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Spread-spectrum telephony with accelerated code acquisition
| Details |
Inventors: Sriram, Sundararajan; Hosur, Srinath;
Assignee: Texas Instruments Incorporated (Dallas, TX)
Primary Examiner: Chin; Stephen
Assistant Examiner: Ha; Dac V.
Attorney, Agent or Firm: Brady, III; Wade James, Telecky, Jr.; Frederick J.
The present application discloses an improved mobile communications architecture, in which each base station broadcasts not only data which has been spread by that station's long code word, but also (intermittently) code identification data which has not been spread. The code identification data is a block code which includes multiple symbols, so that multiple intermittent transmissions are required to complete the transmission of the code identification data. This transmission lets the mobile station shorten the search for the base station's long code word in two ways: the code identification data gives at least some information about the long code itself; and the phase of the block code gives at least some information about the phase of the long code word. |
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DETAILED DESCRIPTION The present invention relates to wireless telecommunications systems which use spread-spectrum methods.
BACKGROUND: SPREAD-SPECTRUM METHODS One of the most important tools in telecommunications is spread spectrum methods.
For example, in a direct-sequence spread-spectrum ("DS-SS") transmission, the signal is spread by a spreading code which is known to both the transmitter and receiver.
(The spreading code is merely a long pseudo-random bit sequence, that is, a sequence of bits which appears random but is determined by the input to a generator and is, therefore, reproducible.
The sequence is generated identically, at both the transmitter and receiver, by custom hardware.
) At the receiving end, digital filtering methods can be used to selectively recognize only the signals which are encoded with the expected pseudo-random bit sequence.
Since the spreading code is used to separate signals which share the same spectrum space, these methods are also known as CDMA (code-division-multiple-access).
Samples of the spread signal are called "chips.
" The chip rate is usually much faster than the bit rate; the ratio is called the "spreading factor" or the "processing gain".
The term "spread spectrum" is also used to refer to two other techniques: "frequency-hopping" systems, in which the transmitter frequency changes in some way which the receiver can predict; and "chirp" modulation or Pulse-FM in which a carrier is swept over a wide band during a given pulse interval.
CDMA methods are commonly used in cell phone systems.
In such a system, adjacent base stations must have different spreading sequences (long pseudo-noise or "PN" codes), and the mobile unit must be able to lock onto the correct long code (spreading sequence) for each base station it may interface to.
The mobile unit will already know the set of possible long codes which it may encounter, but will not know a priori which long code it will encounter when switched on.
In most systems the mobile unit will also not know what the received long code offset is, that is, the timing of the transmission of the long code is not known
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