Apparatus and methods for controlling short code timing...

Details Apparatus and methods for controlling short code timing... Apparatus and methods for controlling short code timing...

2000-06-26

2004-11-02

Nguyen, Chau (Department: 2663)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S342000, C370S350000, C370S503000

Reexamination Certificate

active

06813257

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to communication systems. More specifically, the present invention relates to code division multiple access (CDMA) communication systems that transmit pseudo noise (PN) codes from base stations to subscriber units over a pilot channel.
BACKGROUND OF THE INVENTION
Terrestrial communication systems have provided convenient wireless communications services for years. These services include, for example, cellular telephone services, paging, Internet access, and data transfer services, among others.
Code division multiple access (CDMA, e.g. IS-95) wireless communication systems have become particularly prevalent, because they have shown significant advantages over other radio spectrum utilization schemes, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) systems. In particular, the increased efficiency of CDMA systems is due to improved coding and modulation density, interference rejection, multipath tolerance, and reuse of the same spectrum in every communication cell. The format of CDMA communication signals also makes it extremely difficult to intercept calls, thereby ensuring greater privacy for callers and providing greater immunity against fraud.
FIG. 1
illustrates a simplified block diagram of a terrestrial CDMA system
100
, in accordance with the prior art. Network
100
includes one or more base station antennas
102
coupled to base transceiver stations (BTS)
103
. Each BTS
103
communicates, via antennas
102
and subscriber links
104
, with subscriber units carried by mobile users
106
. Essentially, the BTS modulates and demodulates the information exchanged on the subscriber links
104
, and it converts signals to and from the format used over the subscriber links.
BTS
103
also are coupled to a mobile switching office (MSO)
110
. MSO
110
includes a switch that interfaces the cellular network and a public switched telephone network (PSTN, not shown). Thus, when network data originates from or is destined for a PSTN, this data is routed through MSO
110
. The connection between a BTS
103
and MSO
110
can be a direct connection (e.g., using fiber optic or telephone (e.g., T1) links
105
), or the connection
108
can be chained through other BTS
103
, via links
108
.
The network
100
also includes an operations and maintenance center (OMC)
111
, which is manned by a human operator who evaluates status and control messages received from MSO
110
, BTS
103
, and/or other network elements or external sources. These messages could indicate, for example, that a piece of network equipment has failed, and/or how the various pieces of network equipment are performing. Based on the received messages, the OMC operator may initiate changes to the network, requesting that various network elements alter their operations.
In a CDMA system, data from multiple subscribers is spread across the same portion of the frequency spectrum. Each subscriber unit's baseband data signal is multiplied by a code sequence, called the “spreading code,” which has a much higher rate than the data. Spreading the data in this manner results in a much wider transmission spectrum than the spectrum of the baseband data signal, hence the technique is called “spread spectrum.”
A spreading code typically is a pseudo noise (PN) code that consists of a certain number of bits. Different length PN codes can be used for various purposes. One particular type of PN code is a “short code,” which each BTS periodically modulates and transmits over the network's pilot channel. A pilot channel frame is as long as the short code, and evenly spaced offsets from the start of each pilot channel frame are identified as “offsets.” Each BTS periodically starts transmitting the short code over the pilot channel at an offset that is different from the offsets at which other BTS start transmitting the short code. These different offsets are typically assigned by OMC
111
or MSO
110
. During each frame of the continuous succession of pilot channel frames, all BTS should start transmitting the short code at their assigned offsets.
FIG. 2
illustrates a timing diagram showing multiple BTS beginning transmission of the short code at different offsets, in accordance with the prior art. In particular, timing diagrams
201
-
205
for short codes transmitted over the pilot channel for five BTS are shown. In addition, a timing diagram
206
is shown, which illustrates search windows
207
, described below, within which a subscriber unit should search for the beginning of a particular BTS' short code. In the example shown in
FIG. 2
, BTS
1
201
begins short code transmission at offset
1
, BTS
2
202
transmits at offset
3
, BTS
3
203
transmits at offset
5
, BTS
4
204
transmits at offset
2
, and BTS
5
205
transmits at offset
4
.
Typically, many more offsets than are illustrated in
FIG. 2
are available for the system's BTS to begin transmitting the short code. For example, the IS-95 protocol specifies that 512 offsets are available within a pilot channel frame, and each offset is separated from the previous offset by 64 chips. This means that the short code is 512×64 chips in length.
Subscriber units monitor the pilot channel and attempt to synchronize with candidate BTS to which the subscriber units can hand off.
FIG. 3
illustrates a flowchart of a method for a subscriber unit to handoff from one BTS to another, in accordance with the prior art. The method begins, in block
302
, when each subscriber unit that is communicating on the network receives, in a control channel from the BTS with which it is currently synchronized, a handoff candidate list that identifies various alternate BTS to which the subscriber unit, theoretically, could hand off. The handoff candidate list also identifies the offset at which each candidate BTS' short code is transmitted. Handoff candidate lists are transmitted periodically to the subscriber units, but
FIG. 3
illustrates receipt of only a single handoff candidate list for ease of description.
In block
304
, the subscriber unit monitors the pilot channel and attempts to synchronize with short codes transmitted by the candidate BTS's. The subscriber unit knows the short code, and attempts to correlate the known short code with the pilot channel signal within a time window, referred to as a “search window,” that corresponds to the offset for each candidate BTS. Because the distance between the subscriber unit and each BTS imposes path delays on the signal, the search window has a width that accounts for path delays resulting from a range of possible distances between the candidate BTS's and the subscriber unit. This range typically is known for a terrestrial network, and thus the search window can be fixed by the system. Generally, the search window is selected to be at least as narrow as the offset length, so that a maximum number of offsets can be allocated to BTS's, and so that the subscriber unit is able to search for a particular short code quickly.
For each handoff candidate for which the subscriber unit finds a high degree of correlation between the known short code and the received signal, the subscriber unit measures the power of the signal, in block
306
. The subscriber unit determines, in block
308
, whether the power is sufficiently greater than the power of the signal transmitted by the current BTS. If not, the procedure iterates as shown. If so, then the subscriber unit requests to be handed off to the candidate BTS, in block
310
, and the method ends. After handoff, since the subscriber unit is synchronized to the new BTS, the subscriber unit can despread data received from the new BTS using another known PN code.
In order for a system to provide a maximum number of offsets (and, thus, candidate BTS's), the search window must be at least as narrow as the offset length. A system that incorporates this limitation makes it impossible for a subscriber unit to synchronize with a BTS that is farther than a certain distance

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