RF transceivers employing spread spectrum techniques are well-known and widely used. Although the applications in which spread spectrum transceivers are used are too numerous to describe in detail, one increasingly popular application is in the field of wireless computer systems, such as wireless local area networks (LANs).
In a typical wireless computer network environment, the "backbone" of the LAN takes the form of one or more central servers that communicate with a number of network base stations, or access points, through a hard-wired connection. Each access point (AP) includes an RF transmitter/receiver pair (i.e., transceiver) for wirelessly communicating with at least one roaming mobile station ("MS"), which also incorporates an RF transceiver. The mobile station may be a point-of-sale terminal (e.g., an electronic cash register), a bar code reader or other scanning device, or a notepad, desktop or laptop computer.
Each MS establishes a communication link with an AP by scanning the ISM (industrial, scientific, medical) band to find an available AP. Once a reliable link is established, the MS interacts with other mobile stations or a network server, or both. This allows the user of the MS to move freely in the office, factory, hospital or other facility where the wireless LAN is based, without the length of a hard-wired connection to the LAN limiting the MS.
Eventually, however, the MS will move out of range of its current AP, usually into the coverage area of a second AP. When the MS senses that the communication link with the current AP is unacceptably weak (or, more generally, degraded), the MS initiates establishes a new communication link with the second AP.
As noted, wireless LAN products frequently employ some type of spread spectrum technique, such as direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS), to communicate between roaming mobile stations and network access points. A distinguishing feature of the spread spectrum technique is that the modulated output signals occupy a much greater transmission bandwidth than the baseband information bandwidth requires.
In DSSS, the spreading is achieved by encoding each data bit in the baseband information using a codeword, or symbol, that has a much higher frequency than the baseband information bit rate. The resultant "spreading" of the signal across a wider frequency bandwidth results in comparatively lower power spectral density, so that other communication systems are less likely to suffer interference from the device that transmits the DSSS signal. It also makes the DSSS signal harder to detect and less susceptible to interference (i.e., harder to jam).
DSSS employs a pseudorandom noise (PN) codeword known to the transmitter and to the receiver to spread the data and to make it more difficult to detect by receivers lacking the codeword. The codeword consists of a sequence of "chips" having values of -1 or +1 (non-return-to-zero(NRZ) or polar signal) or 0 and 1 (non-polar signal) that are multiplied by (or Exclusive-ORed with) the information bits to be transmitted. Accordingly, a logic "0" information bit may be encoded as a non-inverted codeword sequence, and a logic "1" information bit may be encoded as an inverted codeword sequence. Alternatively, a logic "0" information bit may be encoded as a first predetermined codeword sequence and a logic "1" information bit may be encoded as a second predetermined codeword sequence. There are numerous well known codes, including M-sequences, Gold codes and Kasami codes.
Many wireless networks conform to the IEEE 802.11 DSSS standard, which employs the well-known Barker code to encode and spread the data. The Barker codeword consists of eleven chips having the sequence "01001000111" (non-polar) or "+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1" (non-return-to-zero (NRZ) or polar signal), in which the leftmost chip is output first in time. One entire Barker codeword sequence is transmitted in the time period occupied by an information-containing symbol. Thus, if the symbol rate is 1 MBaud, the underlying chip rate for the eleven chips in the Barker sequence is 11 MHz.
Numerous techniques may be employed to increase this exemplary data transfer rate above the 1 MBaud symbol rate, including quadrature phase-shift keying (QPSK) modulation, wherein a 2 Mbps data bit stream in the transmitter is grouped in pairs of bits and, depending on the values of the two bits in each pair, two signals are generated. The first signal (called the "in-phase (I) signal" or "I channel") is a first phase-shift modulation signal for a cosinusoidal carrier and the second signal (called the "quadrature (Q) signal" or "Q channel") is a second phase-shift modulation signal for a 90 degree phase-shifted sinusoidal carrier at the same frequency. The combined I and Q channels increase the effective data transfer rate to 2 Mbps. The data transfer rate may be further improved by employing code position modulation, which encodes an additional N bits of data by time-shifting (delaying or advancing) each symbol (Barker sequence) among one of 2.sup.N time positions within a fixed reference time frame established by the transmitter. For example, an additional three (3) bits of data can be transmitted per symbol in each of the I channel and Q channel by time-shifting each symbol among one of eight (8) time positions in each fixed reference time frame. This increases the total data transfer rate to 8 Mbps.
By using the 11 MHz chip rate signal to modulate the carrier wave, rather than the original 1 MBaud information signal, the spectrum occupied by the transmitted signal is eleven times greater. Accordingly, the recovered signal in the receiver, after demodulation and correlation, comprises a series of sign-inverted Barker sequences representing, for example, binary logic "1" information bits, and non-inverted Barker sequences representing, for example, binary logic "0" information bits.
As is well known, the IEEE 802.11 standard for wireless LANs using DSSS techniques employs a training preamble to train a receiver to a transmitter. Each transmitted data message comprises an initial training preamble followed by a DATA field. The 192-bit preamble comprises a 128-bit SYNC (synchronization) field, a 16-bit SFD (start of frame delimiter) field, an 8-bit SIGNAL field, an 8-bit SERVICE field, a 16-bit LENGTH field, and a 16-bit CRC field which provides a CRC check for the SERVICE, SIGNAL, and LENGTH fields. The actual number of bits in the DATA field depends on the values stored in the LENGTH field and the SIGNAL field. The DATA field may contain up to 2346 bytes.
The DATA field may be transmitted using DQPSK (differential quadrature phase-shift keying) modulation of both carrier channels (i.e., I channel and Q channel) or DBPSK (differential binary phase shift keying) modulation of only one carrier channel. The preamble, including the 128 symbol SYNC field, is transmitted at a 1 MBaud symbol rate in differential binary phase-shift keying (DBPSK) modulation in which the I channel and the Q channel contain the same information. The receiver detects the synchronization symbols and aligns the receiver's internal clock(s) to the symbols in the SYNC field in order to establish a fixed reference time frame with which to interpret the preamble field (SFD, SERVICE, SIGNAL, LENGTH and CRC) following the SYNC field and the DATA field, which follows the preamble. In the case of a 1 MBaud symbol rate, the fixed reference time frame consists of successive contiguous one microsecond time frames (windows) synchronized to the time frames during which the 11-chip Barker sequences are transmitted.
The preamble, including the SYNC field, is transmitted at the start of every message (data packet). The DATA field within each transmitted message is kept relatively short (up to about 1500 bytes, for example) for a number of reasons. Many wireless protocols, including the IEEE 802.11 DSSS standard, require retransmission of an entire frame (preamble plus DATA field) if an error is detected. Re-transmission of an extremely long frame would be wasteful of bandwidth. Furthermore, it is necessary to share the available bandwidth with other users on the network, but an extremely long frame will effectively slow down the data transfer rates of other users. Finally, channel conditions could change frequently over time by displacement, but, in some modes (such as code position modulation), the channel conditions are only estimated during transmission of the preamble. If an overly long period occurs between preambles, changed channel conditions may lead to increased error rates. For these reasons, it may be necessary to divide a large block of information over many messages in order to complete transmission.
A key performance parameter of any communication system, particularly computer networks and cellular telephone systems, and the like, is the transfer rate of data between devices in the communication system. Wireless LANs are no exception. It is therefore important to maximize the rate at which data may be exchanged between access points and mobile stations in a wireless LAN in order to maximize the LAN performance.
One way to maximize data transfer rates is to minimize the "overhead" associated with each packet of data that is sent from a transmitter to a receiver. Part of the "overhead" of each data packet is the training preamble, including SYNC field, that precedes each DATA field. Minimizing the duration of the training preamble used to synchronize the receiver portion of a spread spectrum transceiver increases the overall throughput of the transceiver.
Furthermore, a receiver has better channel performance when it can be trained well under poor received signal conditions, such as low signal-to-noise ratio (SNR), high multipath fading, and heavy distortion. In other words, the better trained is the receiver, the more accurately the receiver can recover the degraded transmitted data. This yields a lower error rate. Since a high error rate often requires that the entire message be retransmitted, lowering the error rate results in less frequent retransmissions of messages and yields a higher overall data transfer rate.
Prior art receivers have employed matched filtering techniques and RAKE receiver techniques to improve the detection of spread spectrum signals. Matched filtering uses an optimum receiver filter which has a frequency transfer function, H*(f), matched to the frequency transfer function, H(f), of the channel, where H*(f) is the complex conjugate of H(f). A RAKE receiver uses an optimum weighting of individual signal contributions. Thus, the RAKE receiver technique is based on the same principle of maximizing the signal-to-noise ratio (SNR) as matched filtering.
Both matched filtering techniques and RAKE techniques require an appropriate training period to accurately acquire the weighting coefficients (taps). During the training period, the detection of the preamble symbol information must be robust before the part of the transmitted signal arrives which contains information. Both techniques use the received signal and a reference signal to determine the matched filtering coefficients or the RAKE weighting coefficients. This determination process may take several tens of symbol intervals to obtain accurate settings if the conditions of the channel are poor.
Accordingly, there is a need in the art for improved RF receivers that can be trained rapidly to the preamble portion of a data packet in order to use a minimum duration preamble. There is a further need for improved RF receivers that can be rapidly and accurately trained to a transmitted signal under poor channel conditions.