Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2001-07-20
2004-02-03
Maung, Nay (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S067110, C455S067140, C455S067160, C455S561000
Reexamination Certificate
active
06687501
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to wireless communications systems and, in particular, to system and method for dynamically calibrating base station timing.
2. Description of Related Art and General Background
Calibrating and maintaining proper timing is an important concern in communication systems. This is particularly true in wireless communications operating under Code Division Multiple Access (CDMA) schemes. CDMA is a digital radio-frequency (RF) channelization technique, defined in Telecommunications Industry Association/Electronics Industries Association Interim Standard-95 (TIA/EIA IS-95), entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM”, published in 1993. Other aspects of CDMA communication systems are defined in well-known standards, such as, for example, TLA/EIA IS-97, TIA/EIA IS-98, cdmaOne, cdma2000, and wideband CDMA (WCDMA) standards.
Wireless communication systems employing CDMA technologies assign a unique code to communication signals and apply pseudorandom noise (PN) modulation to spread these communication signals across a common wideband spread spectrum bandwidth. In particular, the communication signals are modulated with PN sequences to spread the signals over a wide bandwidth. CDMA systems employ two short PN code sequences (i.e., “I” and “Q”) and one long PN code sequence. The short PN codes are used for quadrature spreading and have unique offsets serving as identifiers for a cell or a sector. At the WD
110
receiver, the received spread spectrum signal is despread in order to recover the original data. As long as the WD
110
receiver has the correct code, it can successfully detect and select its communication signal from the other signals concurrently transmitted over the same bandwidth. The encoding/decoding, modulation/demodulation, and spreading/despreading processes depend on accurate timing for synchronization and proper system operation.
FIG. 1
(Prior Art) illustrates a simplified block diagram of CDMA wireless communication system
100
. System
100
allows mobile station or wireless communication device (WD)
110
to communicate with an Interworking Function (IWF)
108
via a base station (BS)
106
. The IWF
108
serves as a gateway between the wireless network and other networks, such as the Public Switched Telephone Network (PSTN) and wireline packet data networks providing Internet- or Intranet-based access. WD
110
communicates with BS
106
, which is associated with a geographic cell or sector, via the wireless interface U
m
on the reverse link transmission path. BS
106
is configured to process the communication signals from WD
110
.
On the forward link transmission path, BS
106
communicates with WD
110
via the wireless interface U
m
. During forward link transmissions, each BS
106
is capable of transmitting information-bearing signals as well as control signals, such as pilot signals. Pilot signals are used to identify the BS
106
best suited to accommodate reverse link transmissions. Pilot signals also provide a time and coherent phase reference to enable WD
110
to obtain initial system synchronization and facilitate coherent demodulation on the forward link. All pilot signals are subjected to the same PN spreading code but with a different code phase offsets to enable WD
110
to distinguish between different pilot signals, thereby identifying the originating BS
106
.
As noted above, proper CDMA system
100
operation requires accurate timing. For example, in accordance with IS-95 and IS-97 standards, each BS 106 is required to use a time base reference from which all time-sensitive transmission components, including pilot PN code sequences and frames, are to be derived. Each BS
106
time base reference is required to be synchronized to CDMA system time. Benefits of synchronized BSs
106
include, for example, improved hand-off speed and reliability, enhanced initial system acquisition (i.e., cell search) speed, increased handset (e.g. WD
110
) stand-by time, and improved reliability and power economy due to common channel hand-off operations.
CDMA system time may employ a Global Positioning System (GPS) time base, which may be synchronized with a Universal Coordinated Time (UTC) reference. GPS and UTC may differ by up to a few seconds to compensate for the number of leap year seconds corrections added to UTC since Jan. 6, 1980. BSs
106
are further required to radiate pilot PN code sequences within ±3 &mgr;s of CDMA System Time and all CDMA channels radiated by BSs
106
are required to be within ±1 &mgr;s of each other. The rate of change for timing corrections may not exceed
⅛ PN chip (
101.725 ns) per 200 ms.
Moreover, in accordance with IS-95 and IS-98 standards, each WD
110
is required to use a time base reference used to derive timing for the transmit chip, symbol, frame slot, and system time. During steady-state conditions, each WD
110
is also required to have a timing reference within ±1 &mgr;s of the time of the earliest arriving multipath component being used for demodulation, as measured at the WD
110
antenna connector. In addition, if WD
110
time reference correction is needed, then it is to be corrected no faster than
¼ PN chip (
203.451 ns) in any 200 ms period and no slower than
⅜ PN chip (
305.18 ns) per second.
These stringent timing requirements are necessary because of the interdependence between BS
106
and WD
110
timing.
FIG. 2
illustrates the timing relationship at various points within system
100
. The start of CDMA System Time is Jan. 6, 1980, 00:00:00 UTC, which corresponds to the start of GPS time, indicated as GPS time stamp zero (GPS TS-0). Because, as noted above, each BS
106
time base reference is to be synchronized to CDMA system time, GPS provides an absolute time reference and each BS
106
transmission includes a GPS time stamp. For convenience, GPS TS-0 will be used heretofore to demonstrate the timing relationships between BS
106
and WD
110
.
As indicated in
FIG. 2
, the interval denoted by reference numeral A
1
, demonstrates the trailing portions of the PN codes sequences conveyed by the pilot signals transmitted by BS
106
during forward link transmissions, prior to the start of CDMA System Time. The notation 0
(n)
denotes a portion of the PN code sequences, which comprise n consecutive zeros. The initial state of the long PN code sequence is configured with a “1” at the most significant bit (MSB), followed by 41 consecutive “0”s. Similarly, the initial state for both, the I and Q short PN code sequences are configured with a “1” at the MSB, followed by 15 consecutive “0”s.
Interval A
2
demonstrates the beginning portions of the pilot PN codes sequences transmitted by BS
106
to WD
110
at GPS TS-0. It is to be noted that BS
106
is synchronized with the absolute time reference provided by GPS in order to transmit pilot signals at exactly 2 second intervals (i.e., even second marks). Even second marks are generally divided into twenty-five 80 ms. periods for CDMA frame boundary timing. Moreover, for the Paging Channel, Forward Traffic Channel, Reverse Traffic Channel, and Access Channel, the 80 ms. period is divided into four 20 ms. frames. For the Sync Channel, the 80 ms. period is divided into three ≈26.66 ms. frames. The pilot PN sequence repeats every ≈26.66 ms. and the ≈26.66 ms. frame boundaries coincide with the pilot PN sequence rollover points, which are offset in the forward CDMA channel to identify the transmitting sector of BS
106
.
Interval B
3
indicates the reception, by WD
110
, of the pilot PN code sequences after a one-way forward link transmission delay (&Dgr;
fl
). The forward link transmission delay &Dgr;
fl
may include delays attributable to the line-of-sight (LOS) propagation delay (&Dgr;
LOS
) between BS
106
and WD
110
, as well as BS
106
and WD
110
processing and hardware delays (&Dgr;
bf
, &Dgr;
wf
, respectively) associated wit
Bachand Richard
Brown Charles
Maung Nay
Qualcomm Incorporated
Sobutka Philip J.
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