Rake receiver for spread spectrum signal demodulation

Details Rake receiver for spread spectrum signal demodulation Rake receiver for spread spectrum signal demodulation

2000-07-07

2002-08-06

Deppe, Betsy L. (Department: 2634)

Pulse or digital communications

Spread spectrum

Direct sequence

C375S150000, C375S346000, C375S347000, C370S342000

Reexamination Certificate

active

06430216

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally directed to a system for receiving a spread spectrum signal and specifically to a system for receiving and demodulating a spread spectrum signal.
BACKGROUND OF THE INVENTION
Spread spectrum techniques are finding larger roles in a variety of applications. In cellular telephony, spread spectrum based systems offer the potential for increased efficiency in the use of bandwidth. The resistance of spread spectrum methods to jamming make them ideally suited for radar and Global Positioning System (GPS) applications. For radar applications, spread spectrum signals have a lower probability of being intercepted due to the noise-like appearance of spread spectrum waveforms. In addition, it may be used to increase the pulse repetition frequency without sacrificing unambiguous range.
In spread spectrum radars, GPS, and cellular telephony applications (e.g., Code Division Multiple Access (CDMA)), each transmitted signal or pulse is assigned a time varying pseudo-random code that is used to spread each bit in the digital data stream (i.e., an interference code), such as the long code and PN code in CDMA applications. In CDMA applications, this spreading causes the signal to occupy the entire spectral band allocated to the Multiple Access System (MAS). The different users in such a system are distinguished by unique interference codes assigned to each. Accordingly, all users simultaneously use all of the bandwidth all of the time and thus there is efficient utilization of bandwidth resources. In addition, since signals are wide-band, the multipath delays can be estimated and compensated for. Finally, by carefully constructing interference codes, base stations can operate with limited interference from adjacent base stations and therefore operate with higher reuse factors (i.e., more of the available channels can be used).
In spread spectrum systems, all other spread spectrum signals contribute to background noise, or interference, relative to a selected spread spectrum signal. Because each user (or radar pulse or GPS satellite signal) uses a noise-like interference code to spread the bits in a signal, all the users contribute to the background noise. In CDMA systems in particular, user generated background noise, while having a minimal effect on the forward link (base-to-mobile) (due to the synchronized use of orthogonal Walsh Codes), has a significant effect on the reverse link (mobile to base)(where the Walsh Codes are commonly not synchronized and therefore nonorthogonal). The number of users a base-station can support is directly related to the gain of the antenna and inversely related to the interference. Gain is realized through the amplification of the signal from users that are in the main beam of the antenna, thereby increasing the detection probability in the demodulator. Interference decreases the probability of detection for a signal from a given user. Although “code” filters are used to isolate selected users, filter leakage results in the leakage of signals of other users into the signal of the selected user, thereby producing interference. This leakage problem is particularly significant when the selected user is far away (and thus the user's signal is weak) and the interfering user is nearby (and thus the interfering user's signal is strong). This problem is known as the near-far problem.
There are numerous techniques for improving the signal-to-noise ratio of spectrum signals where the noise in the signal is primarily a result of interference caused by other spread spectrum signals. These techniques primarily attempt to reduce or eliminate the interference by different mechanisms.
In one technique, the interfering signals are reduced by switching frequency intervals assigned to users. This technique is useless for the intentional jamming scenario in which jammers track the transmitter frequencies. Frequency switching is not an option for the CDMA standard for cellular telephones. In that technology, all users use all of the frequencies at all times. As a result there are no vacant frequency bands to switch to. In another technique, the interfering signals are selectively nulled by beam steering. Classical beam steering, however, does not provide, without additional improvements, the required angular resolution for densely populated communications environments.
The above techniques are further hampered due to the fact that signals rarely travel a straight line from the transmitter to the receiver. In fact, signals typically bounce off of buildings, trees, cars, etc., and arrive at the receiver from multiple directions. This situation is referred to as the multipath effect from the multiple paths that the various reflections that a signal takes to arrive at the receiver.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a system architecture for increasing the signal-to-noise ratio (SNR) of a spread spectrum signal. Another objective is to provide a system architecture for removing the interference from a spread spectrum signal, particularly the interference attributable to spread spectrum signals generated by other sources. Yet another objective is to provide a system architecture for removing the interference from a spread spectrum signal that does not employ beam steering. Specific related objectives include providing a demodulating/decoding system for efficiently demodulating/decoding spread spectrum signals generated by far away sources in the presence of spread spectrum signals generated by near sources and/or effectively accounting for the various multipaths of a spread spectrum signal.
These and other objectives are addressed by the spread spectrum system architecture of the present invention. In a first configuration of the present invention, the system includes: (i) an antenna adapted to receive a signal that is decomposable into a first signal segment and a second signal segment, the first signal segment of the signal being attributable to a first source and the second signal segment of the signal being attributable to a source other than the first source; and (ii) an oblique projecting device, in communication with the antenna, for determining the first signal segment. The signal can be any structured signal, such as a spread spectrum signal. A “structured signal” is a signal that has known values or is created as a combination of signals of known values.
The oblique projecting device determines the first signal segment by obliquely projecting a signal space spanned by the signal onto a first space spanned by the first signal segment. As used herein, the “space” spanned by a set “A” of signals is the set of all signals that can be created by linear combinations of the signals in the set “A”. For example, in spread spectrum applications, the space spanned by the signals in set “A” are defined by the interference codes of the one or more selected signals in the set. Thus the space spanned by interfering signals is defined by all linear combinations of the interfering signals. The signal space can be obliquely projected onto the axis along a second space spanned by the second signal segment. The estimated parameters of the first signal segment are related to the actual parameters of the first signal segment and are substantially free of contributions by the second signal segment. Through the use of oblique projection, there is little, if any, leakage of the second signal segment into computed parameters representative of the first signal segment.
For spread spectrum applications where noise characteristics are quantifiable, oblique projection is preferably performed utilizing the following algorithm:
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