OTFS faces inter-symbol interference (ISI) due to delay and/or Doppler spreads, which requires compensation with an equalizer. In addition, in multiple access scenarios where users traverse distinct channels, OTFS also encounters multi-user interference (MUI). Hence, MUI is more challenging to mitigate than intra-user ISI and becomes more severe in high-mobility scenarios, where OTFS is typically deployed. This paper proposes a novel method called pre-equalized multi-user OTFS (PreEQ-MU-OTFS), which applies pre-equalization at the transmitter, using linear precoders for each user to fundamentally mitigate MUI. Specifically, the precoder is derived by initializing with the modified Wiener filter and employing a single step of iterative minimization. Additionally, the receiver in this method requires only per-user scaling, instead of full equalization. By leveraging the property of OTFS whereby fast-varying channels exhibit nearly time-invariant characteristics in the delay-Doppler (DD) domain, PreEQ-MU-OTFS performs effectively in high-mobility channels. The proposed method demonstrates superior bit error rate (BER) performance compared to conventional post-equalization receivers such as minimum mean square error (MMSE), MMSE-successive interference cancellation (MMSE-SIC), message passing (MP), and maximum-ratio combining (MRC) receivers. This method is validated in realistic channel scenarios with moving reflectors and practical channel aging levels.

In this paper, we propose a novel high-mobility-resilient waveform referred to as SWRI-OTSM (Small Walsh- Hadamard Transform (WHT) and Random Interleaving Orthogonal Time Sequency Multiplexing). This method reduces the peak-to-average power ratio (PAPR) without requiring side information (SI) while preserving time domain diversity to enhance bit error rate (BER) performance. A recently proposed PAPR reduction method for OTFS, the discrete Fourier transformspread- Orthogonal Time Frequency Space (DFT-s-OTFS), applies DFT precoding along the Doppler axis to lower PAPR without SI. However, this approach sacrifices time domain diversity effect, leading to performance degradation in high-mobility channels. In contrast, the proposed SWRI-OTSM divides the delay-sequency (DS) domain data matrix into sub-blocks along the sequency axis and applies a small size inverse WHT (IWHT) to each sub-block, transforming the data matrix into the delay-time (DT) domain. While this sub-block-based approach reduces time domain diversity, inter-sub-block column interleaving compensates for the diversity loss. We analyze the fundamental principle behind the performance improvement of SWRI-OTSM and extend it into a generalized version that maintains equivalent performance. Simulation results demonstrate that sub-block-based IWHT reduces the number of superposed signals in the DT domain, thus lowering the PAPR relative to DFT-s-OTFS and OTSM. Moreover, the interleaving process effectively preserves time domain diversity, resulting in superior BER performance. The reduced computational complexity makes our proposed method especially practical for real-world implementations.

We propose a novel low peak-to-average power ratio (PAPR) reduction technique for orthogonal time sequency multiplexing (OTSM) using an adaptively scrambled Walsh-Hadamard transform (S-WHT). Conventional PAPR reduction techniques such as selective mapping (SLM) and partial transmit sequence (PTS) operate in the frequency domain and are less effective for two-dimensional (2D) waveforms like OTSM and orthogonal time-frequency space (OTFS). While discrete Fourier transform-spread OTFS (DFT-s-OTFS) mitigates PAPR by applying Doppler-axis precoding, it sacrifices time domain diversity, degrading performance in high-mobility environments. To address these limitations, we propose a PAPR reduction method that adaptively selects the optimal S-WHT matrix for each row of the delay-sequency (DS) domain data matrix, preserving the inherent time diversity of OTSM. A systematic procedure is developed to construct a globally optimized S-WHT set that minimizes PAPR while maintaining signal orthogonality. Furthermore, leveraging the constellation invariance property of the proposed waveform, we introduce a novel phase rotation-based side information (SI) embedding and detection scheme, eliminating the need for explicit SI transmission. Simulation results show that the proposed low-PAPR OTSM substantially outperforms conventional OTFS, OTSM, DFT-s-OTFS, and SLM-based OTSM in both PAPR and bit error rate (BER) performance.

In cognitive radio (CR) networks, non-contiguous orthogonal frequency division multiplexing (NC-OFDM) is a widely adopted overlay-based CR technique. However, it suffers from serious performance degradation in high-speed mobile environments due to its susceptibility to high Doppler shifts, a vulnerability inherited from OFDM. To overcome this drawback, we introduce non-contiguous orthogonal time frequency space (NC-OTFS). Unlike OFDM, OTFS spreads data symbols across all sub-carriers using the inverse symplectic finite fourier transform (ISFFT), making it challenging to deactivate unwanted sub-carriers. To address this issue, we propose two types of NC-OTFS schemes: Type 1, which generates individual OTFS signals for each disjoint contiguous sub-carrier block, and Type 2, which creates a single OTFS signal across all available non-contiguous sub-carrier blocks to achieve the maximum frequency diversity. Furthermore, we present an enhanced time domain block minimum mean squared error (MMSE) demodulation algorithm to mitigate primary user (PU) interference in the proposed NC-OTFS receivers. In addition, we provide an implementation of Type 2 NC-OTFS transceiver by adding simple pre-processing and post-processing steps to the conventional OTFS transceiver, which allows reusing all the transceiver algorithms for the conventional OTFS. Numerical results demonstrate the remarkable superiority of the proposed NC-OTFS over the conventional NC-OFDM, especially in high Doppler environments.

Low-complexity overhead and very small side information (SI) are virtues of the low peak-to-average-power ratio (PAPR) filter bank multicarrier [Low PAPR FBMC (LP-FBMC in short)], which has been recently proposed. However, the SI is an annoying burden because it needs a complicated system design, irrespective of its size. In this paper, we propose an SI-embedded LP-FBMC that does not separately transmit SI but embeds it in the transmitted signal in a specific way while preserving the PAPR and bit error rate (BER) performance. The approach is motivated by a property of the LP-FBMC where demodulation by even an incorrect SI does not change the signal constellation. We embed the SI in the intentional phase rotation of the constellation without additional energy or bandwidth. The phase rotation differs according to the SI and is common to all data symbols in each LP-FBMC subframe. In the receiver, we remove data symbol-dependent phase terms of the decision variables (DVs) by taking the fourth power and detect the SI embedded in the remaining phase term. Simulation results reveal that the BER of the SI-embedded LP-FBMC is almost the same as that of the perfect SI case when there are an acceptable number of data symbols for SI estimation. We also devised a complexity-reduced implementation for the additional processing required for the proposed SI embedding and detection schemes.

Recently, Low PAPR FBMC (LP-FBMC) was proposed to resolve high peak-to-average power ratio (PAPR) issue of filter bank multicarrier with offset quadrature amplitude modulation (FBMC-OQAM). The LP-FBMC has very small side information (SI) burden (two bits per subframe). However, irrespective of SI size, SI itself is a very annoying burden because sending and reception of SI demands complicated system design and very high reliability. In this paper, we propose a modified version of LP-FBMC which gets rid of necessity of sending SI while preserving BER performances. We embed SI information in the phase term throughout the subframe without additional energy or bandwidth. The receiver detects the SI by checking the phase offset of the decision variables with a fractional increase of computational complexity. Simulation results reveal that if the number of data symbols for SI detection is practically large, SI detection error is negligible and the BER is close to the case of perfect SI. In addition, as the modified LP-FBMC is free from SI burden, we can set the LP-FBMC subframe size to minimum, i.e., 1, which maximizes PAPR reduction gain of LP-FBMC.

So far, the solutions for the two major issues of filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM), i.e., 1) high PAPR and 2) incompatibility with MIMO have been devised exclusive of each other; and thus, the signal formats and processings are also exclusive. Therefore, how to constructively combine their solutions into a single system is another challenging issue. As a specific solution to this, we extend the idea of DFT spreading-based low peak to average power ratio (PAPR) FBMC scheme to the ICI-free Alamouti-coded FBMC. In order to inherit the merits of the two schemes, we first provide the required modifications for combining the two schemes. A mathematical analysis confirms that even with frequency reversal Alamouti code arrangement and subblock partitioning, the proposed scheme still achieves the single carrier effect of DFT spreading. The simulation results reveal that the proposed scheme achieves substantially lower PAPR than the previous Alamouti-coded FBMC scheme. Moreover, owing to the additional frequency diversity via DFT spreading, the proposed scheme with properly chosen subblock size achieves even lower BER than the theoretical Alamouti-coded BER with a diversity order of two. Considering the PAPR, BER, and data rate loss, we conclude that the proposed scheme is a competitive solution for Alamouti-coded multicarrier system.

To further enhance frequency-asynchronous distributed Alamouti-coded (FADAC) orthogonal frequency division multiplexing (OFDM), we propose a new scheme which combines the partial maximum likelihood detection (PMLD) to the residual intercarrier interference cancellation (RIC). In order to decrease the performance gap from intercarrier interference- (ICI-) free level after single time iteration of the RIC, the final stage of the proposed scheme performs the PMLD limited to the symbols of less-reliable decision variables. We show that with the practically acceptable candidate symbol set size, a single iteration for RIC is enough to achieve the ICI-free performance. Moreover, the proposed scheme substantially expands the allowable ranges of the three undesirable terms, i.e., the timing and frequency offsets between the transmit antennas and the multipath delay spreads.

Unlike single carrier-frequency division multiple access (SC-FDMA), just combining discrete Fourier transform (DFT) spreading and filter bank multicarrier with offset quadrature amplitude modulation (FBMC-OQAM) results in only marginal peak to average power ratio (PAPR) reduction. To utilize the single carrier effect of DFT spreading, a special condition of the coefficients at each subcarrier's in-phase and quadrature-phase (IQ) channels should be satisfied. As a starting point, we first derive this condition, which we call the identically-time-shifted-multicarrier (ITSM) condition. Then, based on this condition, we propose a new type of FBMC for low PAPR. The main features of the proposed scheme are summarized as follows. First, in order to further enhance the amount of PAPR reduction, we generate the four candidate versions of the DFT-spread and ITSM-conditioned FBMC waveform and select the one with minimum peak power. Even with multiple candidate generation, the major computation parts, such as DFT and IDFT are shared and need to be performed only once, unlike the conventional side information (SI)-based PAPR reduction schemes. Consequently, with a fractional complexity overhead compared with the previous DFT-spread FBMC, the proposed scheme achieves a PAPR reduction comparable to that of SC-FDMA. Second, the proposed scheme transmits only two bit SI per data block consisting of multiple FBMC-OQAM symbols. Hence, the SI overhead is significantly low compared with the usual SI-based schemes, such as selective mapping or partial transmit sequence.

The authors propose a dual mode transmit diversity (TD) scheme switching between co‐located TD (CTD) and distributed TD (DTD) based on the quasi‐static channel parameters. The authors first compare CTD and DTD according to spatial correlation coefficient (SCC) of CTD and distributed mean channel gain (MCG) of DTD. By using asymptotic bit error rate (BER) formula, the authors derive a compact and very accurate border equation of DTD‐preferred and CTD‐preferred regions in MCG ratio and SCC space. Based on the derived border, the authors propose a dual mode TD scheme switching between CTD and DTD according to MCG ratio and SCC. It is shown that the signal‐to‐noise ratio (SNR) gain contours and the border line for TD switching are irrespective of SNR and modulation scheme. The results show that the proposed dual mode scheme outperforms the single modes not only in the location‐specific performance but also in the averaged performance over the cell area. In the averaged BER over the entire cell area, performance improvement of the dual mode scheme over CTD is significant if SCC is roughly larger than 0.6 irrespective of cell shape, spatial correlation model of CTD, path loss exponent and shadowing model.

Unlike the conventional distributed Alamouti coded orthogonal frequency division multiplexing (OFDM), over‐sampling substantially improves the performance of frequency asynchronous distributed Alamouti coded (FADAC) OFDM. This exclusive effectiveness of over‐sampling in FADAC‐OFDM comes from two factors. One is the unique characteristics of residual inter carrier interference term in FADAC‐OFDM and the other factor is moving away the cyclic harmonic inter‐carrier interference (ICI) by over‐sampling. In addition, the authors confirm that over‐sampling factor of 2, i.e. 2 N point fast Fourier transform (FFT) is sufficient to move the cyclic harmonic ICI. From this investigation, the authors propose 2 N point FFT FADAC‐OFDM. The proposed scheme achieves the significantly improved performance not only in the flat fading channel but also in the selective fading channels. For example, while the non‐oversampled FADAC‐OFDM has the unacceptable error rate level near to or above 0.1 at the band edges even with high signal to noise ratio, the proposed scheme achieves near intersymbol interference‐free performance. This leads the proposed scheme to achieve 50% reduction of the required number of null subcarriers to achieve ICI free performance compared with the non‐oversampled FADAC‐OFDM. Moreover, in terms of implementation feasibility and computational complexity, the proposed scheme is shown to be acceptable.

In the paper “Energy-efficient Uplink Multi-user MIMO,” it was mentioned that the employed linear receiver does not change the variance of the elements of the noise vector. Then, the performance was calculated based on this proposition. In this note, we show that this proposition does not hold by providing the counter examples. In addition, we analyze how the performance will be affected by fixing this proposition.

The authors propose a generalised framework that analytically compares different sub‐carrier allocation (SA) or sub‐channelisation schemes (such as interleaved SA (ISA), localised SA (LSA) or hybrid schemes) in uplink (UL) orthogonal frequency division multiple access (OFDMA). The ultimate goal of the proposed framework is to systematically determine the best SA scheme among the considered candidates for a given condition of inter‐user frequency offsets (IUFOs) and the target bit error rate (BER). As an illustration, two typical SA schemes are considered, that is, ISA and LSA for comparison. First, based on the well‐known fact that multiple access interference by IUFO and frequency diversity gain are both dictated by the employed SA scheme, the authors propose a semi‐analytic approach to derive coded BER curve formula as a function of frequency offset bound (FOB) for each scheme. Then, the signal‐to‐noise ratio gain of ISA over LSA for various target BERs and FOBs is derived. Finally, the cutoff FOB over which ISA obtains worse than LSA is obtained and its functional relationship with the target BER is investigated. By following the overall procedure in the proposed framework, the best SA scheme among the various SA candidates for the general UL OFDMA systems with arbitrary system parameters can be chosen.

ABSTRACT We propose a very simple and efficient soft linear multi‐input multi‐output (MIMO) detection scheme. The detection process is divided into two separate problems. The proposed scheme first detects MIMO symbols using conventional linear detection methods and produces soft bit information using a simple soft demapping method. Next, we refine the soft information by accounting for uneven post‐detection noise variance across MIMO layers. From the simulation result investigated in this paper, we first emphasize that powerful channel coding may suppress the differences of diversity gains among various MIMO detection schemes. This implies that the channel decoding operation may not be transparent to performance gain that resulted from MIMO detection process. The proposed scheme concentrates on accurate estimation of soft post‐MIMO detected information in a very simple manner, rather than concentrating on a complex MIMO detection scheme prior to decoding process. In combination with turbo codes, the proposed scheme produces comparable performance to maximum likelihood detection, even with the simplest scheme such as zero forcing detection, with drastically reduced complexity. Copyright © 2011 John Wiley & Sons, Ltd.

We propose a new Ad hoc OFDMA (Orthogonal Frequency Division Multiple Access) scheme with greatly enhanced tolerance of timing offset among the nodes. In terms of data frame structure, the proposed scheme is similar to code spread OFDMA or MC DS CDMA. The main different aspect is that we employ a cyclic extended PSW (Properly Scrambled Walsh-Hadamard) code as the spreading sequences for the Ad hoc nodes. Exploiting the property that PSW code has zero correlation despite ±1 chip timing offset, the proposed Ad hoc OFDMA achieves internode- interference free performance with the timing offset up to ±1 OFDM symbol duration. In contrast, the other existing schemes in comparison undergo severe performance degradation even with small timing offset under multipath fading channel.

In this paper, we propose a simple and efficient soft detection scheme for turbo coded multi-input multi-output (MIMO) system using minimum mean square error (MMSE) scheme. Although the maximum performance can be achieved using a maximum likelihood (ML) soft detection scheme, the complexity increases exponentially by the number of antennas and modulation orders. In this paper, we divide the soft detection process into three separate sequential problems. In combination with turbo codes, the proposed soft MMSE scheme produces comparable performance to ML detection with drastically reduced complexity.

An optimum power and rate allocation rule for cellular code division multiple access (CDMA) downlink networks is derived; it minimises mean transmission delay of the active users with different channel qualities under the constraints of total transmission power and the required signal-to-interference ratio. The proposed rule states that the power assigned to a user is inversely proportional to the square root of the user's channel quality factor, whereas the rate assigned to a user is proportional to the square root of the user's channel quality factor. The amount of reduction in average mean transmission delay by the proposed optimum allocation rule against the equal rate allocation with power control and the equal power allocation with rate control is also derived for the case when each user undergoes shadowing, path loss and inter-cell interference. It is shown that the proposed allocation rule not only achieves the minimum mean transmission delay, but also minimises the probability of the data rate being less than a tolerable threshold, against the equal rate or equal power allocation schemes.

We study two power allocation (PA) schemes for distributed transmit diversity systems. We first derive the performance of the instantaneous channel gain feedback-based PA (ICG-PA) scheme in the presence of channel variation during feedback delay. We then study channel gain variance feedback-based PA (CGV-PA) to mitigate the performance degradation of ICG-PA caused by feedback delay. Finally, we derive design rules for optimum CGV-PA from a compact and accurate performance expression derived.

Inversion of channel matrix is required for commonly used detection schemes in multiple-input multiple-output (MIMO) systems. In time-varying fading channels, frequent matrix inversion is computationally intensive for mobile terminals operating at high data rates. Several existing papers have addressed this problem for MIMO orthogonal frequency division multiplexing systems by employing interpolation since the channel coefficients are correlated in the frequency domain. The correlation of the channel in the time domain, i.e., the channel matrices at consecutive symbol intervals vary only slightly, could also be exploited. We propose an algorithm that exploits second-order extrapolation in the time domain to lower the computational complexity of matrix inversion. While existing schemes are mainly designed for linear MIMO detection, the proposed algorithm can be applied for both non-linear detection such as ordered successive interference cancelation (OSIC) and linear detection. The proposed scheme can be efficiently implemented with only addition and integer multiplication. Simulation results of the proposed scheme applied in MIMO OSIC detection demonstrate that it can significantly reduce the matrix inversion complexity while maintaining the system performance.

We propose a computationally efficient lattice reduction (LR) algorithm for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in frequency-selective fading channels. In our proposed algorithm, we exploit the inherent feature of unimodular transformation matrix P that remains the same for frequency components which have relatively high amplitude correlation. We eliminate the redundant calculations by reducing brute-force LR iterations among adjacent subcarriers. We simulate the error performance and complexity of the proposed algorithm under various MIMO-OFDM system configurations. The results demonstrate that the proposed algorithm could significantly reduce the LR complexity by up to 90% multiplications and 99% divisions of brute-force LR while maintaining the system performance.

In this paper, we present a soft zero forcing (ZF) scheme for turbo-coded multi-input multi-output (MIMO) schemes. The complexity of maximum likelihood (ML) soft detection for turbo coded MIMO schemes increases exponentially by the number of antennas and modulation orders, and thus it is a NP hard problem. In this paper, we divide the soft detection process as three sequential steps and this highly reduces the computational complexity. The three sequential steps include ZF detection of the transmitted modulation symbol, soft demapping, and application of proper channel gain. We demonstrate the simulation results on fast and slow fading channels and show that the proposed soft ZF scheme can achieve just a dB power loss with dramatically reduced detection complexity compared to ML detection.

We present a very low complexity QRD-M algorithm for MIMO systems. The original QRD-M algorithm decomposes the MIMO channel matrix into upper triangular matrix and applies a limited tree search. To accomplish near- MLD(Maximum Likelihood Detection) performance for QRD-M algorithm, number of search points at each layer must be the modulation size. In the proposed scheme, each of survival branches are extended only to the corresponding QR decomposition (QRD)-based detection symbol in the next layer and its neighboring symbols in the constellation. Using this approach, we can significantly decrease the complexity of conventional QRD-M algorithm. Simulation results show that the proposed algorithm scheme achieves the detection performance near to that of the MLD with negligibly low complexity.

This paper proposes a new adaptive K-best algorithm for MIMO systems. The proposed scheme controls the number of survivor paths, K based on the degree of the reliability of zero-forcing (ZF) estimates at each K-best step. The critical drawback of the fixed K-best detection is that the correct path's metric may be temporarily larger than K minimum paths metrics due to imperfect interference cancellation by the incorrect ZF estimates. So, the conventional variable K-best schemes control K according to measured SNR value. However, these schemes still have the problem that needs to accurately and dynamically measure SNR for optimal setting of K. In the proposed variable scheme, we accomplish adaptation of K without necessity of SNR measurement. It is found that the ratio of the minimum path metric to the second minimum is a good reliability indicator for the channel condition. By adaptively changing K based on this ratio, the proposed scheme effectively achieves the performance of large K-best system while maintaining the overall average computation complexity much smaller than that of large K-best system without the necessity of SNR value estimation.

We propose a new adaptive K-best algorithm for MIMO systems where the number of survivor paths, A is changed based on the degree of the reliability of Zero-Forcing (ZF) estimates at each K-best step. The critical drawback of the fixed K-best detection is that the correct path's metric may be temporarily larger than K minimum paths metrics due to imperfect interference cancellation by the incorrect ZF estimates. So, the conventional variable K-best schemes control A according to measured SNR value. However, these schemes still have the problem that needs to accurately and dynamically measure SNR for optimal setting of A. In the proposed variable scheme, we accomplish adaptation of K without necessity of SNR measurement. It is found that the ratio of the minimum path metric to the second minimum is a good reliability indicator for the channel condition. By adaptively changing K based on this ratio, the proposed scheme effectively achieves the performance of large K-best system while maintaining the overall average computation complexity much smaller than that of large K-best system without the necessity of SNR value estimation.

We propose a modified iterative turbo decoder, which inherently compensates the residual frequency offset remained in the decoder input. Based on the recently proposed phase offset compensation scheme which embeds phase compensation into iterative turbo decoding, we extend it to cover up to the frequency offset compensation. At each iteration, we estimate the residual frequency offset remained in the soft decoder output and compensate it for the next iteration. As iteration goes on, frequency offset is gradually and drastically reduced. Consequently, the severe turbo decoding failure due to even small residual frequency offset can be significantly recovered and then, almost the ideal decoding performance is achieved. Especially for power limited communications under very low SNR environment where the even tiny frequency offset is critical to turbo decoding, the proposed scheme is efficient since it does not require additional transmission power for compensation.

Orthogonal spreading code based on scrambling on Walsh sequence is proposed for quasi-synchronous (QS) CDMA. We derive scramble patterns which achieve zero cross correlation within lower and upper half rows of Walsh-Hadamard sequence matrix for single chip timing offset. By assigning the proposed codes to the users in the QS CDMA, multi-access interference free communication is possible with the number of simultaneous users less than one half of the code length, which is the nominal uplink simultaneous user density in frequency reuse- efficient cellular systems. Even with more than one chip timing offset, the proposed scheme achieves still lower BER compared to the conventional spreading codes. Furthermore, there exist numerous multiple scramble patterns satisfying zero periodic cross correlation among user spreading codes. This allows us to utilize the intrinsic function of long code such as inter-cell interference randomization in multi-cell environment.

In this paper, we propose the enhanced carrier recovery algorithm for very low SNR environment. Especially, in order to increase an accuracy of residual frequency and phase offset estimation under very low SNR with constraints of power-limited communication system, it is achieved by comparing the phases between soft decision outputs of code symbols and the channel symbols at the each turbo decoding iteration. As the number of iteration increases, residual freq. and phase offset are compensated and thus the reliability of the soft decision output at next iteration is increased. In consequence, the complementary cooperation between iteration decoding and carrier recovery improves the final coded BER performance-The proposed techniques can fulfill the stringent up-link link budgets at Ka band by operating at very low SNR.

The effect of changing sampling timing in intermediate frequency (IF) sampling receivers is analyzed for the case when symbol timing recovery is performed by controlling the IF sampling clock. We note that changing the sampling timing in IF sampling causes a phase discontinuity which is detrimental to the stable operation of the phase recovery loop. We propose a new IF sampling scheme which makes it possible to control sampling timing while maintaining the continuous phase of the output samples so that the subsequent phase recovery properly works.

Optimum values for the modulation order M, code rate r, and the number of frequency-hop slots q maximizing the network throughput are obtained based on simulations for frequency-hopped spread-spectrum multiple-access networks, where L Q-ary Reed-Solomon (RS) code symbols are transmitted per hop, and each Q-ary RS code symbol is transmitted using log/sub M/Q M-ary frequency-shift-keying-modulated signals. Network throughput is evaluated under additive white Gaussian noise and Rayleigh fading channels. For the case when the received RS symbol is not interfered by multiple-access interference (MAI), a closed-form expression for the symbol-error probability is derived, and for the case when the symbol is interfered by MAI, simulated symbol-error probabilities are used. It is shown that the optimum M is four or eight, irrespective of the channel environment and the number of users. The optimum code rate is determined primarily based on the channel environment and does not show much dependence on M or Q. It is also shown that for the case of synchronous hopping under Rayleigh fading at high signal-to-noise ratios, the difference in instantaneous power among the interfering users significantly improves the performance, compared with the case when there is no fading. We also consider the case when the receiver erases the symbols that are interfered and compare the performance with the case of the hard decisions receiver.

Optimization for system parameters including the modulation order, diversity order and traffic density to maximize the throughput of FHMA system with multi-level FSK is performed. Theoretical limit on the throughput and optimal system parameters are derived. The derived results can be applied to the conventional synchronous fast FHMA system with MFSK modulation based on the equivalent characteristic in multiple access channel model between both systems.

An adaptive serial search pseudonoise (PN) code acquisition scheme is proposed, in which the detection threshold is scaled by the instantaneous received power measured prior to PN code correlation. We observe that the proposed scheme achieves significantly improved mean acquisition times compared to the conventional nonadaptive schemes under Rayleigh fading and pulsed Gaussian noise jamming. Furthermore, the proposed scheme is shown to be optimum under pulsed Gaussian noise jamming in the sense that it forces the worst case jamming fraction to unity.

The performance of asynchronous slow frequency-hop spread-spectrum multiple-access networks where each user transmits L, M-ary symbols per hop using M-ary frequency-shift keying (FSK) modulation with noncoherent demodulation is investigated. Expressions for the decision variables are derived for a given multiple FSK (MFSK) symbol within a hop hit by K' interfering users under additive white Gaussian noise and Rayleigh fading channel models. For the special case when M=2, an accurate analytic approximation for the average error probability is derived as a function of L and K' and semianalytic Monte Carlo simulations are performed to estimate the probability of error for M larger than 2. The results are used to investigate the dependence of the average symbol error probability on L and M. Finally, the effect of enforcing phase transition between the MFSK symbols within a hop is investigated.

Uncoded antijamming capability of the multicarrier direct-sequence spread-spectrum system proposed by Kondo and Milstein (see ibid. vol.44, p.238-46, 1996 ) is analyzed. Joint pulse/partial-band noise jamming is considered with various subcarrier demodulator output combining schemes. We find that full-band pulse jamming maximizes the error probability for all cases considered. With Rayleigh fading and maximal-ratio combining, the multicarrier system achieves performance very close to that of the single-carrier system without fading even with a very small number of subcarriers.

The main concern of this paper is to estimate the symbol error probabilities of synchronous and asynchronous frequency-hop spread-spectrum multiple-access (FHSS-MA) networks through semi-analytic Monte Carlo simulations. We concentrate on systems transmitting one M-ary (M/spl ges/2) FSK modulated symbol per hop with noncoherent demodulation. The usual practice when analyzing the performance of such networks is to upper-bound the probability of symbol error when a hop is hit by K/sup '//spl ges/1 interfering users with (M-1)/M or 1. Previous work on the derivation of accurate approximations to this probability for the case when M=2 has indicated that the aforementioned bound not only gives excessively pessimistic results but may also lead to wrong tradeoff decisions. Using the simulated values for the error probabilities, we show that a similar argument holds for the cases when M>2 as well. Also, by employing a normalized throughput measure that takes into account the bandwidth and time expansion associated with the modulation order M, we find that there exists an optimum value of M that achieves the maximum possible throughput for the cases when binary and M-ary error correcting codes are employed. Throughput results are also given for the case when the signals from the active users in the network suffer from independent Rayleigh fading.