Advanced signal-processing algorithms for energy-efficient wirelesscommunications

Substantial progress has been made in the receiver signal-processing algorithms for wireless communications to minimize the requirements on signal-to-noise (and/or interference) power ratio and computational complexities for the same quality of service. In cellular infrastructure systems, one of the key system design objectives in the base stations is to maximize the receiver sensitivity, so that the required signal level from the mobile stations can be minimized. The use of advance signal-processing algorithms, based on maximum a posteriori (MAP) estimation, iterative (turbo) channel estimation, equalization, and decoding, allows for a reduction of the required transmitter power by one-third to one-half. Lower computational complexities in the terminals, which implies a reduced power drain on the digital circuits, can be achieved by using techniques that adapt the state complexity of the receiver to the propagation channel. We give an in-depth review of these algorithms, and discuss their performance and implementation requirements     

A digital signal-processing codec filter for PCM applications

A programmable signal-processing codec filter (SICOFI) which performs its main filter functions by means of digital signal processing (DSP) is described. Besides PCM coding and band limitation, the circuit provides programmable adjustment of level control, impedance matching, hybrid balancing, and frequency response correction. Circuit performance, chip area, and power consumption have been optimized and a set of consistent CAD tools were developed for complete verification. Progress in circuit-design simulation techniques and an advanced CMOS technology allowed the economic integration of the analog and digital parts consisting of 36000 transistors on a 37.5-mm/SUP 2/ die.     

宽带高隔离度双极化多层微带阵列天线研制

一种新型阵列天线采用多层印制板形式、通过寄生贴片加多级阻抗匹配实现了宽频带工作,对矩形贴片相互垂直的两条边采用"H"缝耦合及微带线边馈实现了双极化。实验结果表明,双层带线馈电网络很大程度上解决了馈电网络间耦合问题,提高了端口隔离度。天线带宽约26.5%,实测增益19 d B左右,隔离度大于37 d B。该天线剖面低、质量小,可作为单元组成更大规模的阵列。     

A diagonal growth curve model and some signal-processing applications

We consider a variation of the growth-curve (GC) model, referred to as the diagonal growth-curve (DGC) model, where the steering vectors and waveforms are both known and the complex amplitude matrix is constrained to be diagonal. A closed-form approximate maximum likelihood (AML) estimator for this model is derived based on the maximum likelihood principle. We analyze the statistical properties of this method theoretically and show that the AML estimate is unbiased and asymptotically statistically efficient for a large snapshot number. Via several numerical examples in array signal processing and spectral analysis, we also show that the proposed AML estimator can achieve better estimation accuracy and exhibit greater robustness than the best existing methods.     

Tools for real-time signal-processing research

The use of digital signal processing (DSP) devices for real-time communication applications is discussed. The authors comment on distinguishing aspects of DSP architecture, describing not so much individual processors as those features common to DSPs and distinct from modern general-purpose processors. They describe three DSP32xx-based machines that support DSP algorithm implementation: SURF-board, HoBo, and DSP3. They also described rtpi, a source-code debugger for workstations and for the AT&T DSP32C signal-processor integrated circuit, and dspx, a collection of subroutines and host programs that provides an execution environment for DSPs akin to the UNIX environment. These tools facilitate the transfer of algorithms from mainframes or workstations to DSP hardware. Included are case studies of two real-time implementations: the low-delay CELP (LD-CELP) speech coder and the decoder side of the perceptual audio coder (PAC), an algorithm that compresses CD-quality audio into a 128-kb/s stream without perceptible distortion     

Generalised Rice-lognormal channel model for wirelesscommunications

The Rice-lognormal (RLN) channel model is generalised to include an additive diffuse-multipath component having constant average power. The generalised RLN (GRLN) model includes as limiting cases many well-known narrowband models, e.g. Rayleigh, Rice, lognormal and Loo. The GRLN probability density functions of both envelope and phase are evaluated     

Compared broad-wall slots for array applications

An overview of the theory, properties, and potential applications of compound slots is given. Methods for analyzing compound slots are discussed. Scattering properties of the compound slot are examined, covering resonance and the equivalent circuit model, resonant length scattering in the main waveguide, the aperture electric field, and off-resonant characteristics. Array characteristics are also considered, including external mutual coupling, internal higher order mode coupling, and some factors to consider in designing array configurations. Problems remaining to be solved are indicated     

An error-protected speech recognition system for wirelesscommunications

Future wireless multimedia terminals will have a variety of applications that require speech recognition capabilities. We consider a robust distributed speech recognition system where representative parameters of the speech signal are extracted at the wireless terminal and transmitted to a centralized automatic speech recognition (ASR) server. We propose two unequal error protection schemes for the ASR bit stream and demonstrate the satisfactory performance of these schemes for typical wireless cellular channels. In addition, a "soft-feature" error concealment strategy is introduced at the ASR server that uses "soft-outputs" from the channel decoder to compute the marginal distribution of only the reliable features during likelihood computation at the speech recognizer. This soft-feature error concealment technique reduces the ASR error rate by more than a factor of 2.5 for certain channels. Also considered is a channel decoding technique with source information that improves ASR performance     

MSW applications for phased array antennas

A potential application of MSW technology lies in the area of time delay for future low-sidelobe wide-bandwidth phased array antennas. High-precision MSW electronically tunable analog time-delay units in transmit/receive modules in phased arrays have the potential of greatly enhancing antenna system capabilities, by increasing instantaneous operating bandwidth and decreasing sidelobe levels, over phased array systems using only phase shifters or switched lines for beam steering and control. This paper provides a status report of MSW time delays for such arrays.     

Decision feedback equalization for CDMA in indoor wirelesscommunications

Commercial interest in Code Division Multiple Access (CDMA) systems has risen dramatically in the last few years. It yields a potential increase in capacity over other access schemes, because it provides protection against interference, multipath, fading, and jamming. Recently, several interference cancellation schemes for CDMA have been proposed but they require information about all interfering active users or some channel parameters. The authors present an adaptive fractionally spaced decision feedback equalizer (DFE) for a CDMA system in an indoor wireless Rayleigh fading environment. This system only uses information about the desired user's spreading code and a training sequence. An analysis on the optimum performance of the DFE receiver shows the advantages of this system over others in terms of capacity improvements. A simulation of this system is also presented to study the convergence properties and implementation considerations of the DFE receiver. Effects on the performance because of sudden birth and death of users in the CDMA system and bit error rate performance of the DFE receiver is also presented     

Low-power direct digital frequency synthesis for wirelesscommunications

A low-power direct digital frequency synthesizer (DDFS) architecture is presented. It uses a smaller lookup table for sine and cosine functions compared to already existing systems with a minimum additional hardware. Only 16 points are stored in the internal memory implemented in ROM (read-only memory). The full computation of the generated sine and cosine is based on the linear interpolation between the sample points. A DDFS with 60-dBc spectral purity 29-Hz frequency resolution, and 9-bit output data for sine function generation is being implemented in 0.8-μm CMOS technology. Experimental results verify that the average power dissipation of the DDFS logic is 9.5 mW (at 30 MHz, 3.3 V)     

IC signal-processing circuit for TV receivers

An IC signal-processing circuit that can be applied in both black-and-white and color receivers is described. The integrated circuit combines the following functions: video preamplifier; keyed AGC detector, operating on top sync level; AGC amplifier for IF and tuner control; noise canceling circuits for AGC and sync circuits; sync separator; automatic horizontal sync; and vertical sync pulse separator. Due to the noise-canceling circuit a stable synchronization is obtained when impulse noise is received. Since the values of the capacitors in the AGC circuit can be rather low without difficulties with instabilities, the performance during fast input signal fluctuations (airplane flutter) is very good.     

A space-time coding modem for high-data-rate wirelesscommunications

This paper presents the theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach, data is encoded by a space-time (ST) channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem, as applied to the IS-136 cellular standard, are presented. We present the frame error rate (FER) performance results as a function of the signal-to-noise ratio (SNR) and the maximum Doppler frequency, in the presence of timing and frequency offset errors. Simulation results show that for a 10% FER, a 32-state eight-phase-shift keyed (8-PSK) ST code with two transmit and two receive antennas can support data rates up to 55.8 kb/s on a 30-kHz channel, at an SNR of 11.7 dB and a maximum Doppler frequency of 180 Hz. Simulation results for other codes and other channel conditions are also provided. We also compare the performance of the proposed STCM scheme with delay diversity schemes and conclude that STCM can provide significant SNR improvement over simple delay diversity     

Packet reservation multiple access for local wirelesscommunications

Simulation work is reported indicating that packet reservation multiple access (PRMA) allows a variety of information sources to share the same wireless access channel. Some of the sources, such as speech terminals, are classified as periodic and others, such as signaling, are classified as random. Packets from all sources contend for access to channel time slots. When a periodic information terminal succeeds in gaining access, it reserves subsequent time slots for uncontested transmission. Both computer simulations and a listening test reveal that PRMA achieves a promising combination of voice quality and bandwidth efficiency     

Channel reuse strategies for indoor infrared wirelesscommunications

We examine systems of fixed-channel reuse for base stations in an indoor infrared wireless communication system. The following techniques are compared: time-division multiple access (TDMA) using on-off keying (OOK) or pulse-position modulation (PPM); frequency-division multiple access (FDMA) using binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK); code-division multiple access (CDMA) using OOK with direct-sequence spreading by m-sequences or optical orthogonal codes (OOCs). We define a parameter γ, which equals the signal-to-noise ratio (SNR) for unit optical path gain and is proportional to the square of the transmitted average optical power. Using measured pathloss data, it is found that in a system using hexagonal cells and a reuse factor of three, for cell radii above 3 m, TDMA with OOK or 2-PPM, and CDMA using OOCs all require approximately the same γ to achieve a worst-case bit-error rate (BER) of 10^-9 within a cell. Using TDMA with 4-PPM results in a 6-dB decrease in the required value of γ. CDMA using m-sequences requires an increase in γ of 5 dB over TDMA using OOK, and FDMA with BPSK requires an increase of 12 dB. For a given reuse factor N in the noise-limited regime, the required value of γ decreases in inverse proportion to N² for TDMA schemes and inversely with N for FDMA and CDMA schemes. For cell radii below 3 m, cochannel interference dominates the systems using TDMA, FDMA, and CDMA with an OOC, resulting in an irreducible BER above 10^-9 at cell radii below 1.5 m. Only CDMA with m-sequences does not develop an irreducible BER, making it the only choice for cell radii below 1.5 m     

A waveguide transverse slot for array applications

The characteristics of a radiating rectangular transverse slot in a rectangular waveguide have been studied. A moment method solution is used with entire basis expansion and testing functions (Galerkin) including the effect of wall thickness. The results are presented in terms of normalized resistance and reactance versus slot length and frequency. Excellent agreement with a previous pulse basis solution is found, as well as with measured data. An array of resonantly spaced transverse slots radiates large grating lobes. A spatial filter using baffles to suppress the grating lobes is described, suppressed. The effect of the baffles on the transverse slot impedance is analyzed for the case of baffles with infinite height     

A sectored phased array for DBF applications

In this paper, we present the idea of using switching techniques in phased arrays in order to reduce the size and cost of the RF/IF circuitry on a smart array system with a given number of array elements. The principle is illustrated using a sectored phased array antenna working on the 5.2 GHz industrial, scientific and medical (ISM) band. The antenna is a four-element dipole array, with two active and two parasitic elements. The array uses a reflective 2P4T switch to route the RF signals to consecutive elements of the array and four pin diode SPST switches for short or open circuiting the antenna elements. The performance of the proposed antenna system surpasses the performance of traditional two-port phased array systems and is comparable to that of four-element planar arrays. Simulated results of the antenna performance and beam-forming capabilities are presented.     

Phase conjugator for active retrodirective array applications

A novel phase conjugator for active retrodirective array applications is presented. The circuit provides conversion gain as well as phase conjugating operation by using active devices, and results in a simple and compact design through the use of a common port for RF and IF signals. Measurement results indicate a 3.2 dB conversion gain with an RF-IF isolation of 20 dB. This phase conjugator can be used as a component in a retrodirective array when combined with an antenna     

A novel array antenna for MSAT applications

The issue of reducing the cost of phased array vehicle antennas through the use of a lens feeding arrangement instead of phase shifters at each element is addressed. In particular, the economic viability of a mobile satellite system (MSAT) is largely dependent on the efficient use of the allocated scarce spectrum and orbit as well as the satellite power. The type of vehicle antenna used will play a critical role in achieving this efficiency. A standard design approach for an electronically steered array uses phase shifters at each element to provide beam steering. A method for reducing the required number of phase shifters by using an R-KR lens feed network is outlined. The authors briefly discuss the phase shifter approach to beam steering, examine various lens feed techniques, and describe the R-KR lens approach. The lens feed network architecture is examined, a computer model for simulation of the array is presented, and the results of analysis of a suggested design for the MSAT application are given. In addition, satellite acquisition and tracking considerations are investigated     

A software environment for digital signal-processing simulations

A signal-processing software system is described which allows the simulation of systems described by block diagrams or signal-flow graphs. A high-level data-flow language describes the interconnection of the components. All configurations of interconnections are allowed, including those containing feedback. Component systems (blocks) are allowed to be multi-input, multi-output, and to be programmed in any language. Blocks are implemented as separate processes running under a UNIX² operating system. Input and output signals are transferred between blocks via the UNIXpipe facility. Thedata type of a signal is arbitrary in the context of the compiler; within a component or system, signals and internal variables can be either floating point or fixed point. The compiler enforces strong or weak type checking of signals according to the characteristics of the blocks generating and receiving the signals. The invariance of the program to implementation data type is accomplished by the use of abstract data types. Fixed-point simulations having differing number of bits per signal and per internal variable in a block are supported. Special display software is used to allow any signal in the system to be displayed on any graphics device.This work was supported primarily by Grant MCS-8121884 from the National Science Foundation. The software system described herein may be obtained from the authors.