Conventionally, data communication systems typically use narrow band modulation techniques, such as amplitude modulation (AM), frequency modulation (FM), frequency shift keying (FSK), binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and quadrature amplitude modulation (QAM). With such systems, demodulation at the receiver may be achieved with a relatively small amount of circuitry. However, these types of systems suffer from several problems including multipath fading and narrow band noise.
In contrast, in spread spectrum communication systems, a data spectrum is spread by a pseudo-noise (PN) code at a transmitter while the PN code and the data are synchronized at a receiver. The PN code is composed of a binary sequence that is often referred to as the “chip sequence”. The binary symbols in the chip sequence are referred to as “chips” and it is appreciated by those skilled in the art that the transmitter and intended receiver both have available the same chip sequence. This technique reduces the adverse effects of multipath fading and narrow band noise. The military has employed spread spectrum communication systems to both combat the intentional jamming and detection of radio and satellite communication links. Accordingly, spread spectrum communication systems have attracted increased attention as a promising technique for radio frequency transmission of binary data in the non-military sector.
One of the two most common spread spectrum techniques, referred to as frequency hopping spread spectrum (FH-DSS), employs the chip sequence to shift, over a wide bandwidth, the carrier frequency of a conventional narrow band transmitter signal. The other common technique, referred to as direct sequence spread spectrum (DS-DSS), directly multiplies a conventional narrow band signal by the chip sequence. The chip rate is typically much higher than the data rate of the conventional narrow band signal. In both of these common spread spectrum techniques, a conventional narrow band signal is viewed as a carrier that is either frequency modulated or directly multiplied by the chip sequence. It is appreciated that other types of spread spectrum systems include combinations of both FH-DSS and DS-DSS in one system.
Spread spectrum signals allow more than one transmission signal in the same frequency and time interval when each signal uses a different chip sequence. This technique is known as code division multiple access (CDMA). An example application of Direct Sequence CDMA (DS-CDMA) is the Global Positioning System (GPS). The GPS system uses DS-CDMA to broadcast time and position data to receivers, which use such data to determine position and navigation information.
The subject of spread spectrum communications is described in a three book series by Marvin K. Simon, Jim K. Omura, Robert A. Scholtz, and Barry K. Levitt, Spread Spectrum Communications, Volume I, II, and III, Rockville, Md.: Computer Science Press, 1985. See also Robert A. Scholtz, The Origins of Spread-spectrum Communications, IEEE Transactions on Communications, COM 30, pp. 822-854, May 1982; Rober A. Scholtz, Notes on Spread-spectrum History, IEEE Transactions on Communications, COM-31, pp. 82-84, January 1983; and Robert Price, Further Notes and Anecdotes on Spread Spectrum Origins, IEEE Transactions on Communications, COM 31, pp. 85-97, January 1983.
In a spread spectrum system, the conventional narrow band signal is spread by a PN code signal that has a wider bandwidth than the conventional narrow band signal. In order to correctly restore the conventional narrow band signal, the demodulation PN code generated at the receiving side is synchronized to the modulation PN code generated at the transmitting side. Proper phase synchronization is typically achieved when the received spread spectrum signal is accurately timed in both its spreading PN code pattern position and its rate of chip generation. The phase synchronization process is preferably accomplished in two stages: an initial synchronization process to find a synchronous phase, and a process to track the detected phase. Known techniques for initial synchronization depend upon both analog and digital sliding correlators, matched filters and other equivalent devices.
In a conventional matched filter spread spectrum receiver, the receiver includes a radio frequency (RF) section that receives the spread spectrum signal having a PN code modulated therein. The receiver converts the received spread spectrum signal into an intermediate-frequency (IF) signal. An in-phase converter and a quadrature-phase converter convert the IF signal into an in-phase (I-channel) spread signal and a quadrature-phase (Q-channel) spread signal. A PN code sync device de-spreads the received PN code modulated from the spread spectrum signal by synchronizing a reference PN code with the received PN code and maintaining the two codes in fine synchronism using, for example, a pair of correlators or a tracking loop based on a matched filter. A data demodulator demodulates the spread spectrum signal into the original baseband (narrow band) signal. Utilizing a matched filter has the advantage that the transmitted spread spectrum signal may be acquired relatively quickly even with relatively large initial errors between the locally generated PN code and the received PN code.
In a DS-CDMA system such as a GPS receiver, a matched filter may selectively be utilized to receive signals from multiple transmitters such as satellites using a single set of receiver samples. However, each signal has a different PN code and a different amount of Doppler shift. If the receiver samples are Doppler corrected before the samples are stored in the matched filter, new signal samples or new Doppler correction samples are required for each received signal. This process lowers the rate at which the matched filter is utilized and also increases the power consumption of the receiver due to the filter loading process. Accordingly, there is a need for a Doppler corrected spread spectrum receiver that avoids the limitations of the prior art, has low power consumption, and is fast.