Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-12-04
2003-09-16
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06621858
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the use of spread spectrum or Code Division Multiple Access (CDMA) communications techniques in cellular radio telephone systems. More particularly, the invention relates to receivers used in direct sequence spread spectrum (DS-SS), or “traditional” direct-sequence CDMA systems.
CDMA or spread spectrum communications have been in existence since the days of World War II. Early applications were predominantly military oriented. However, today there has been increasing interest in using spread spectrum systems in commercial applications. Examples include digital cellular radio, land mobile radio, and indoor and outdoor personal communications networks. Commercial operations of the cellular telephone industry continue to grow, and users continue to demand flexible data transfer rates as a key feature in newer communications systems.
CDMA allows signals to overlap in both time and frequency, as illustrated in FIG.
1
. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other. In a CDMA system, an information data stream (e.g., speech) to be transmitted is impressed upon a much higher bit rate data stream known as a spreading code signal, a signature sequence or a code sequence. The signature sequence, which has a random appearance, can be generated by a pseudorandom code generator, and replicated in an authorized receiver. The information data stream can be combined with the signature sequence by effectively multiplying the two bit streams together. Combining the higher bit rate signature sequence with the lower bit rate information data stream is called coding or spreading the information data stream signal. Each information data stream or channel is allocated a unique spreading code or signature sequence. A plurality of coded information signals are modulated and transmitted on a radio or carrier wave as a modulated composite signal. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. The modulated composite signal of multiple coded signals is received at a receiver and is demodulated into a baseband frequency. The demodulated composite signal, or baseband signal, can also be referred to as a complex signal because it typically contains both real and imaginary components. A coded signal is extracted and isolated from the demodulated composite signal by correlating the coded signal using the same signature sequence that was used to create the coded signal.
Typically, the information data stream and the signature sequence are binary with the bits of the signature sequence being known as “chips”. In traditional direct-sequence CDMA or spread spectrum systems, a signature sequence having N chips is used to represent one bit or data symbol of the information data stream. An entire transmitted N-chip sequence is referred to as a transmitted symbol.
In particular,
FIGS. 2 and 3
illustrate how information signals in a CDMA system are encoded and decoded. Two different data streams (a) and (d) are shown graphically in
FIG. 2
, and represent digitized information to be communicated over two separate communication channels as Signal
1
and Signal
2
, respectively. Signal
1
is modulated using a unique signature sequence having a high bit rate, and is thereby encoded as shown in signal graph (b) of FIG.
2
. The term “bit” refers to one digit of the information signal. The term “bit period” refers to the time period between the beginning and the end of the bit signal. Accordingly, the chip period refers to the time period between the beginning and the end of one digit of the high rate signature sequence signal. The bit period is much greater than the chip period. The result of this modulation, which is essentially the product of the signature sequence and the data stream, is shown in signal graph (c) of FIG.
2
. In Boolean notation, the modulation of two binary waveforms is essentially an exclusive-OR operation. A similar series of operations is carried out for Signal
2
as shown in signal graphs (d)-(f) of FIG.
2
. In practice, of course, many more than two coded information signals are spread across the frequency spectrum available for cellular telephone communications.
Each coded signal is used to modulate an RF carrier using any one of a number of modulation techniques, such as Quadrature Phase Shift Keying (QPSK). Each modulated carrier is transmitted over an air interface. At a radio receiver, such as a cellular base station, all of the modulated carrier signals that overlap in the allocated frequency bandwidth are received together, and are effectively added to form a composite of the modulated carrier signals, or a composite transmission signal. The composite of modulated carrier signals is demodulated to the appropriate baseband frequency, and the result is a composite or sum of the individually coded signals. For example, signal graph (c) of
FIG. 3
is a composite or sum of the individually coded signals of signal graphs (a) and (b) of
FIG. 3
, i.e., is a composite baseband signal. The composite baseband signal can have in-phase and out-of-phase components, i.e., real and imaginary components, and can also be referred to as a complex baseband signal.
The original data streams can be extracted or decoded from the composite baseband signal. For example, signal
1
can be decoded by multiplying the composite baseband signal in the signal graph (c) of
FIG. 3
with the unique signature sequence used originally to encode signal
1
, as shown in the signal graph (d) of FIG.
3
. The resulting signal is analyzed to decide the polarity (high or low, +1 or −1, “1” or “0”) of each information bit period of the signal.
These decisions can be made by taking an average or majority vote of the chip polarities during one bit period. Such “hard decision” making processes are acceptable as long as there is no signal ambiguity. For example, during the first bit period in the signal graph (f), the average chip value is +0.67 which readily indicates a bit polarity +1. Similarly, during the subsequent bit period, the average chip value is −1.33. As a result, the bit polarity was most likely a −1. Finally, in the third bit period, the average is +0.80 which indicates a bit polarity of +1. However, whenever the average is zero, the majority vote or averaging test fails to provide an acceptable polarity value.
In most situations, a “soft decision” making process is used to determine the bit polarity. For example, an analog voltage proportional to the received signal after despreading can be integrated over the number of chip periods corresponding to a single information bit. The sign or polarity of the net integration result indicates that the bit value is a +1 or −1.
CDMA receivers often contain a RAKE receiver. In mobile communication systems, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion, caused for example by signal reflections from large buildings or nearby mountain ranges. Multipath dispersion occurs when a signal proceeds to the receiver along not one but many paths so that the receiver hears many echoes having different and randomly varying delays and amplitudes. Typically a RAKE receiver “rakes” all the multipath contributions together. A CDMA RAKE receiver individually detects each echo signal using a correlation method, corrects for different time delays, and adds the detected echo signals algebraically (with the same sign).
Sliding correlators can be used in spread spectrum or CDMA receivers to perform the correlation/extraction process, and are typically capable of doing so relatively quickly. In particular, a conventional sliding correlator can correlate the baseband signal with a portion of a signature sequence used to spread the signal. The signature sequence portion is also known as a local code section, and is correlated at a
Barrow David
Bottomley Greg
Ramesh Rajaram
Roberts Clarence V.
Sourour Essam
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Tran Khai
LandOfFree
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