scheme

The lifting scheme is well known to be an efficient tool for constructing second generation wavelets and is often used to design a class of biorthogonal wavelet filter banks. For its efficiency, the lifting implementation has been adopted in the international standard JPEG2000. It is known that the orthogonality of wavelets is an important property for many applications. This paper presents how to implement a class of infinite-impulse-response (IIR) orthogonal wavelet filter banks by using the lifting scheme with two lifting steps. It is shown that a class of IIR orthogonal wavelet filter banks can be realized by using allpass filters in the lifting steps. Then, the design of the proposed IIR orthogonal wavelet filter banks is discussed. The designed IIR orthogonal wavelet filter banks have approximately linear phase responses. Finally, the proposed IIR orthogonal wavelet filter banks are applied to the image compression, and then the coding performance of the proposed IIR filter banks is evaluated and compared with the conventional wavelet transforms.     

Coherent and Differential Downlink Space-Time Steering Aided Generalised Multicarrier DS-CDMA

This paper presents a generalised multicarrier direct sequence code division multiple access (MC DS-CDMA) system invoking smart antennas for improving the achievable performance in the downlink. In this contribution, the MC DS-CDMA transmitter employs an antenna array (AA) and steered space-time spreading (SSTS). Furthermore, the proposed system employs both time and frequency (TF) domain spreading for extending the capacity of the system, which is combined with a user-grouping technique for reducing the effects of multi-user interference (MUI). Moreover, to eliminate the high- complexity multiple input multiple output (MIMO) channel estimation required for coherent detection, we also propose a Differential SSTS (DSSTS) scheme. More explicitly, for coherent SSTS detection MVN_r number of channel estimates have to be generated, where M is the number of transmit AAs, V is the number of subcarriers and N_r is the number of receive antennas. This is a challenging task, which renders the low-complexity DSSTS scheme attractive.     

Initial synchronization of DS-CDMA via bursty pilot signals

We consider initial timing acquisition in discrete-sequence code-division multiple-access (DS-CDMA) when propagation is affected by multipath and fading, and where the base-station broadcasts a synchronization pilot signal in the form of bursts of modulated chips transmitted periodically and separated by long silent intervals. Subject to certain simplifying assumptions, we derive the maximum-likelihood (ML) estimator by solving a constrained maximization problem. Our ML timing estimator has constant complexity per observation sample. The relation to other estimation methods is addressed, and performance comparisons are provided by simulation. The proposed estimator yields good performance independently of the multipath-intensity profile of the channel, provided that the delay spread is not larger than a given maximum spread. Moreover, our estimator is fairly robust to the mismatch in the fading Doppler spectrum and provides good performance for both fast and slow fading     

A 10.7-MHz CMOS SC radio IF filter using orthogonal hardwaremodulation

FM radio receivers require an IF filter for channel selection, customarily set at an IF center frequency of 10.7 MHz. Up until now, the limitations of integrated radio selectivity filters in terms of power dissipation, dynamic range, and cost are such that it is still required to use an external ceramic 10.7-MHz bandpass filter. This paper demonstrates a CMOS switched-capacitor IF filter that can be integrated with most of the rest of the FM receiver, eliminating external components and printed circuit board area. This is made possible through a combination of two techniques: orthogonal hardware modulation, and delta-charge redistribution. It exhibits a tightly controlled center frequency with a Q of 55 and also contains a programmable gain. The filter occupies an area of 0.7 mm² in a 0.6 μm CMOS process with poly-poly capacitors. The new filter requires only 16 mW of power, and this is offset by elimination of the power needed in current designs to drive off-chip filters     

An investigation into time-domain approach for OFDM channelestimation

A time-domain based channel estimation for OFDM system with pilot-data multiplexed scheme is investigated. As an approximation to linear minimum mean square estimator (LMMSE), a time-domain based channel estimation is proposed where intra-symbol time-averaging and most significant channel taps selection are applied. The relation and differences of the proposed method to DFT-based LMMSE methods are discussed. The performances of the proposed method, DFT-based LMMSE method and the methods of Chini, Wu, El-Tanany and Mahmoud (see IEEE Trans. on Broadcasting, vol.44, no.1, p.2-11, 1998) and of Yeh and Lin (see IEEE Trans. on Broadcasting, vol.45, no.4, p.400-409, 1999) are evaluated in multipath fading channels. The simulation results show that proposed method achieves almost the same performance as DFT-based LMMSE method and better BER performance than the other methods while keeping less complexity     

Multidimensional orthogonal filter bank characterization and design using the Cayley transform.

We present a complete characterization and design of orthogonal infinite impulse response (IIR) and finite impulse response (FIR) filter banks in any dimension using the Cayley transform (CT). Traditional design methods for one-dimensional orthogonal filter banks cannot be extended to higher dimensions directly due to the lack of a multidimensional (MD) spectral factorization theorem. In the polyphase domain, orthogonal filter banks are equivalent to paraunitary matrices and lead to solving a set of nonlinear equations. The CT establishes a one-to-one mapping between paraunitary matrices and para-skew-Hermitian matrices. In contrast to the paraunitary condition, the para-skew-Hermitian condition amounts to linear constraints on the matrix entries which are much easier to solve. Based on this characterization, we propose efficient methods to design MD orthogonal filter banks and present new design results for both IIR and FIR cases.     

Multiuser detection for DS-CDMA systems using evolutionary programming

This article introduces a new multiuser detection scheme which uses evolutionary programming (EP) to detect the user bits based on the maximum-likelihood decision rule. The major advantage of the proposed detector is that it has a lower computational complexity compared to other popular evolutionary-algorithm-based detectors. The simulation results show that the EP has always converged to the optimum solution with a small number of generations. The simulated average computational time performance demonstrates that this approach achieves practical ML performance with polynomial complexity in the number of users.     

Performance of DS-CDMA over measured indoor radio channels usingrandom orthogonal codes

Direct sequence spread spectrum, with its inherent resistance to multipath interference, has become a commercial reality for indoor wireless communications and has been proposed for personal communication networks. To allow multiple users within the limited bandwidths allocated by the FCC, code-division multiple-access (CDMA) is needed. This paper analyzes the performance of CDMA systems using random orthogonal codes over fading multipath indoor radio channels using channel measurements from five different buildings. The effect of RAKE receiver structure is studied, as is the effect of average power control. The average probability of error as a function of signal-to-noise ratio is used as the performance criteria. Results are compared with models for Rayleigh fading channels     

Self-adjusting orthogonal digital filter for identification of unknown processes

A digital filter with independent adjustable parameters which may be used for online identification of linear or nonlinear unknown processes is described. Algorithms for constructing the components of the filter and computing the optimum values of the adjustable parameters are given.     

An improved orthogonal digital filter structure

A novel structure is derived for digital filter implementation. This structure is actually an improved version of an existing one in terms of implementation efficiency and reducing finite word length (FWL) effects. Expression of roundoff noise gain is obtained for the proposed structure. Design examples are given to demonstrate the performance of this structure and to compare it with the existing one and the classical minimum roundoff state-space realizations. Numerical examples show that the proposed structure outperforms the others in terms of minimizing roundoff noise as well as implementation efficiency.     

Least mean squares algorithm for fractionally spaced blind channelestimation

The authors consider the problem of blind estimation and equalisation of digital communication finite impulse response (FIR) channels using fractionally spaced samples. The system input is assumed to be a deterministic but unknown data sequence. Exploiting the periodicity of the transmitted data sequence in the frequency domain in the noise free case, it is shown that it is possible to form a linear system in terms of the unknown channel impulse response. In the presence of noise, a least mean squares (LMS) criterion is used to resolve the channel. The resulting algorithm has an appealing interpretation and can be considered as a single channel counterpart of the multi-channel cross-relation (CR) method. Finally, it is shown that the latter can be derived from the proposed algorithm