Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-06-28
2001-10-16
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06304592
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to spread spectrum communications, and more particularly to a matched filter and a phase-locked-loop circuit which can be used for synchronizing to, and despreading of, a direct sequence, spread-spectrum signal.
DESCRIPTION OF THE RELEVANT ART
Spread-spectrum communications require that an incoming spreading chip-sequence signal embedded in a spread-spectrum signal, and the local spreading chip sequence signal at a receiver, be phase synchronized prior to processing of information transfer. Phase synchronization of the spreading chip sequence signal is commonly known as code acquisition. Code acquisition is one of the most difficult issues facing the system designer.
Code acquisition is followed by the tracking process. Due to imperfect frequency references, the incoming spreading chip sequence signal and the local spreading chip sequence signal tend to lose phase synchronization. Retaining the phase synchronization, or tracking, is a difficult process that typically employs feedback loops.
Conventional spread-spectrum systems implemented without: the benefit of a matched filter employ additional circuits, such as delay locked loops (DLLs), dedicated to achieving and sustaining fine-grain-phase synchronization between the local spreading chip-sequence signal and the incoming spreading chip-sequence signal to within a unit of time which is much less than the duration of one chip. The circuits for sustaining fine-grain-phase synchronization are difficult to design and implement.
In wireless environments, minimizing the performance degradation due to long or short duration attenuation of the incoming signal caused by changing propagation channel conditions is highly desirable. As the quality of the channel degrades, the quality of the detected signal degrades, often below acceptable levels.
Typical systems combat this condition by employing any of a variety of techniques collectively known as diversity processing. The diversity processing techniques have in common the ability to independently manipulate the information received through separate propagation paths or channels. The benefit derived from diversity processing is that when a given propagation channel degrades, the information can be recovered from signals received via other channels. A common, though suboptimum, diversity technique is to employ two or more separate antennas and process the signal via two or more processing chains in parallel. Although necessary, the use of two or more antennas and dual processing is a difficult and costly undertaking, requiring two or more times the number of circuits required for one path as well as additional circuits and processing for insuring that the individual channel outputs are synchronized.
A better approach is to employ a wideband signal of bandwidth W. If the multipath spread were T
M
then the receiver can recover L=T
M
(W+1) replicas of the incoming signal. If the receiver properly processes the replicas, then the receiver attains the performance of an equivalent L
th
order diversity communication system. For wideband systems the value of L can become very large and it becomes unfeasible to implement L processing paths. Thus a non-matched-filter spread spectrum receiver cannot attain the best possible performance.
The coherent demodulation of information signals requires that the phase of the carrier, at the radio frequency (RF), intermediate frequency (IF) or other frequency at which the demodulation is to take place, be known. The extraction of the phase of the carrier information requires that additional feedback loops be employed, such as phase-locked loops (PLLS), Costas loops, n
th
power loops or other devices capable of extracting the carrier phase information. In the wireless environment, where signals propagate through a multitude of separate and independent channels, each path processed by the receiver requires its own carrier phase information and therefore its own means to extract it. This requirement greatly increases the potential complexity of the system. The need to limit system complexity acts so as to limit the system performance.
Conventional receivers, for spread-spectrum reception or other coherent systems, employ circuits dedicated to extracting the carrier phase. These techniques, e.g., phase-locked loops (PLLs), Costas loops, n
th
power loops, etc., exhibit design and implementation complexities that are well documented throughout the professional literature. A separate and independent set of these circuits is implemented for each individual signal path, or channel, that is received. Practical limits on system complexity force the system to receive a small subset of the L=T
M
(W+1) independent signal replicas.
A complex matched filter consists of two identical branches, in-phase (I) and quadrature-phase (Q), used to process in-phase and quadrature-phase signals. Each branch has a local signal reference register, an incoming signal register, a multiplication layer and an adder tree. The multiplication layer and the adder tree contained in the in-phase and quadrature-phase branches are identical and may contain the majority of the gates used to implement the matched filter. To implement a matched filter it is preferable to reduce the size of the structure as much as possible.
Processing multiple signals, whether quadrature-phase-shift keying (QPSK) or binary-phase-shift keying (BPSK) modulated, simultaneously by matched filtering is desirable. An example of a requirement for processing multiple signals is the simultaneous matched filter processing of the I & Q components of the BPSK or QPSK spread-spectrum signal and then the combining of such signals. This normally requires the implementation of two or more matched filter structures, one per signal. Matched filters are large, costly and often difficult structures to build. Thus, limiting the size and complexity of the devices as much as possible is desirable.
SUMMARY OF THE INVENTION
A general object of the invention is a matched filter and phase-locked-loop circuit for use with a spread-spectrum receiver which does not require the use of a pilot channel for synchronization.
Another object of the invention is a matched filter and phase-locked-loop circuit for use with a spread-spectrum receiver which can maintain synchronization in spite of channel fading.
According to the present invention, as embodied and broadly described herein, a spread-spectrum, phase-locked-loop apparatus is provided comprising symbol-matched means, a voltage-controlled oscillator, a first product device, a second product device, a third product device, a fourth product device, a fifth product device, a first combiner, and a second combiner. The symbol-matched means may include an in-phase portion and a quadrature-phase portion, and may be embodied as an in-phase-symbol-matched filter and a quadrature-phase-symbol-matched filter, respectively. Alternatively, the symbol-matched means may be embodied as a matched filter which is time shared between the in-phase signals and the quadrature-phase signals.
The spread-spectrum-phase-locked loop apparatus can be used as part of a spread-spectrum receiver, for receiving a received-spread-spectrum signal. A received-spread-spectrum signal, as used herein, is a spread-spectrum signal arriving at the input of the spread-spectrum receiver. The received-spread-spectrum signal is assumed to include a plurality of packets, although the invention alternatively may be used with a continuous spread-spectrum signal having frames or blocks, with each frame or block having a header. Each packet has a header followed in time by data. The header and data are sent as a packet, and the timing for the data in the packet is keyed from the header. The data may contain information such as digitized voice, signalling, adaptive power control (APC), cyclic-redundancy-check (CRC) code, etc.
A symbol-sequence signal is assumed to include a header and data. The header portion of the symbol-sequence signal is denoted herein as a header
Davidovici Sorin
Kanterakis Emmanuel
Schilling Donald L.
Bocure Tesfaldet
Chartered David Newman
Golden Bridge Technology Inc.
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