The following applications of common assignee may contain common disclosure with the present application:
U.S. patent application Ser. No. 09/638,192 entitled xe2x80x9cA METHOD FOR SPECIFYING NON-TEMPORAL PULSE CHARACTERISTICSxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/638,046 entitled xe2x80x9cA METHOD AND APPARATUS FOR APPLYING CODES HAVE PREDEFINED PROPERTIESxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/637,878 entitled xe2x80x9cA METHOD AND APPARATUS FOR POSITIONING PULSES USING A LA YOUT HAVING NON-ALLOWABLE REGIONSxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/638,150 entitled xe2x80x9cA METHOD AND APPARATUS FOR POSITIONING PULSES IN TIMExe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/638,151 entitled xe2x80x9cA METHOD AND APPARATUS FOR MAPPING PULSES TOA NON-FIXED LAYOUTxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/638,152 entitled xe2x80x9cA METHOD AND APPARATUS FOR SPECIFYING PULSE CHARACTERISTICS USING CODE THAT SATISFIES PREDEFINED CRITERIAxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/638,153 entitled xe2x80x9cA METHOD FOR SPECIFYING PULSE CHARACTERISTICS USING CODESxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/638,154 entitled xe2x80x9cA METHOD FOR SPECIFYING NON-ALLOWABLE PULSE CHARACTERISTICSxe2x80x9d, filed on Aug. 15, 2000.
U.S. patent application Ser. No. 09/708,025 entitled xe2x80x9cA METHOD AND APPARATUS FOR GENERATING A PULSE TRAIN WITH SPECIFIABLE SPECTRAL RESPONSE CHARACTERISTICSxe2x80x9d, filed on Nov. 8, 2000.
The above-listed applications are incorporated herein by reference in their entireties.
The present invention relates to impulse radio systems and, more particularly, to a method and apparatus for receiving time spaced signals.
As the availability of communication bandwidth in the increasingly crowded frequency spectrum is becoming a scarce and valuable commodity, Ultra Wideband (IWB) technology provides an excellent alternative for offering significant communication bandwidth, particularly, for various wireless communications applications. Because UWB communication systems are based on communicating extremely short-duration pulses (e.g., pico-seconds in duration), such systems are also known as impulse radio systems. Impulse radio systems are described in a series of patents, including U.S. Pat. Nos. 4,641,317 (issued Feb. 3, 1987), 4,813,057 (issued Mar. 14, 1989), 4,979,186 (issued Dec. 18, 1990), and 5,363,057 (issued Nov. 8, 1994) to Larry W. Fullerton, and U.S. Pat. Nos. 5,677,927 (issued Oct. 14, 1997), 5,687,169 (issued Nov. 11, 1997), and 5,832,035 (issued Nov. 3, 1998) to Larry W. Fullerton, et al. These patents are incorporated herein by reference.
Multiple access impulse radio systems are radically different from conventional Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) systems. Unlike such systems, which use continuous sinusoidal waveforms for transmitting information, a conventional impulse radio transmitter emits a low power electromagnetic train of short pulses, which are shaped to approach a Gaussian monocycle. 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, impulse radio transmitters may 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.
In applications where the modulating information signal is a binary information signal, each binary state may modulate the time position of more than one pulse to generate a modulated, coded timing signal that comprises a train of identically shaped pulses that represent a single data bit. The impulse transmitter applies the generated pulses to a specified transmission medium, via a coupler, such as an antenna, which electromagnetically radiates the pulses for reception by an impulse radio receiver. The impulse radio receiver typically includes a single direct conversion stage. Using a correlator, the conversion stage coherently converts the received pulses to a baseband signal, based on a priori knowledge of the time-hopping code. Because of the correlation properties of the selected time-hopping codes, the correlator integrates the desired received pulses coherently, while the undesired noise signals are integrated non-coherently such that by comparing the coherent and non-coherent integration results, the impulse receiver can recover the communicated information.
Conventional spread-spectrum code division multiple access (SS-CDMA) techniques accommodate multiple users by permitting them to use the same frequency bandwidth at the same time. Direct sequence CDMA systems employ pseudo-noise (PN) codewords generated at a transmitter to xe2x80x9cspreadxe2x80x9d 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 xe2x80x9cSPREAD-SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d by Gilhousen et al.
Unlike direct sequence spread-spectrum systems, impulse radio communications systems have not employed time-hopping codes for energy spreading, because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the impulse radio systems use the time-hoping codes for channelization, energy smoothing in the frequency domain, and interference suppression. The time-hoping code defines a relative position of each pulse within a group of pulses, or pulse train, such that the combination of pulse positions defines the communications channel. In order to convey information on such communication channel, each state of a multi-state information signal may vary a relative pulse position by a predefined time shift such that a modulated, coded timing signal is generated comprising a train of pulses, each with timing corresponding to the combination of the time position coding and the multi-state modulation. Alternative modulation schemes may also be used instead of time modulation or in combination with it.
In one conventional binary approach, pulses are time-modulated forward or backward about a nominal position. More specifically, each pulse is time modulated by adjusting its position within a time frame to one of two or more possible times. For example, in order to send a xe2x80x9c0xe2x80x9d binary bit during the time frame, the pulse may be offset from a nominal position of the time frame by about xe2x88x9250 picoseconds. For a xe2x80x9c1xe2x80x9d binary state, the pulse may be offset from the nominal position by about +50 picoseconds. Conventional coders that generate the time-hoping code do so in response to a periodic timing signal that corresponds to the data-rate of the multi-state information signal. The data rate of the impulse radio transmission may for example be a fraction of a periodic timing signal that is used as a time base or time reference.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. Because each data bit modulates the time position of many pulses of the periodic timing signal, a modulated, coded timing signal is produced comprising a train of identically shaped pulses for each single data bit. The impulse radio receiver integrates multiple pulses to recover the transmitted information.
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-hoping code should be steeply peaked, with small side-lobes. Maximally peaked time-hopping code autocorrelation yields 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 subframe or frame shift.
McCorkle in U.S. Pat. No. 5,847,677 discloses a random number generator for generating a pseudorandom code for use with jittered pulse repetition interval radar systems. The code is generated by a random number generator that possesses certain attributes desirable for jittered radar. As disclosed, the attributes related to a flat frequency spectrum, a nearly perfect spike for an autocorrelation function, a controllable absolute minimum and maximum interval, long sequences that do not repeat, and a reasonable average pulse rate.
One known coding technique for an impulse radio is disclosed by Barrett in U.S. Pat. No. 5,610,907, entitled xe2x80x9cUltrafast Time Hopping CDMA-RF Communications: Code-As-Carrier, Multichannel Operation, High data Rate Operation and Data Rate on Demand.xe2x80x9d According to the disclosed techniques, two levels of coding are used: major orthogonal codes are applied to provide multiple channels, and forward error correction (FEC) codes are applied to information data before transmission. The disclosed system relies on dividing time into repetitive super-frames, frames and subframes. As disclosed, a super-frame corresponds to a time interval of about 1 millisecond, representing one repetition of a code pattern, where as a frame is defined as a time interval of about 1 microsecond divided according to a code length. A subframe corresponds to a short time interval of about 1 nanosecond during which a pulse is time positioned.
It is well known that communicated signals over a wireless transmission medium can be subject to various types of interference. In communicating voice messages, data messages, control messages, or other types of messages, interference causes problems by corrupting information intended to be conveyed by the transmission message. As a result, noise, or electromagnetic interference can interfere with efficient communication using impulse radio technology.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods. In some multi-user environments where there is a high density of users in a coverage area or where data rates are so high that processing gain is marginal, power control may be used to reduce the multi-user background noise to improve the number of channels available and the aggregate traffic density of the area. Briefly stated, power control generally refers to adjusting the transmitter output power to the minimum necessary power to achieve acceptable signal reception at an impulse radio receiver.
Another known method for mitigating adverse effects of interference in impulse radio communication varies transmission rates (bit rates), signal strength, packet sizes and frequency of packet repetition, and other parameters associated with conveying a transmission message, whatever the source of the interference may be. Some noise, or interference, may be periodic in nature, with a constant period or with a period that varies over time (i.e., quasi-periodic). Such periodic or quasi-periodic noise may be substantially predicted, at least within a limited time period. Other electromagnetic interference may be intermittent and less predictable. Usually no one type of interference occurs alone; most likely an impulse radio communication system will encounter a variety of interference, having varying periodicity or unpredictability, varying strengths, and other varying parameters.
Because UWB technology is applicable to a wide variety of applications including communications, radar, and positioning, transmitted pulse trains may be subject to interfering signals, for example, periodic interference that degrades received signal quality periodically. For example, significant portions of impulse radio system pulse trains may be adversely affected due to exposure to periodic interference resulting in data errors. Therefore, there exists a need for improving received signal quality of impulse radio systems in the presence of periodic interference.
The present invention provides a method of receiving a signal comprising a plurality of time spaced signals that convey an intelligence signal comprising a series of data bits. The intelligence signal may have been produced by multiplexing a plurality of input signals. The time spaced signals, which may be pulses or bursts, can be positioned uniformly in time or positioned according to a time hopping code over a time layout. Additionally, the time spaced signals can be modulated in accordance with a modulation technique to represent various types of information, e.g., voice or data, as a part of the intelligence signal. Time shift modulation, amplitude modulation, frequency modulation and phase modulation are some of the modulation techniques that may be used to convey the information, for example, as binary data bits.
Reception of time-spaced signals may be a coherent detection process. Coherent detection may be accomplished by mixing the received signals with template signals spaced in time such that they coincide or correlate with the received signals. Generally, the correlation process can include performing short-term integration of the time spaced signals. In accordance with the present invention, the coherently detected signals are contributed to a plurality of intermediate signals in accordance with a defined order, for example a predefined pulse interleaving order. In one embodiment the predefined pulse interleaving order is a sequential order. In another embodiment the predefined pulse interleaving order is a pseudorandom order. Each of the plurality of intermediate signals can then be separately integrated, for example, relative to or independent from the pulse train sequence, to produce the intelligence signal. More specifically, the intermediate signals can be subject to a long-term integration that produces a series of data bits in parallel.
Once derived from the intermediate signals, the series of data bits can be further ordered in accordance with a predefined bit order to convey the intelligence signal. In one embodiment the predefined bit order is a sequential order. In another embodiment the predefined bit order is a pseudorandom order. When multiple input signals are multiplexed in the transmitter to produce the intelligence signal, in one embodiment, the predefined bit order also specifies the demultiplexing of the data bits to multiple output signals that correspond to the input signals.
In a further embodiment, the coherently detected signals can be contributed to the plurality of intermediate signals in accordance with code elements of a pulse interleaving code. In one exemplary embodiment, the pulse interleaving code may be a pseudorandom code having code elements that specify the order in which coherently detected signals are contributed to the intermediate signals. Thus, each coherently detected signal may be contributed to a different one of the plurality of intermediate signals based on a selected pulse interleaving code.
Similarly, in another embodiment, the order in which data bits derived from intermediate signals are applied to the at least one intelligence signal can be specified in accordance with code elements of a bit ordering code, for example, a pseudorandom code.
In accordance with one of the more detailed features of the present invention, various parameters for transmission and reception of the time spaced signals can be modified or delayed based on a quality measure to satisfy a received signal quality criterion. For example, based on the quality measure, the pulse interleaving code may be dynamically modified, thereby changing the contribution of the coherently detected signals to the intermediate signals in a dynamic manner. Under this arrangement, a quality measure for the intermediate signals or the recovered intelligence signals can be determined. Then, the contribution of the coherently detected signals to the intermediate signals may be modified based on the quality measure. The contribution can be varied based on at least one of a statistical redistribution, a random redistribution, and an optimal order search algorithm. Furthermore, a time hopping code can be modified or the time spaced signals can be delayed to satisfy the received signal quality criterion. The time hopping code may be modified or the time spaced signals may be delayed based on a relationship between a plurality of codes in a code family, to satisfy the received signal quality criterion. Of course, any change to the pulse interleaving code, change to the time hopping code, or delay of the time spaced signals is coordinated between a transmitter and a receiver.
Furthermore, according to yet another embodiment, one intermediate signal can be compared to another intermediate signal, for example, as an amplitude reference and/or a time reference to support amplitude modulation or signal acquisition. Also, an order of the plurality of intermediate signals may be varied according to at least one of a pulse interleaving code and a bit ordering code.