Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-11-29
2004-07-06
Ghayour, Mohammad H. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S229000, C370S341000
Reexamination Certificate
active
06760366
ABSTRACT:
TECHNICAL FIELD
The present invention is related to detection of pilot signals by a digital cellular communications mobile unit. Specifically, the present invention relates to a method and apparatus of using a matched filter in a digital cellular communications mobile unit to search for and detect pilot signals generated by cellular base stations.
BACKGROUND
In a code division multiple access (CDMA) spread spectrum communication system, a shared frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Standard TIA/EIA-95-B entitled “Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System”, incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,901,307 entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITES OR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both; of which are assigned to the assignee of the present invention and incorporated herein by reference.
Radio Frequency (RF) signals are exchanged between a respective mobile unit and one or more base stations. Mobile units do not communicate directly with one another. Base stations communicate with a base station cellular or personal communication system controller, referred to herein as a base station controller (BSC) using various media such as ground based wires or a microwave link, for example. The BSC can route calls to a public switching telephone network (PSTN) or can route packets to a packet switched network, such as the Internet. The base station also coordinates the operation of base stations within the system during soft handoff for example.
TIA/EIA-95 is one example of a CDMA communication system. Communication from a mobile unit to one or more base stations in a TIA/EIA-95 CDMA system takes place over shared frequency channels each of which occupies approximately 1.25 MHz of radio frequency bandwidth. More specifically, communication signals occupying a given frequency band are discriminated at a receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and mobile units. Signals from different base stations can be separately received at a given mobile unit either by the discrimination of different PN codes, and/or by the discrimination of shifted versions of the same PN code. The high rate PN spreading also allows a receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Patent No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS” and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The various channels within a given “forward” (base station to mobile unit) TIA/EIA-95 CDMA channel include data channels, a synchronization channel, a pilot channel, and a set of paging channels, all transmitted from the base station to mobile units. The pilot channel carries a reference signal, commonly known as the pilot signal. The pilot signal is a regularly repeated digital pattern of “chips”, wherein each chips is represented by a single binary digit. In the exemplary embodiment, the pilot signal is a pattern that is 32,768 “chips” in length, which repeats at a chip rate of 1.2288 MHz. Thus, the pattern repeats itself every 26.6 milliseconds (ms).
The pilot provides for time reference and for amplitude and phase tracking. The pilot signal allows mobile units to identify and become synchronized with the relative phase of a base station that is within range of the mobile units' communication capability. Synchronization with a base station allows the mobile unit to further refine its timing and receive data signals from the base station.
However, as the mobile unit moves, the base stations with which it is synchronized may become more distant or become blocked, and the signal from various stations may become too weak for continued reception. Further, as the mobile unit moves, a closer base station that was previously blocked may become unblocked. The more powerful signal from the closer base station may then suppress the reception of the weaker signal from the more distant, synchronized base station.
Accordingly, a mobile unit must periodically perform searches for pilot signals transmitted from other, alternative base stations in order to identify a base station with a stronger or higher power pilot signal with which to synchronize. In general, in order to facilitate these searches, the synchronized base station sends signals to the mobile unit that identify phase offsets of pilot channels for base stations neighboring the synchronized base station. Typically, to avoid pilot signal overlap, the pilot signals of neighboring base stations are phase shifted by at least 64 chips from each other. Thus, if a mobile unit is synchronized with a base station transmitting a pilot signal at a relative phase shift of 128 chips, the synchronized base station could have neighboring base stations broadcasting at relative phase shifts of 64 chips, 192 chips and, perhaps, 256 chips, for example. The mobile unit can then search for neighboring base station pilot signals around the specific phase offsets identified by the currently synchronized base station (e.g. 64, 172 and 256) on a periodic basis to determine whether to synchronize with another base station.
FIG. 1
is a block diagram of an earlier signal detection circuit or “searcher”
10
that can be used in a mobile unit to check the power of pilot signals at certain given phase offsets or to search for received pilot signals over an entire sequence of phase offsets. Searcher
10
includes a despreader
12
, a correlator
14
, an energy storage and sorting module
16
, and a processing control
18
.
The base station creates a pilot signal having two components: an in-phase, or “I” component; and a quadrature, or “Q” component. Using these two components, the base station modulates or “spreads” the pilot signal. Most often, the specific protocol used in spreading a CDMA signal is referred to as Quadrature Phase Shift Keying (QPSK) spreading. QPSK spreading is discussed in detail, for example, in R. Prasad,
CDMA for Wireless Personal Communications
, (Artech House, 1996). After receiving a signal and passing the signal through an analog to digital converter (not shown), despreader
12
performs a mathematical algorithm on the I- and Q-components of a signal to ensure that the correct signal magnitude is detected. The mathematical algorithm used for PN despreading involves the exclusive-oring (XORing) of expected I- and Q-components with the I and Q components received. The specifics of the mathematical algorithm, as well as the specific components used in a typical despreader are well known in the art.
Correlator
14
compares an input despread signal from despreader
12
and compares it with a reference signal, commonly termed an expected signal. The expected signal can include a portion of the 32,768 chip pattern of the PN pilot signal provided to the correlator at a certain phase offset. Correlator
14
produces an energy output indicative of the level of correlation between the input despread signal and the reference signal. For example, while an exact match of all compared chips will yield a high energy output, and a match of 50% or less of all compared chips will yield a low energy output, various energy outputs between the high and low range will be yielded in accordance with a match that falls between these levels.
FIG. 2
is a schematic diagram of a greatly simplified correlator
14
. A detected sig
Maloney John E.
Wheatley III Charles E.
Al-Beshrawi Tony
Baker Kent D.
Ghayour Mohammad H.
Phillips Marc
Qualcomm Incorporated
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