As the communication bandwidth in the crowded frequency spectrum becomes an increasingly scarce and valuable commodity, significant efforts are being applied to devise methods that enable more efficient use of available bandwidth. At the same time, demand for greater data transfer rates via both wireless and wired connections increases at a rate that tends to out-pace the state of the art.
Transmission of data at a high rate in radio communication has been plagued by problems of multipath interference and mutual interference. Multipath interference occurs when a signal interferes with reflected versions of itself. Mutual interference occurs when a signal is interfered with by a signal from a different transmitter.
Attempts to increase the number of concurrent users (channelization) include well-known techniques of Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) and Frequency Division Multiple Access. Various coding techniques have been used in these access techniques to accommodate multiple users by permitting them to use the same frequency bandwidth at the same time. For example, conventional spread-spectrum code division multiple access (SS-CDMA), or direct sequence CDMA systems, employ pseudo-noise (PN) codeword generated at a transmitter to “spread” the bandwidth occupied by transmitted data beyond the minimum required by the data. The conventional SS-CDMA systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence synchronized to the family of codes. Each user is assigned one of the spreading codes as a spreading function. One such spread-spectrum system is described in U.S. Pat. No. 4,901,307 entitled “Spread-Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters” by Gilbousen et al.
Dent et al. in U.S. Pat. No. 5,353,352 disclose a multiple access coding technique for radio communications, such as cellular systems, in which individual information signals are encoded with a common block error-correction code. The information signals are assigned a unique scrambling mask, or signature sequence, taken from a set of scrambling masks having selected correlation properties. The set of scrambling masks is selected such that the correlation between the modulo-2 sum of two masks with any codeword in the block code is a constant magnitude, independent of the mask set and the individual masks being compared. In one embodiment, when any two masks are summed using modulo-2 arithmetic, the Walsh transformation of that sum results in a maximally flat Walsh spectrum.
Awater et al. in U.S. Pat. No. 5,862,182 disclose enhancing encoding/transmission of information in an OFDM Wireless Local Area Network (WLAN) system using complementary codes. The complementary codes are converted into phase vectors and the resulting phase vectors are then used to modulate respective carrier signals. The modulated result is then transmitted to a receiver which decodes the received signals to recover the encoded information.
Awater et al. in U.S. Pat. No. 6,005,840 disclose a combined complementary encoder and modulation WLAN system for an OFDM transmitter system that combines complementary coding and modulation and exploits the similarity of their mathematical structure to reduce implementation complexity. Additionally, the combined complementary encoder and modulation system can be modified to provide scalability, which allows a transmitter to operate in various transmission environments.
The above-described techniques typically experience degraded performance in severe multipath environments, unless significant processing measures are taken to mediate multipath effects. In addition, such systems experience a limited data rate. For example, such techniques are not optimal for wireless video communications.
Efforts to provide greater usable communications bandwidth without interference have led to the development of impulse radio techniques that use ultra wideband (“UWB”) signals. Impulse radio systems were first described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990), and U.S. Pat. No. 5,363,057 (issued Nov. 8, 1994) to Larry W. Fullerton, and U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997), and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Larry W. Fullerton, et al. These patents are incorporated herein by reference.
Impulse radio refers to a radio system based on short, low duty cycle pulses. In the widest bandwidth embodiment, the resulting waveform approaches one cycle per pulse at the center frequency. In more narrow band embodiments, each pulse consists of a burst of cycles usually with some spectral shaping to control the bandwidth to meet desired properties such as out of band emissions or in-band spectral flatness, or time domain peak power or burst off time attenuation. As a result, the impulse radio transmitter uses very little power to generate noise-like communication signals for use in multiple-access communications, radar and positioning applications, among other things. In the multi-access communication applications, the impulse radio systems depend, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high achievable processing gains, the impulse radio systems are relatively immune to unwanted signals and interference, which limit the performance of systems that use continuous sinusoidal waveforms. The high processing gains of the impulse radio systems also provide much higher dynamic ranges than those commonly achieved by the processing gains of other known spread-spectrum systems.
Impulse radio communication systems transmit and receive the pulses at precisely controlled time intervals, in accordance with a time-hopping code. As such, the time-hopping code defines a communication channel that can be considered as a unidirectional data path for communicating information at high speed. In order to communicate the information over such channels, typical impulse radio transmitters use position modulation, which is a form of time modulation, to position the pulses in time, based on instantaneous samples of a modulating information signal. The modulating information signal may for example be a multi-state information signal, such as a binary signal. Under this arrangement, a modulator varies relative positions of a plurality of pulses on a pulse-by-pulse basis, in accordance with the modulating information signal and a specific time-hopping code that defines the communication channel.
Unlike conventional spread-spectrum systems, impulse radio systems typically do not employ time-hopping codes for the purpose of energy spreading. This is because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the impulse radio systems use the time-hopping codes primarily for channelization, energy smoothing in the frequency domain, and interference suppression.
In practice, decoding errors are minimized using distinctive time-hopping codes with suitable autocorrelation and cross-correlation properties. The cross-correlation between any two time-hopping codes should be low for minimal interference between multiple users in a communications system or between multiple target reflections in radar and positioning applications. At the same time, the autocorrelation property of a time-hopping code should be steeply peaked, with small side-lobes. Maximally peaked time-hopping code autocorrelation perform best in channels with multipath as well as yielding optimal acquisition and synchronization properties for communications, radar and positioning applications.
Various coding schemes with known correlation characteristics are available. For example, algebraic codes, Quadratic Congruential (QC) codes, Hyperbolic Congruential (HC) codes and optical codes have been suggested in the past for coding in impulse radio systems. Generally, based on known assumptions, the coding schemes guarantee a maximum number of pulse coincidences, i.e., hits, for any defined time frame or time frame shift during which the codes are repeated. For example, HC codes are guaranteed a maximum of two hits for any sub-frame or frame shift.
Specialized coding techniques can be employed to specify temporal and/or non-temporal pulse characteristics to produce a pulse train having certain spectral and/or correlation properties. As stated above, coding provides a method of establishing independent communication channels. Specifically, families of time-hopping codes can be designed such that the number of pulse coincidences between two pulse trains produced by any two codes will be no more than some maximum number (e.g., four), regardless of the time offset between the two pulse trains simultaneously arriving at an impulse radio receiver. Coding can also be used to facilitate signal acquisition. Specifically, coding techniques can be used to produce pulse trains with a desirable main-lobe-to-side-lobe ratio and to reduce acquisition algorithm search space.
A simple form of ultra wideband radio carries information in the relative time position of UWB impulses or in the polarity, i.e., initial direction of amplitude, of UWB impulses, or both. If it is desired to carry more information in a UWB channel, the obvious technique to try is to send the UWB impulses more often. However, if the impulses are sent with a time interval between pulses that is shorter than the impulse response of the multipath environment through which the pulses are traveling, the pulses interfere with each other at the receiver causing demodulation errors. Paradoxically, if it is attempted to send more information through a channel by sending pulses more often with less than a certain spacing of UWB impulses, the channel carries less information due to this self-jamming effect. This is the multipath problem.
The basic difficulty in the multipath problem comes from the interference of a signal and a time shifted copy of itself. The interfering copy of the signal is time shifted because it took a longer or shorter path from the transmitter to the receiver than the copy of signal we are trying to demodulate.
If it is desired to have several systems all on the air at the same time in an uncoordinated fashion, then signals from the different channels interfere with each other at a particular receiver and cause bit errors. This is similar to the multipath problem and again the paradox is that if it is attempted to move more total data through a particular space by putting more channels on the air, less data may actually be received due to this jamming effect. This is known as the channelization problem. The basic difficulty in the channelization problem comes from interference of a desired signal and signals being used by other channels.
Several well-known code methods produce code sequences that are orthogonal to one another, meaning that for a given code sequence, there would be a limited amount of correlation between it and a time-shifted copy of itself. Such methods include Barker codes, Kasami codes, Gold codes and Minimum side-lobe radar codes. While orthogonality of the code sequences produced by these methods is perhaps beneficial to signal acquisition in a mulitpath environment, such methods produced a limited number of codes of a given length. Thus, the number of channels using such codes is limited. Former attempts to increase channelization involved increasing the length of code sequences used. This provides an increased number of channels by providing an increased number of possible codes that can be used. However, the greater the length of the code, the greater the scan time required to acquire the signal.
Walsh-Hadamard codes have been investigated for use in this area. It is well known that Walsh-Hadamard codes display desirable correlation properties when aligned in time, having both perfect auto-correlation and cross-correlation values. However, when misaligned in time, auto-correlation properties are degraded.
Additionally, time-hopping codes used for modulating and demodulating data signals require timing mechanisms on both transmit and receive ends. Such mechanisms increase the complexity, cost and power usage of the device. Problems of signal acquisition and transmit/receive synchronization increase in complexity.
Thus, a novel communications code system is needed that can provide a set of short code sequences where the set of sequences provides improved channelization options in intense multipath environments for high data throughput operations.
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