Timing-synchronization analysis for IEEE 802.11a wireless LANs in frequency-nonselective Rician fading environments

This paper derives and computes the probability of synchronization failure P/sub fail/ for IEEE 802.11a wireless LANs on frequency-flat Rician fading channels. For a frequency offset within /spl plusmn/232 kHz, it is shown that its effect on the synchronization performance is minor. The E/sub ds//N/sub 0/ ratios required to achieve P/sub fail/=10/sup -3/ and 10/sup -4/ are computed, where E/sub ds/ is the data-symbol energy. We find that E/sub ds//N/sub 0/ ratios over 20 dB are generally required for channels with Rician factors K/spl les/6 dB. In particular, E/sub ds//N/sub 0/ ratios that yield P/sub fail/=10/sup -4/ exceed 30 dB for K/spl les/4 dB.     

Impact of RF impairments on a DS-CDMA receiver

This letter provides an analysis of the impact of classical radio frequency (RF) front-end impairments (RF mismatch and direct current (dc) offset) on the performance of direct-sequence code-division multiple-access (DS-CDMA) receivers; such issues are of particular importance in the case of zero intermediate-frequency (Zero-IF) receivers. Using a standard expression of the baseband signals as well as basic properties of spreading codes used in DS-CDMA communications, we show that the consequence of such RF mismatch and dc offset is essentially a degradation of the wanted signal's level and an increase of the noise power. We give closed-form expressions (in E/sub b//N/sub 0/ form) for both. The results are then shown to be in accordance with numerical simulations utilizing the codes of the universal terrestrial radio access in the frequency-division duplex mode downlink standard.     

Curves on a sphere, shift-map dynamics, and error control for continuous alphabet sources

We consider two codes based on dynamical systems, for transmitting information from a continuous alphabet, discrete-time source over a Gaussian channel. The first code, a homogeneous spherical code, is generated by the linear dynamical system s/spl dot/=As, with A a square skew-symmetric matrix. The second code is generated by the shift map s/sub n/=b/sub n/s/sub n-1/(mod 1). The performance of each of these codes is determined by the geometry of its locus or signal set, specifically, its arc length and minimum distance, suitably defined. We show that the performance analyses for these systems are closely related, and derive exact expressions and bounds for relevant geometric parameters. We also observe that the lattice /spl Zopf//sup N/ underlies both modulation systems and we develop a fast decoding algorithm that relies on this observation. Analytic results show that for fixed bandwidth expansion, good scaling behavior of the mean squared error is obtained relative to the channel signal-to-noise ratio (SNR). Particularly interesting is the resulting observation that sampled, exponentially chirped modulation codes are good bandwidth expansion codes.     

Log-likelihood ratio (LLR) conversion schemes in orthogonal code hopping multiplexing

Log-likelihood ratio (LLR) conversion schemes are proposed to reduce the effect of perforations that occur in orthogonal code hopping multiplexing (OCHM), which was previously proposed to accommodate more downlink channels than the number of orthogonal codewords. The proposed LLR conversion schemes greatly reduce the required signal-to-noise ratio (SNR) in channel decoding even when the perforation probability is high. The performance of the proposed schemes is evaluated by simulation in terms of the required E/sub b//N/sub 0/ for a 1% block error rate.     

Approaching the capacity of multiple access channels using interleaved low-rate codes

We show that the capacity of a Gaussian multiple access channel can be approached by interleaved low-rate codes together with a simple chip-by-chip iterative decoding strategy. Based on a rate /spl ap/ 1/69 code and with a total of 35 simultaneous users (the aggregate rate /spl ap/ 1/2), performance of BER=10/sup -5/ is observed at E/sub b//N/sub 0/ /spl ap/ 1.4 dB, which is close to the corresponding capacity limit (E/sub b//N/sub 0/ /spl ap/ 0 dB).     

A novel non-data-aided symbol timing recovery technique for OFDM systems

A new highly efficient non-data-aided technique to recover symbol timing of orthogonal frequency-division multiplexing systems is proposed. The algorithm in the proposed work exploits the interference that results due to the loss of orthogonality between subcarriers, where the second-order statistics of the resulting interference is proportional to the offset from the optimum sampling point. The presented technique does not require prior fine carrier synchronization, and it is capable of extracting symbol timing at low E/sub s//N/sub 0/ values with large carrier frequency offsets (CFOs). The system performance was investigated in multipath fading channels with large CFOs and additive white Gaussian noise.     

Experiments on real-time 1-Gb/s packet transmission using MLD-based signal detection in MIMO-OFDM broadband radio access

This paper presents experimental results on real-time packet transmission of greater than 1 Gb/s using 4-by-4 multiple-input-multiple-output (MIMO) multiplexing and maximum-likelihood detection (MLD)-based signal detection with a decreased level of computational complexity in orthogonal frequency-division multiplexing (OFDM) radio access. We apply our previous algorithm called adaptive selection of surviving symbol replica candidates (ASESS) based on the maximum reliability in MLD employing QR decomposition and the M-algorithm (QRM-MLD) to reduce the extremely high level of computational complexity in the conventional MLD. The experimental results using multipath fading simulators are in good agreement with the computer simulation results. The loss in the required received signal energy per bit-to-background noise power spectrum density ratio (E/sub b//N/sub 0/) is suppressed to within approximately 1-2 dB. Therefore, through experiments, we demonstrate that the QRM-MLD employing ASESS is very beneficial in reducing the influence of hardware implementation loss, as well as in decreasing the required received E/sub b//N/sub 0/. We further show that the extremely high-speed real-time packet transmission of greater than 1 Gb/s in a 100-MHz channel bandwidth (i.e., 10 bit/s/Hz) is achieved at the average received E/sub b//N/sub 0/ per receiver antenna of approximately 12 dB using 16QAM modulation and turbo coding with the coding rate of 8/9 in 4-by-4 MIMO multiplexing.     

Power- and bandwidth-efficient communications using LDPC codes

We apply low-density parity-check (LDPC) codes to a bandwidth-efficient modulation scheme using multilevel coding, multistage decoding, and trellis-based signal shaping. Performance results based on density evolution and simulations are presented. Using irregular LDPC component codes of block length 10/sup 5/ and a 64-quadrature amplitude modulation signal constellation operating at 2 bits/dimension, a bit-error rate of 10/sup -5/ is achieved at an E/sub b//N/sub 0/ of 6.55 dB. At this value of E/sub b//N/sub 0/, the Shannon channel capacity, computed assuming equally likely signaling, is below 2 bits/dimension.     

Collision mitigation by log-likelihood ratio (LLR) conversion in orthogonal code-hopping multiplexing

Log-likelihood ratio (LLR) conversion schemes are proposed to mitigate the effect of collisions (or perforations) that occur in orthogonal code-hopping multiplexing (OCHM), which was previously proposed to accommodate more downlink channels than the number of orthogonal codewords. The proposed LLR conversion schemes greatly reduce the required SNR in channel decoding even when the perforation probability is high. The perforation probability in some LLR conversion schemes is estimated and several estimation methods are proposed. An LLR conversion scheme without estimation of the perforation probability is also proposed to avoid accurate estimation. The performance of the proposed schemes is evaluated by simulation in terms of the required E/sub b//N/sub 0/ for a 1% block error rate (BLER).     

Asymptotic expression for the equivocation probability of the NDA FF carrier synchroniser

Equivocation in nonlinear feedforward carrier synchronisers considerably degrades the symbol error rate performance. A simple asymptotic (high E/sub s//N/sub 0/) expression for the equivocation probability is derived for two types of averaging filter and general nonlinearity. The expression is proven accurate in the E/sub s//N/sub 0/ region of practical interest. The results extend previous work.<>     

Optimum erasure threshold in Reed-Solomon coded differential 16 star-QAM modulation

The performance of a Reed-Solomon (RS) coded differential 16 star-QAM modulation scheme with a decision-directed estimation (DDE) technique as the channel-fading parameter in a frequency hopping (FH) system is simulated. The optimum erasure threshold for an RS decoder is obtained. It is shown that with the optimum erasure threshold a bit-error probability of 10/sup -5/ can be achieved at E/sub n//N/sub 0/=21.2 dB for using RS(63,31) and E/sub b//N/sub 0/=21.5 dB for RS(31,15).<>     

Non Data Aided SNR Estimation for OFDM Signals in Frequency Selective Fading Channels

This paper deals with the problem of non data aided (NDA) signal to noise ratio (SNR) estimation of OFDM signals transmitted through unknown multipath fading channel. Most of present day’s SNR estimators are based on the knowledge of pilot sequences which is not applicable in some contexts such as cognitive radio for example. Moreover in Multipath fading channels SNR also depends on frequency offset which is caused by mismatch between the oscillator in the transmitter and that in the receiver. Previous NDA SNR estimation schemes assumed a perfect synchronization at reception (i.e. τ = 0 and ${\varepsilon = 0}$ ) which results estimation of SNR with less accuracy. The frequency offset attenuates the desired signal and causes intercarrier interference, thus reducing the SNR. In this paper we propose a new NDA SNR estimator which uses periodic redundancy induced by the cyclic prefix, considering SNR degradation due to frequency offset ( ${\varepsilon}$ ).     

Visual and quantitative evaluation of selected image combination schemes in ultrasound spatial compound scanning

Multi-angle spatial compound images are normally generated by averaging the recorded single-angle images (SAIs). To exploit possible advantages associated with alternative combination schemes, this paper investigates both the effect of number of angles (N/sub /spl theta//) as well as operator (mean, median, mean-excluding-maximum (mem), root-mean-square (rms), geometric mean and maximum) on image quality (tissue delineation and artifacts), speckle signal-to-noise ratio (SNR/sub s/) and contrast. The evaluation is based on in vitro SAI (/spl plusmn/21/spl deg/ in steps of /spl Delta//spl theta/=7/spl deg/) of formalin fixed porcine tissue containing adipose, connective and muscular tissue. Image quality increased with number of angles up to /spl plusmn/14/spl deg/ after which the improvements became debatable. The mem and median operators, which try to render the images more quantitatively correct by suppressing strong echoes from specular reflectors, provide some improvement in this regard. When combining the SAI with the mean operator, the SNR/sub s/ increases-in general-with N/sub /spl theta//. For N/sub /spl theta//=2, the SNR/sub s/ increases with /spl Delta//spl theta/ as expected. When N/sub /spl theta//=7, the highest SNR/sub s/ is obtained for the mem, rms, and geometric mean operators, while the lowest SNR/sub s/ is obtained for the maximum operator. When comparing SNR/sub s/ for adipose and fibrous tissue, the level is close to 1.91 for adipose tissue but only 1.7 for fibrous tissue which contain relatively few organized scattering structures.     

Simultaneous estimation of echo path and channel responses usingfull-duplex transmitted training data sequences

To reduce the system initialization time in high-speed full-duplex data transmission over two-wire lines, we propose an efficient full-duplex fast training algorithm which can simultaneously estimate the impulse responses of echo paths and channels at both ends. Two mutually orthogonal periodic sequences are designed and used to co-estimate the near echo, the far echo with bulk delay, and the channel response. The new algorithm can reduce the tap-setting time to half of that required by the traditional half-duplex fast training schemes. The effects of channel noise and symbol rate offset between two ends are examined in terms of both mean-square error (MSE) and signal-to-noise ratio (SNR). Both the theoretical analysis and computer simulation show that there are three degrading factors due to symbol rate offset. The SNR is inversely proportional to the sum of the estimated coefficients, the half period of the training sequences, and the square of symbol rate offset. If the far-signal-to-channel-noise ratio is 30 dB, then the degradation is significant when the symbol rate offset is more than 10 ^-4. If it is 40 dB, then the degradation is significant when the symbol rate offset is above 3×10^-5     

Constellation mappings for two-dimensional signaling of nonuniform sources

The design of two-dimensional constellation mappings for the transmission of binary nonuniform memoryless sources over additive white Gaussian noise channels using standard M-ary PSK and QAM modulation schemes is investigated. The main application of this problem is the incorporation of an adaptive mapping assignment in modem devices that employ fixed PSK/QAM modulation schemes for the transmission of heterogenous data (such as multimedia information) containing various levels of nonuniformity. In general, the optimal mapping depends on both the probability distribution of the input signals and the signal-to-noise ratio (SNR) in the channel, in addition to the geometry of the signal constellation. We show that constellation mappings which follow the objective of minimizing the average symbol energy and, given this, maximizing the decoding probability of the most likely signals, can yield symbol-error-rate and bit-error-rate performance that is substantially better than Gray encoding maps. Gains as high as 3.5 dB in SNR E/sub b//N/sub 0/ are obtained for highly nonuniform sources. Finally, we note that the mappings techniques result in nonzero mean constellations and briefly consider their performances when they are converted to zero mean constellations by shifting. In this case, we observe that the shifted zero-mean Gray map outperforms our shifted maps for small- to medium-sized constellations (M/spl les/32), but not for larger sizes.     

Performance analysis of various 16 level modulation schemes under Rayleigh fading

The average bit error rate (BER) performances of 16 square/star QAM and 16 CPSK/DPSK under Rayleigh fading are theoretically analysed and compared. 16 star QAM is inferior to 16 square QAM, which achieves the best performance, by approximately 4 dB in the required average E/sub b//N/sub 0/; however, when compared with differentially encoded square QAM, the performance inferiority is less than 2 dB.<>     

On the low-rate Shannon limit for binary intersymbol interference channels

For a discrete-time, binary-input, Gaussian channel with finite intersymbol interference, we prove that reliable communication can be achieved if, and only if, E/sub b//N/sub 0/>log2/G/sub opt/, for some constant G/sub opt/ that depends on the channel. To determine this constant, we consider the finite-state machine which represents the output sequences of the channel filter when driven by binary inputs. We then define G/sub opt/ as the maximum output power achieved by a simple cycle in this graph, and show that no other cycle or asymptotically long sequence can achieve an output power greater than this. We provide examples where the binary input constraint leads to a suboptimality, and other cases where binary signaling is just as effective as real signaling at very low signal-to-noise ratios.     

Adaptive orthogonal signaling with diversity in a frequency-nonselective Rayleigh fading channel

Power and rate adaptations with diversity are considered for M-ary orthogonal signals in a frequency-nonselective Rayleigh fading channel. We derived the minimum required E~/sub b//N/sub 0/ for a given probability of error, the maximum average data rate, and the outage probability, when maximal-ratio combining is performed with an imperfect channel estimation. We analyzed the effects of channel estimation errors on the required E~/sub b//N/sub 0/ and outage probability, as a function of diversity order and adaptation methods. The relative benefits of rate and power control techniques are further presented in terms of channel estimation error, diversity order, and outage probability. The minimum required E~/sub b//N/sub 0/ with Reed-Solomon codes is also presented for comparison purposes.     

On the capacity and normalization of ISI channels

We investigate the capacity of various intersymbol interference (ISI) channels with additive white Gaussian noise (AWGN). Previous papers showed a minimum E/sub b//N/sub 0/ of -4.6 dB, 3 dB below the capacity of a flat channel, is obtained using water-pouring capacity formulas for the 1+D channel. However, these papers did not take into account that the channel power gain can be greater than one when water-pouring is used. We present a generic power normalization method of the channel frequency response, namely, peak bandwidth normalization (PBN), to facilitate the fair capacity comparison of various ISI channels. Three types of ISI channel, i.e., adder channels, RC channels, and magnetic recording channels, are examined. By using our channel power gain normalization, the capacity curves of these ISI channels are shown.     

Downlink transmission of broadband OFCDM Systems-part I: hybrid detection

The broadband orthogonal frequency and code division multiplexing (OFCDM) system with two-dimensional spreading (time and frequency domain spreading) is becoming a very attractive technique for high-rate data transmission in future wireless communication systems. In this paper, a quasianalytical study is presented on the downlink performance of the OFCDM system with hybrid multi-code interference (MCI) cancellation and minimum mean square error (MMSE) detection. The weights of MMSE are derived and updated stage by stage of MCI cancellation. The effects of channel estimation errors and sub-carrier correlation are also studied. It is shown that the hybrid detection scheme performs much better than pure MMSE when good channel estimation is guaranteed. The power ratio between the pilot channel and all data channels should be set to 0.25, which is a near optimum value for the two-dimensional spreading system with time domain spreading factor (N/sub T/) of 4 and 8. On the other hand, in a slow fading channel, a large value of the channel estimation window size /spl gamma/N/sub T/, where /spl gamma/ is an odd integer, is expected. However, /spl gamma/=3 is large enough for the system with N/sub T/=8 while /spl gamma/=5 is more desirable for the system with N/sub T/=4. Although performance of the hybrid detection degrades in the presence of the sub-carrier correlation, the hybrid detection still works well even the correlation coefficient is as high as 0.7. Finally, given N/sub T/, although performance improves when the frequency domain spreading factor (N/sub F/) increases, the frequency diversity gain is almost saturated for a large value of N/sub F/ (i.e., N/sub F/ /spl ges/ 32).