Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-04-23
2002-03-26
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C375S152000
Reexamination Certificate
active
06363102
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to wireless communications. More specifically, this invention relates to systems for digital wireless communications that employ coherent detection.
2. Description of Related Art and General Background
1) Spread Spectrum and Code-division Multiple-access
Spread spectrum communication techniques are robust to noise, allow for the use of low transmission power, and have a low probability of intercept. For such reasons, much of the early development of spread spectrum technology was performed by military researchers. Recently, however, the advantages of this technology have led to its increasing use for consumer applications as well: most notably, in advanced digital cellular telephone systems.
Whereas most other communication techniques modulate a carrier signal with one or more data signals alone, spread spectrum techniques also modulate the carrier with a pseudorandom noise or ‘pseudonoise’ (PN) signal. In the frequency-hopping variant of spread spectrum systems, the value of the PN signal at a particular instant determines the frequency of the transmitted signal, and thus the spectrum of the signal is spread. In the direct sequence spread spectrum (DSSS) variant, the bit rate of the PN signal (called the ‘chip rate’) is chosen to be higher than the bit rate of the information signal, such that when the carrier is modulated by both signals, its spectrum is spread.
Communication systems that support multiple individual signals over a single channel must employ some technique to make the various signals distinguishable at the receiver. In time-division multiple-access (TDMA) systems, the individual signals are transmitted in nonoverlapping intervals such that they are orthogonal (and thus separable) in time space. In frequency-division multiple-access (FDMA) systems, the signals are bandlimited and transmitted in nonoverlapping subchannels such that they are orthogonal in frequency space. In code-division multiple-access (CDMA) systems, the signals are spread through modulation by orthogonal or uncorrelated code sequences such that they are orthogonal or nearly orthogonal in code space and may be transmitted across the same channel at the same time while remaining distinguishable from one another at the receiver. An exemplary CDMA system is described in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” issued Feb. 13, 1990 and assigned to the assignee of the present invention, and the disclosure of which is hereby incorporated by reference.
In a CDMA DSSS system, then, each individual carrier signal is modulated by a data signal and a pseudonoise (PN) signal that is at least nearly orthogonal to the PN signals assigned to all other users, thus spreading the spectrum of the transmitted signal while rendering it distinguishable from the other users' signals. Before spreading and modulation onto the carrier, the data signal typically undergoes various encoding and interleaving operations designed, for example, to increase data redundancy and allow error correction at the receiver. The data signals may also be encrypted to provide extra security against eavesdroppers. The generation of CDMA signals in a spread spectrum communications system is disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” issued Apr. 7, 1992 and assigned to the assignee of the present invention, and the disclosure of which is hereby incorporated by reference.
2) Phase Modulation
In a DSSS telecommunications system, the baseband information signal is typically spread by the PN sequences to have a bandwidth of 1 MHz or more. In order to transmit the spread baseband signal over a radio channel, it is necessary to modulate it onto an RF carrier of the desired frequency.
Various methods of modulating digital baseband signals onto RF carriers exist. These methods typically operate by varying the amplitude, phase, and/or frequency of one or both of the in-phase (I) and quadrature (Q) components of the carrier according to the data symbol to be transmitted at any particular instant. DSSS systems commonly use a variant of either phase-shift keying (PSK), in which the phase states in the carrier components correspond to data symbols being transferred, or quadrature amplitude modulation (QAM), in which both the phases and the amplitudes of the carrier components are modulated.
In an exemplary system using binary PSK (BPSK) modulation, a transition of the carrier from a base phase state (defining a phase of zero) to a second phase state which is different by 180 degrees (i.e. a phase shift of &pgr; radians away from zero) may be designated to indicate a transition from a data symbol
0
to a data symbol
1
. The converse phase shift of &pgr; radians back to zero would then be designated to indicate a transition from a data symbol
1
to a data symbol
0
. Between these transitions, the phase of the carrier indicates whether a data symbol
0
is being transmitted (phase of zero) or a data symbol
1
instead (phase of &pgr; radians). An improved ratio of data rate to bandwidth may be obtained by using quadrature PSK (QPSK) modulation, in which the data symbols are encoded into 180-degree shifts in both the I and Q components. These and other variants of PSK modulation are well known in the art.
Note that in PSK modulation, all phase states have significance only in relation to the base phase state. If this reference state is unknown, then only the points of phase state transition may be identified, and the actual identities of the symbols cannot be determined. In the BPSK system described above, for example, a phase shift of &pgr; radians indicates either a transition from 0 to 1 or from 1 to 0. Unless one knows the relation between the base phase state and either the starting or the ending phase state, it is impossible to determine which transition was meant.
This phase ambiguity problem may be addressed in several different ways. One approach has been to avoid it by using a modulation which is suitable for noncoherent detection and does not require knowledge of the base phase state, such as differential PSK (DPSK). A more power-efficient method uses orthogonal signalling sets to encode the data symbols in a manner which is unambiguous regardless of the base phase state. The Hadamard-Walsh functions are one suitable signalling set, as discussed in Chapter 4 of CDMA:
Principles of Spread Spectrum Communications
by Andrew J. Viterbi, Addison Wesley Longman, Reading, Mass., 1995, which chapter is herein incorporated by reference. However, by providing the informational redundancy necessary to avoid the phase ambiguity problem, these methods may also reduce the achievable data throughput of the channel. An alternative approach has been to use a coherent detection scheme.
3) Coherent Detection and Phase Noise
In pilot-assisted coherent detection, the base phase state is derived from a pilot signal, a signal of known form which is transmitted along with the data signal to provide a phase and magnitude reference. One method of transmitting pilot and data channels over the same carrier is to cover the channels with different orthogonal codes (e.g., with different Walsh functions). At the receiver, the pilot channel may be used to establish carrier synchronization and enable coherent detection by, for example, using a phase-locked loop to keep the output of a local oscillator at a constant phase angle with respect to the received pilot.
Unfortunately, carrier synchronization is often complicated by the presence of phase noise. Phase noise may have two components, one being random and the other being more determinable. The random component is primarily due to Doppler effects caused by relative motion between the transmitter and receiver (or apparent motion between the two, as might be caused by a reflector). The maximum magnitude f
d
—
max
of such a Doppler shift is defined as
f
d
—
max
Black Peter J.
Easton Kenneth D.
Ling Fuyun
Baker Kent D.
Greenhaus Bruce W.
Qualcomm Incorporated
Tse Young T.
Wadsworth Philip R.
LandOfFree
Method and apparatus for frequency offset correction does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for frequency offset correction, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for frequency offset correction will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2821272