J.H. Cafarella

Acoustoelectric convolvers for programmable matched filtering in spread-spectrum systems

Acoustoelectric convolvers for a spread-spectrum communication application age described with a 100-MHz bandwidth. The use of convolvers as programmable matched filters provides the ability to change the coding waveform from bit-to-bit, thus offering improved multipath performance, security against decoding, and protection against repeat jamming. In this paper, we describe design and performance details of a prototype convolver which processes signals of 10-µs duration with a 100-MHz bandwidth. A dynamic range of 50 dB is obtained, and error signals are 30 dB below the output signal with input signal levels of +14 dBm. A test circuit is described which creates typical spread-spectrum signals in which each bit is encoded into 512 chips and data is encoded by inverting the phase of an entire bit at a data rate of 100 kbits/s.

Wide-band packet radio technology

Advances in signal processing and architectural design for high-performance packet radio are described. The scope of the work roughly encompasses the data-link and physical levels of standardized layered-network architectures. A hardware-function layering approach is used, including the purposeful design of an interface to provide a structured control environment for a demonstration packet radio. The advanced signal processing provides a robust, flexible data link to service demanding network environments, and uses 100-MHz-bandwidth surface-acoustic-wave (SAW) convolvers as large time-bandwidth product matched filters for communication with nonrepeating pseudonoise waveforms. The convolvers are combined with a binary-quantized postprocessor to implement a hybrid correlator which provides high processing gain for detection, demodulation, and ranging measurements. Data rates can be selected, in response to varying channel conditions, over a range from 1.45 Mbits/s down to 44 bits/s with an almost ideal tradeoff in processing gain for interference rejection and privacy ranging from 18 dB up to 61 dB. Future enhancements are proposed that will advance both the signal processing and the architecture.

Programmable Matched Filtering with Acoustoelectric Convolvers in Spread-Spectrum Systems

Abstract : Acoustoelectric convolvers for a spread-spectrum communication application are described with a 100-MHz-bandwidth capability. The use of convolvers as programmable matched filters provides the ability to change the coding waveform from bit-to-bit, thus offering improved multipath performance, security against decoding, and protection against repeat jamming. In this paper we describe design and performance details of a prototype convolver which processes signals of 10 microseconds duration with a 100-MHz bandwidth. A dynamic range of 50 dB is obtained and error signals are 30 dB below the output signal with input signal levels of +14 dBm. A test circuit is described which creates typical spread-spectrum signals in which each bit is encoded into 512 chips and data is encoded by inverting the phase of an entire bit at a data rate of 100 kbits/s. Examples of typical convolver outputs will be described, including spurious artifacts due to a pseudorandom-shift-register code, with a code-cycle time of 36 minutes. (Author)

Application of SAW Convolvers to Spread-Spectrum Communication

Surface-acoustic-wave convolvers and support circuitry have been developed to provide matched filter acquisition of wideband waveforms having time-bandwidth products up to lo3 and continuously changing spreading codes. This technology can provide correlation of waveforms having timebandwidth products of lo6 or more with a search window of microseconds for a 100-MHz-bandwidth signal. It can also perform antimultipath processing for data demodulation and for reliable estimates of signal arrival time in a diffuse-multipath environment.

Hybrid Convolver/Binary Signal Processor Achieves High Processing Gain

A hybrid analog/binary signal-processing technique has been developed which offers processing gains of 50 to 60 dB for spread-spectrum comnunication systems by cross-correlatin g a signal with a number of time-shifted replicas of a reference waveform. This hybrid concept employs convol vers followed by binary integration circuits. The convolvers precondition any interference presented to the binary integrator so as to avoid serious degradation which would otherwise occur with binary quantization for some types of interference. In turn, the binary integrators overcome the dynamic-range limitation of the convolver. The convolver output is converted to I-and-() video, quantized via analog comparators, and then fed to binary i ntegrators for further time integration. Because the integration is done in programable counters, the processing gain can be dynamically adjusted depending on signal -to-jamner ratios. A prototype signal processor using these concepts has been developed, using a 10-v~ acoustoelectric convolver operated at a signal bandwidth of 50 MHz to provide a processing gain of 27 dB. An 8-bit binary-integrato r array provided an additional 22 dB, for a total processing gain of 49 dB. Experimental results will be presented for both CW and white-noise interference.

Device Requirements For Spread-Spectrum Communication

Spread-sprectrum techniques are widely used in state-of-the-art communication systems for suppression of interference. In this paper we will establish the characteristics required for signal-processing devices used in such systems. Typical spread-spectrum waveforms will be described to emphasize those characteristics which lead to good system performance. The synchronization problem will be reviewed to show that this is often the most difficult aspect of a communication system design and strongly influences the choice of devices. The performance of any device in a system will reflect some features of the device which are undesirable. Spurious responses can cause errors in detection or demodulation circuitry. A limited dynamic range for a signal-processing device might have the effect of lowering the apparent interference rejection of the system.

Programmable Transversal Filters: Applications and Capabilities

Transversal filters, which have found wide application in signal processing, consist of a delay medium and taps to replicate an input signal at various lags, and means for weighting and summing of these replicas to provide an output signal. The pattern of the tap weights constitutes a filtering function; a structure which enables this function to be changed readily is referred to as a programmable transversal filter. This paper will describe PTF parameters and attributes in light of various systems applications. Examples of parameters that must be considered when selecting a component are bandwidth, number of taps, dynamic range, accuracy, linearity, and programming rate. After discussing applications, various SAW approaches to implementing PTFs will be reviewed, as will alternate approaches.