Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-01-26
2003-08-19
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S148000
Reexamination Certificate
active
06608858
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for frequency tracking of multipath signals which have been subjected to doppler shifts.
II. Description of the Related Art
Frequency tracking loops are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1
provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units
10
a-d
conduct wireless communication by establishing one or more RF interfaces with one or more base stations
12
a-d
using CDMA modulated RF signals. Each RF interface between a base station
12
and a subscriber unit
10
is comprised of a forward link signal transmitted from the base station
12
, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO)
14
and public switch telephone network (PSTN)
16
. The links between base stations
12
, MTSO
14
and PSTN
16
are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
Each subscriber unit
10
communicates with one or more base stations
12
by utilizing a rake receiver. A RAKE receiver is described in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilot from neighboring base stations, and two or more fingers for receiving and combining information signals from those base stations. Searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled “MULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS”, filed Sep. 30, 1994, assigned to the assignee of the present invention and incorporated herein by reference.
In any passband digital communication system, such as that described above in relation to
FIG. 1
, there is a need for carrier synchronization. The sender modulates information onto a carrier at frequency f
c
, and the receiver must recover this frequency so that the received signal constellation does not rotate and degrade the signal to noise ratio (SNR) of the demodulated symbols. In the following discussion, the sender is a CDMA base station and the receiver is a CDMA subscriber unit.
Although the receiver knows the nominal carrier frequency, there are two main sources of error that contribute to the frequency difference between the received carrier from the base station and the carrier produced at the subscriber unit. First, the subscriber unit produces the carrier using a frequency synthesizer that uses a local clock as its timing reference. An example RF/IF section of a conventional heterodyne CDMA receiver is shown in
FIG. 2. A
signal received at antenna
200
is passed through low-noise amplifier (LNA)
202
and filtered in filter
204
before being mixed down to IF by RF mixer
206
. This IF signal is filtered in filter
208
, passed through variable-gain amplifier (VGA)
210
and is then mixed down to baseband by IF mixer
212
. The baseband signal is then filtered in filter
214
and passed through analog to digital converter
216
to produce IQ symbols at baseband.
The carrier waveforms sent to RF and IF mixers
206
and
208
are produced using frequency synthesizers
218
and
220
, respectively, that use the subscriber unit's local clock as a timing reference. This clock has an unknown timing error, typically expressed in parts per million (ppm). In the exemplary implementation, this clock is voltage-controlled temperature compensated crystal oscillator (VCTCXO)
222
, whose frequency is 19.68 MHz and is rated at +/−5 ppm. This means if the desired waveform is a cellular 800 MHz carrier, the synthesized carrier applied to the RF mixer can be 800 MHz+/−4000 Hz. Similarly, if the desired waveform is a 1900 MHz PCS carrier, the synthesized carrier can be 1900 MHz+/−9500 Hz. To correct this error, CDMA receivers use a frequency tracking loop that monitors the frequency error and applies a tuning voltage to VCTCXO
222
to correct it.
The second source of error is due to frequency doppler created from movement of the subscriber unit station. The doppler effect manifests as an apparent change in the frequency of a received signal due to a relative velocity between the transmitter and receiver. The doppler contribution can be computed as
f
D
=
v
λ
cos
θ
=
vf
c
cos
θ
where v is the velocity of the subscriber unit, x is the wavelength of the carrier, f is the carrier frequency, and c is the speed of light. The variable &thgr; represents the direction of travel of the subscriber unit relative to the direction of the received path from the base station. If the subscriber unit is travelling directly toward the base station, &thgr;=0 degrees. If the subscriber unit is travelling directly away from the base station, &thgr;=180 degrees. So the carrier frequency received at the subscriber unit changes depending on the speed and direction of the subscriber unit relative to the received signal path.
As mentioned above, CDMA systems use RAKE receivers that combine symbol energy from different paths. Each strong path is tracked by a finger that performs despreading, walsh decovering and accumulation, pilot time and frequency tracking, and symbol demodulation. An exemplary finger architecture is shown in
FIG. 3
, where each of N fingers
3
A-
3
N outputs pilot and data symbols obtained for the path it is tracking to digital signal processor (DSP)
300
. DSP
300
performs symbol demodulation and implements the time and frequency tracking loops. IQ baseband samples are despread in PN despreaders
310
A-
310
N, and I and Q pilot and data samples are produced in walsh decover and accumulate blocks
320
A-
320
N and
330
A-
330
N, respectively.
An exemplary IS-95A CDMA receiver has four fingers to track four paths, whereas an exemplary cdma
2
OOO CDMA receiver has
12
fingers to handle the
3
x multicarrier case. Cdma
2
OOO is described in TIA/EIA/IS-
2000
-
2
, entitled “PHYSICAL LAYER STANDARD FOR CDMA
2000
SPREAD SPECTRUM SYSTEMS”, incorporated herein by reference. A subscriber unit can be tracking paths from different base stations (in soft handoff), as well as time-delayed paths from the same base station, created from reflections off of local objects. Since the angle &thgr; can be differ
Agrawal Avneesh
Butler Brian K.
Challa Raghu
Roh Mark
Sih Gilbert C.
Brown Charles D.
Chin Stephen
Pauley Nicholas J.
Qualcomm Incorporated
Wadsworth Philip R.
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