Advantage and Implementation Considerations of Shaped OFDM Signals
by Dragan Vuletic, fred harris
This paper presents the structure and measured performance of a MODEM designed to implement a variant of OFDM known as shaped OFDM. Individual complex sub-carriers matching the mutually orthogonal tones of a Fourier transform with time span of MT seconds form a standard OFDM signal. The amplitudes of the orthogonal sinusoids are obtained by a DFT of uniformly spaced samples of the time function. An interfer-ing tone within the frequency span of the OFDM signal may interfere with all the OFDM channels. The interference can be isolated to a few OFDM channels by replacing the rectangle envelope with a shaped envelope. The shaping controls the side lobe levels of the individual channel spectra and thus suppress the projection of an inter channel interfering signal. To maintain a fixed system throughput, the time span of the OFDM frame is lengthened and the adjacent frames are overlapped.
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Digital Receivers and Transmitters Using Polyphase Filter Banks for Wireless Communications
by fred j. harris, Chris Dick, Michael Rice
This paper provides a tutorial overview of multichannel wireless digital receivers and the relationships between channel bandwidth, channel separation, and channel sample rate. The overview makes liberal use of figures to support the underlying mathematics. A multichannel digital receiver simultaneously down-convert a set of frequency division multiplexed (FDM) channels residing in a single sampled data signal stream. In a similar way, a multichannel digital transmitter simultaneously up-converts a number of baseband signals to assemble a set of FDM channels in a single sampled data signal stream. The polyphase filter bank [1] has become the architecture of choice to efficiently accomplish these tasks. This architecture uses three [2, 3] interacting processes to assemble or to disassemble the channel-ized signal set. In a receiver these processes are an in-put commutator to effect spectral folding or aliasing due to a reduction in sample rate, a polyphase M-path filter to time align the partitioned and resampled time series in each path, and a discrete Fourier transform to phase align and separate the multiple base-band aliases. In a transmitter these same processes operate in a related manner to alias baseband signals to high order Nyquist zones while increasing the sample rate with the output commutator.
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Synthesizing and Processing Wide Bandwidth Time Signals With Stepped Frequency FM Chirps
by fred j. harris
In the same way that synthetic aperture radar (SAR) synthesizes an extended length spatial array by coherently processing data collected while tine sharing a single transmitter and receiver (but using a short array), a wide bandwidth pulse can be synthesized by coherent processing of a sequence of narrow bandwidth pulses. The synthesis is performed with a set of narrowband pulses which Span the desired frequency range using a sequence of scheduled frequency shifts. As in SAR processing, each return is preprocessed to form an IQ pair, which for this synthesis task, is equivalent to a spectral sample of the desired wideband waveform. These spectral samples are transformed in a DFT to form the compressed pulse. The preprocessing entails a heterodyne to remove the complex carrier from the signal returns. The detected IQ pair is then modified to compensate for known variations in the return related to the medium and data collection process. The equalized IQ data is then windowed and Is finally compressed by a discrete Fourier transform. This processing is seen to be a time sequential narrowband equivalent to the processing technique performed on a wideband linear FM sweep known as STRETCH.
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A Novel Use of The Sigma-Delta Modulator to Insert Arbitrary Training Signals Into a 1-bit (Clipped) Data Stream For 1-bit Beamforming Processing
by fred j. harris, Don Ransbury
This paper describes a novel application of a sigma-delta modulator. The system described here forms the sum of a 1 6-bit training signal and a 1 -bit tactical data stream obtained as the hard-limited output of a set of hydrophones and returns the sum as another 1-bit sequence. The modified 1-bit data streams are processed by a 1-bit beamformer and the inserted signals (which are used for operator training) must not affect the normal operation of the i-bit beamformer and the insertion process must be transparent to the operator. A description of the system which accomplishes these tasks, an analysis of its noise performance, and a demonstration of its non-linear merging capabilities is included in this paper.
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On The Design, Implementation, and Performance of a Microprocessor Controlled AGC System For a Digital Receiver
by fred j. harris, Gregory Smith
Microprocessor management of the automatic gain control (AGC) loop for a DSP based radio receiver brings the flexibility of an intelligent controller to the task of satisfying operational and signal dependent requirements such as variable attack, hold, and decay times, We present details of the design, some important hardware and software considerations, and the measured performance of an AGC loop controller we have implemented and inserted in a high performance digital surveillance receiver. The algorithms, based on an EMS adaptive filter, are implemented in a TMS-32020 DSP chip.
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Multirate All-Digital Modem For Support of Universal Multiplex Transport Layer For Digital Compression
by fred j. harris, Tony Wechselberger
With the approaching adoption of digital compression and the supporting development of transport systems to effect the delivery of alldigital television, there is intense activity surround ing the search for low cost hut high performance modems for digital signals. This paper discusses recent advances in the state-of-the-art of modulator/demodulator technology for the satellite channel. An all-digital modem implementation is described, yielding benefits of near-theoretical performance, low cost, high reliability and variable rate operation from I to 60Mbps.
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On Structure and Implementation of Algorithms for Carrier and Symbol Synchronization in Software Defined Radios
by fred j. harris, Chris Dick
Synchronization techniques based on DSP implementations are often digital emulations of their analog prototypes. Such solutions do not include structures and algorithms responsive to DSP system considerations and implementation strengths and weaknesses. We present a number of unconventional algorithms and structures used in carrier and timing recovery schemes. Multirate signal processing, polyphase filter structures, and CORDIC subsystems are at the heart of efficient first principle DSP based solutions to carrier recovery, matched filtering, timing recovery, and phase detection tasks required for synchronization.
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Signal Processing in Next Generation Digital Receivers and Transmitters
by fred j. harris
Digital signal processing has become the mainstay of high-performance modulators and demodulators in transmitters and receivers used in cable, wire-line, satellite, and wireless communication systems. As expected, DSP has been applied to the traditional tasks of spectral shaping, modulation, and equalization. A pleasant surprise is the many other tasks to which DSP has been applied. These tasks include timing recovery, carrier recovery, automatic gain control, DC canceling, I-Q gain and phase balancing, built in test, pre-equalization, and power amplifier liberalization. An eye-opening realization is that DSP solutions to traditional signal conditioning tasks are very different from their analog predecessors. This is partly due to new tools not previously available (did anyone here order an FFT?) and in no small part, to a change in perspective related to multi-rate processing and newer measures of complexity. This presentation will illustrate a number of traditional signal-conditioning problems in receivers and transmitters with new perspective DSP based solutions.
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