Multistage multiuser detection for FHMA

A multistage multiuser detector (MMD) is presented for frequency-hopping/code-division multiple access (FH/CDMA.) The MMD reduces the bit-error rate (BER) over the conventional detector by exploiting prior knowledge of the addresses and energies of the user signals. This detector is a conservative multiuser detector which is robust to unknown users and has a complexity that is only linear in the number of users. The performance analysis of the synchronous MMD includes both theoretical and simulation BERs for the noiseless case as well as simulation results that include the presence of noise. The MMD is then extended to the fully asynchronous case which makes this work the first to propose an FH/CDMA multiuser detector for asynchronous communications. This asynchronous MMD is compared via simulation to the conventional detector     

基于分组密码的跳频序列多址接入性能分析

基于3DES迭代型分组密码产生的跳频序列,构造了一种跳频组网的随机多址接入方案模型。在此基础上分析了跳频码分多址的系统接入性能,从理论上推导出跳频图案碰撞引起的误分组率、跳频组网的吞吐量和归一化吞吐率,证明数据分组长度M与可用频隙数q的比值是决定跳频码分多址系统性能的重要因素。计算机仿真结果验证了理论的正确性,同时也证明了基于分组密码的跳频序列具有较好的跳频多址组网性能。     

基于扩频与跳频的多带OFDM-UWB多址技术

根据多带OFDM-UWB通信系统的特点,提出一种基于扩频与跳频的多址技术。该多址技术将多载波码分多址和跳频多址相结合,构成一种新颖的多址技术-正交频分多址(OFDMA),实现多载波码分多址技术和跳频多址技术的优势互补,此接收机的复杂性明显小于常规时频码的多用户接收机。仿真结果表明,基于OFDMA多址技术方案的UWB系统在系统误比特率性能上优于常规时频码方案约5dB。     

Analysis of FHMA performance on block cipher based frequency-hopping sequences

Based on the triple Data Encryption Standard (DES) block cipher frequency-hopping (FH) sequences, a kind of packet protocol for frequency-hopping multiple-access (FHMA) network is suggested in this paper. In theory, the performances of both the packet error and the throughput are derived, which shows that the ratio of the packet length M to the frequency slots number q is most key parameter to determine the FHMA system performance under the condition of large user number. Computer simulation results of 3-DES FH sequences compared with those of the chaotic FH sequences are also included to support our developments.     

Multiuser detection for DS-CDMA UWB in the home environment

We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that the adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate the adaptive MMSE is able to reject a narrowband IEEE 802.11a OFDM interferer, even for signal-to-interference ratio as severe as -30 dB. We show the adaptive MMSE exhibits only a 6 dB penalty relative to the single user case for the heavy multi-access interference (number of asynchronous users equal to spreading code length). The practical RAKE receivers were incapable of effectively rejecting either the strong narrowband interference or the heavily loaded wideband interference. Even more moderate levels of interference caused significant degradation in the performance of the practical RAKE receivers.     

Multiuser demodulation and iterative decoding for frequency-hoppednetworks

Demodulation and decoding for frequency-hopped spread-spectrum multiple-access (FH/SSMA) systems have been traditionally conducted by conventional single-user (noncollaborative) demodulation and error- and erasure-correcting decoding techniques. In this paper, we study the demodulation and decoding aspects of collaborative multiuser reception for FH/SSMA and propose methods which increase the number of users the system can support. In particular, we propose and analyze the optimum maximum a priori probability demodulation of multiple symbols or type, and the use of iterative multiuser decoding after the demodulation. Since hits from one or two other users are the most likely hit events in FH/SSMA, the joint demodulation of two or of three users is performed based on likelihood ratio tests. M-ary frequency-shift keying modulation with noncoherent demodulation and Reed-Solomon codes with hard-decision minimum distance decoding are used in the FH/SSMA system. Results are derived for both synchronous and asynchronous frequency-hop systems. The performance of the proposed multiuser detector in additive white Gaussian noise and flat Rayleigh fading channels is evaluated. Scenarios when all simultaneous users or only a subset of them are collaboratively demodulated and decoded are simulated     

一种多信道跳频通信系统信道控制方法

提出了一种多个物理信道共同工作的无线跳频通信系统的跳频同步及信道分配控制方法,采用独立的控制信道实现多个物理信道间的跳频同步控制,并在多个信道保持跳频同步的基础上利用高效的分段轮询方式实现控制信道和业务信道的时隙分配,同时利用控制信道实现多个业务信道间的用户调度,提高了信道的利用率,可广泛应用于各类多信道无线跳频通信系统,尤其是窄带信道的战术无线通信系统中。     

On the Probability Density Function of the Squared Envelope of a Sum of Random Phase Vectors

A recursive solution is presented for the probability density function of the sum ofNindependent, random phase vectors. The recursion parameter isN, the number of vectors in the sum. This approach allows one to rapidly compute a complete set of these density functions for values ofN = 2, 3, ..., N_{max}, where Nmaxtypically corresponds to the total number of system users in a multiuser FHMA/MFSK application, or one plus the total number of jamming tones in an FH/MFSK spread-spectrum application. Such evaluations are necessary for exact calculation of the average error probability performance of such systems.     

Semiblind channel estimation for pulse-based ultra-wideband wireless communication systems

This paper proposes an H/sub /spl infin// based semiblind channel estimation algorithm for pulse-based ultra-wideband (UWB) wireless communication systems. In the proposed scheme, sparsely inserted periodic pilot symbols are exploited to adapt to not only the time-varying channel fading and noise processes but to their changing statistics and potential external disturbances, such as interference. While the existing optimal filtering-based channel estimation schemes, which are optimized mostly for traditional narrowband or wideband systems, require a priori knowledge of the channel and noise statistics, the proposed scheme does not. By further making full use of the channel characteristics unique in UWB systems, the proposed method is thus especially useful for robust operation in the highly frequency-selective UWB indoor channels for which the channel statistics are environment-dependent, and the noise processes do not necessarily satisfy the white Gaussian distribution in the presence of potential narrowband and multiuser interferences. Performance gain of the proposed scheme over the least square method, an existing technique that could also be applied to UWB channels with unknown statistics, and the Wiener filter-based algorithm is also provided.     

On the UWB system coexistence with GSM900, UMTS/WCDMA, and GPS

This paper evaluates the level of interference caused by different ultra-wideband (UWB) signals to other various radio systems, as well as the performance degradation of UWB systems in the presence of narrowband interference and pulsed jamming. The in-band interference caused by a selection of UWB signals is calculated at GSM900, UMTS/wideband code-division multiple-access (WCDMA), and Global Position System (GPS) frequency bands as a function of the UWB pulsewidth. Several short-pulse waveforms, based on the Gaussian pulse, can be used to generate UWB transmission. The two UWB system concepts studied here are time hopping and direct sequence spread spectrum. Baseband binary pulse amplitude modulation is used as the data modulation scheme. Proper selection of pulse waveform and pulsewidth allows one to avoid some rejected frequency bands up to a certain limit. However, the pulse shape is also intertwined with the data rate demands. If short-pulses are used in UWB communication the high-pass filtered waveforms are preferred according to the results. The use of long pulses, however, favors the generic Gaussian waveform instead. An UWB system suffers most from narrowband systems if the narrowband interference and the nominal center frequency of the UWB signal overlap. This is proved by bit-error rate simulations in an additive white Gaussian noise (AWGN) channel with interference at global system for mobile communication (GSM) and UMTS/WCDMA frequencies.     

Iterative multiuser detection for turbo-coded FHMA communications

An iterative receiver structure Is proposed for turbo-coded frequency-hop multiple access (FHMA) systems. In FHMA systems, the adjacent channel interference (ACI) is the major contributor of multiple access interference (MAI) if orthogonal hopping patterns are used. The ACI is a function of the tone spacings of the adjacent subchannels and the rolloff factor of the pulse-shaping filter. The calculation of the ACI for a square-root raised-cosine pulse-shaping filter in an FHMA system is presented in this paper. In addition, a low complexity iterative multiuser detector is developed to mitigate the degradation caused by ACI in the FHMA systems. The iterative receiver structure is based on a modified turbo decoding algorithm which makes use of the a posteriori log-likelihood ratio (LLR) information of the systematic bits to obtain the a posteriori information of the turbo-encoded parity bits. Iterations of the receiver/decoder are used as the mechanism to estimate and mitigate the MAI in the FHMA system. The properties of both soft and hard interference suppressors based on the modified turbo decoding algorithm are examined and an efficient recursive implementation is derived. Compared to maximum-likelihood multiuser detection, the proposed system is more practical and its complexity is only a linear function of the number of users. Simulation results show that the proposed iterative receiver structure offers significant performance gain in bandwidth efficiency and the required signal-to-noise ratio (SNR) for a target bit-error rate (BER) over the noniterative receiver structure. Moreover, the single user performance can be achieved when imperfect power control exists     

Mathematical Models for Cochannel Interference in FH/MFSK Multiple-Access Systems

FH/MFSK has been Proposed for a multiuser spread spectrum digital communication system to combat both self-jamming and intentional jamming. An independence assumption at the energy detector outputs is used to evaluate such a system for a digitized voice mobile radio system [11]. This paper presents a correlated model and compares its performance to an independence model under the symbol error probability criterion. A Gaussian process model is also developed for comparison. It is found that the conventional Gaussian approximation is inadequate in predicting the number of users that can be accommodated by an FH/MFSK system using the conventional receiver. Furthermore, for the cases we evaluated, the independence model provides an excellent approximation to the correlated model. Additionally, a conditional Chernoff bound is presented for the more general case of frequency-hopping systems with multiple hops per symbol.     

基于分组密码的跳频序列族构造

基于迭代型分组密码的理论体系,本文从工程实现的角度提出了一种用于跳频码分多址通信系统的新型跳频序列族构造方法.该算法基于密码学的加密机制,具有好的安全性和高的计算复杂度;算法的设计遵循了密码学的"混淆"和"扩散"准则,生成序列具有各项优异的性能指标.本文从安全性、随机性、均匀性、复杂度、组网特性及跳频间隔特性等各方面对产生的跳频序列进行了全面的理论分析,证明该算法具有理想的综合系统性能指标.在此基础上,利用VHDL语言设计并开发出相应的跳频加密芯片.经测试其性能稳定、运算速度快、输入方式灵活多样,已应用于实际的高速跳频通信系统中.     

基于认知的UWB窄带干扰抑制

提出一种基于认知无线电的自适应超宽带窄带干扰抑制方法。基于软频谱思想,利用近似椭球波函数良好的时限-带限特性,设计出一种能够抑制窄带干扰的脉冲波形。通过调整脉冲参数实现对授权频段的动态避让。仿真结果表明：自适应脉冲具有良好的窄带干扰抑制能力,能够自适应地随着感知结果动态地规避授权用户,从而实现UWB系统与其它通信系统的共存。     

An ultra-wideband CMOS low noise amplifier for 3-5-GHz UWB system

An ultra-wideband (UWB) CMOS low noise amplifier (LNA) topology that combines a narrowband LNA with a resistive shunt-feedback is proposed. The resistive shunt-feedback provides wideband input matching with small noise figure (NF) degradation by reducing the Q-factor of the narrowband LNA input and flattens the passband gain. The proposed UWB amplifier is implemented in 0.18-/spl mu/m CMOS technology for a 3.1-5-GHz UWB system. Measurements show a -3-dB gain bandwidth of 2-4.6GHz, a minimum NF of 2.3 dB, a power gain of 9.8 dB, better than -9 dB of input matching, and an input IP3 of -7dBm, while consuming only 12.6 mW of power.     

Optimal frequency hopping sequences: a combinatorial approach

Frequency hopping multiple access (FHMA) spread-spectrum communication systems employing multiple frequency-shift keying (MFSK) as data modulation technique are investigated from a combinatorial approach. A correspondence between optimal frequency hopping (FH) sequences and partition-type difference packings is first established. By virtue of this correspondence, FHMA systems with a single optimal FH sequence each are constructed from various combinatorial structures such as affine geometries, cyclic designs, and difference families. Combinatorial recursive constructions are also presented. Many new infinite series of optimal FH sequences are thus obtained. These new FH sequences are also useful in ultra wideband (UWB) communication systems.     

Performance analysis of multicarrier frequency-hopping (MC-FH) code-division multiple-access systems: uncoded and coded schemes

In this paper, we provide multiuser performance analysis of a multicarrier frequency-hopping (MC-FH) code-division multiple-access system as first introduced in the work of Lance and Kaleh. We propose to use a practical low-rate convolutional error-correcting code in this system, which does not require any additional bandwidth than what is needed by the frequency-hopping spread-spectrum modulation. We provide multiuser exact performance analysis of the system for both uncoded and coded schemes in additive white Gaussian noise and fading channels for a single-user correlator receiver. We also derive the performance analysis of the system based on a Gaussian distribution assumption for multiuser interference at the receiver output. Our numerical results first indicate that the coded scheme significantly increases the number of users supported by the system at a fixed bit error rate, in comparison with the uncoded MC-FH scheme. Moreover, it shows that the Gaussian analysis in some cases does not accurately predict the number of users supported by the system.     

MIMO-assisted space-code-division multiple-access: linear detectors and performance over multipath fading channels

In this contribution, we propose and investigate a multiple-input-multiple-output space-division, code-division multiple-access (MIMO SCDMA) scheme. The main objective is to improve the capacity of the existing direct-sequence (DS)-CDMA systems, for example, for supporting an increased number of users, by deploying multiple transmit and receive antennas in the corresponding systems and by using some advanced transmission and detection algorithms. In the proposed MIMO SCDMA system, each user can be distinguished jointly by its spreading code signature and its unique channel impulse response (CIR) transfer function referred to as spatial signature. Hence, the number of users might be supported by the MIMO SCDMA system and the corresponding achievable performance are determined by the degrees of freedom provided by both the code signatures and the spatial signatures, as well as by how efficiently the degrees of freedom are exploited. Specifically, the number of users supported by the proposed MIMO SCDMA can be significantly higher than the number of chips per bit, owing to the employment of space-division. In this contribution, space-time spreading is employed for configuring the transmitted signals. Three types of low-complexity linear detectors, namely, correlation, decorrelating, and minimum mean-square error (MMSE) are considered for detecting the MIMO SCDMA signals. The bit-error rate performance of the MIMO SCDMA system associated with these linear detectors are evaluated by simulations, when assuming that the MIMO SCDMA signals are transmitted over multipath Rayleigh-fading channels. Our study and simulation results show that MIMO SCDMA assisted by multiuser detection is capable of facilitating joint space-time despreading, multipath combining, and receiver diversity combining, while simultaneously suppressing the multiuser interfering signals.     

Coarse Acquisition Performance of Spectral-Encoded UWB Communication Systems in the Presence of Narrow-Band Interference

It is known that a major practical implementation challenge of ultra-wideband (UWB) receivers is the design of the coarse acquisition stage. Due to the fine time resolution of UWB signals, the acquisition stage has to acquire a large number of low-energy multipath components, with no or little knowledge of the state of the channel. In addition, the complexity further increases with the presence of narrowband interference due to the proposed spectral overlay. Our goal in this paper is to evaluate the affects of the lack of a priori knowledge of the channel state and the presence of narrowband interference during acquisition. Maximum-likelihood and maximum a posteriori procedures for estimation in the presence of narrowband interference are formulated, and two different interference mitigation techniques are evaluated. In particular, this paper considers UWB communication systems that use spectral encoding as both the multiple access scheme and the interference suppression technique. The qualitative results are, however, believed to be valid for any UWB system implementation. It is shown that the acquisition performance strongly depends on the amount of a priori knowledge of the channel state at the receiver, and on whether or not interference suppression is employed.      

Cross-layer multiple-access optimization scheme for linear precoded OFDM UWB systems

With the growing demand for new wireless applications accompanied with high expectations for better QoS fulfillment especially for multimedia and real-time applications, the performance of the radio resource management in a multiuser context is ensured by the ability to provide an efficient and optimized spectrum sharing scheme that should respect the wireless channel conditions and satisfy the different users’ demands. From the physical layer perspective, metrics such as spectrum efficiency and minimum BER are the most important criteria to be considered. On the other hand, from a user perspective, QoS as well as fairness among the competing users are the main metrics because they determine how much end-users are satisfied and how efficient the available resources are shared among the existing users. Based on the use of the linear precoded orthogonal frequency division multiplexing (LP-OFDM) solution proposed as an evolution of the well-known multiband OFDM (MB-OFDM) solution supported by the WiMedia Alliance for future high-rate ultra-wideband (UWB) systems, the objective of this paper is twofold. First, we study a multiuser optimization spectrum sharing scheme for LP-OFDM systems. Second, based on the optimization study, we define a novel multiple-access solution which jointly considers the frequency resource allocation and the time scheduling for the high-rate LP-OFDM UWB systems. Simulation results demonstrate the efficiency of the use of the LP-OFDM transmission technique in the multiuser spectrum sharing scheme. Besides, the novel multiuser time–frequency sharing scheme shows its capacity to provide a high performance level for high-priority users.