Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-12-18
2002-04-02
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S371000
Reexamination Certificate
active
06366604
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to a code division multiple access (CDMA) communication system having a circuit for providing compensation for phase errors due to clock jitter.
2. Background Art
Code Division Multiple Access (CDMA) is a form of digital cellular phone service that assigns a code to all speech bits, sends a scrambled transmission of the encoded speech over the air and reassembles the speech to its original format. CDMA combines each phone call with a code which only one cellular phone plucks from the air.
CDMA operates in conjunction with spread spectrum transmission. A transmitter takes the original information signal and combines it with a unique correlating code to produce a radio frequency (RF) signal that occupies a much greater bandwidth than the original signal. RF signals from several transmitters are spread across the same broad frequency spectrum. The dispersed signals are pulled out of the background noise by a receiver which knows the code. By assigning a unique correlating code to each transmitter, several simultaneous conversations can share the same frequency allocation.
A typical CDMA system comprises a plurality of cells or designated regions, a base station associated with each cell and a plurality of mobile units. CDMA systems require transmission schemes which efficiently use an allocated frequency band so that a maximum number of mobile units can be accommodated with a minimum amount of interference. In accordance with CDMA standards, a communication link from a mobile unit to a base station is called a reverse link, and a communication link from a base station to a mobile unit is called a forward link. Communication in the reverse link is particularly difficult because a base station must be able to distinguish among all of the information signals transmitted from mobile units located within its particular cell. To provide communication in the reverse link, a CDMA mobile unit has a transmitter that produces an RF carrier signal based on an information signal.
As shown in
FIG. 1
of the drawings, a typical transmitter
20
of a CDMA mobile telephone set has a data input
21
for supplying a binary information sequence to be transmitted. For example, input data may be encoded using a Non-Return to Zero (NRZ) encoding scheme in -which ones and zeroes are represented by opposite and alternating high and low voltages. To utilize the entire available channel bandwidth, the phase of the carrier should be shifted pseudo-randomly according to pseudo-noise (PN) sequences. In CDMA cellular systems, pseudo-random data spreading is defined in the Telecommunication Industry Association (TIA)/Electronic Industry Association (EIA) Interim Standard TIA/EIA/IS-95-A (May 1995) entitled Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, and incorporated herein as a reference.
To provide quadrature data spreading, input data is processed in in-phase (I) and quadrature-phase (Q) channels of the transmitter
20
using in-phase and quadrature-phased pilot pseudo-noise (PN) sequences PNI and PNQ, respectively, defined in the TIA/EIA/IS-95-A Standard. In accordance with this standard, the PNI and PNQ sequences are periodic signals generated at a rate of 1.2288 Mchip/sec based on characteristic polynomials.
Multipliers
22
and
24
are respectively arranged in I- and Q-channels to multiply the input data by PNI and PNQ sequences. Up-sampling circuits
26
and
28
up-sample output values of the multipliers
22
and
24
, respectively, by a factor of 8. In addition, the Q-channel contains a delay circuit for delaying the output of the circuit
28
by ½ chip equal to 4 samples. The outputs of circuits
26
and
29
are supplied to n-tap finite impulse response (FIR) filters
30
and
32
respectively arranged in the I- and Q-channels. The up-sampling circuits
26
and
28
, delay circuit
29
and the FIR filters
30
and
32
are defined in the TIA/EIA/IS-95-A standard for a factor 4 up-sampling.
Outputs of the FIR filters
30
and
32
are supplied to digital-to-analog (D/A) converters
34
and
36
, respectively. For example, the outputs of the FIR filters
30
and
32
may be represented by 10-bit digital signals. The D/A converters
34
and
36
produce analog signals Vi and Vq respectively supplied via anti-aliasing low-pass filters (LPF)
38
and
40
to I and Q inputs of a transmitting circuit
42
that performs offset quadrature phase-shift keying (QPSK) to produce a modulated radio-frequency signal transmitted to a base station using an antenna
44
.
To synchronize signal processing in the I- and Q-channels, the transmitter
20
comprises a digital synthesizer
46
that produces an internal clock signal in response to an external clock signal. The internal clock signal is supplied to the FIR filters
30
and
32
and to the D/A converters
34
and
36
.
To support signal processing in the I- and Q-channels, the internal clock frequency of the transmitter
20
must be equal to a chip rate multiplied by 8. As the standard chip rate is equal to 1.2288 Mchip/sec, the internal clock must be produced at a frequency fx8 equal to 9.8304 MHz. However, to meet the frequency plan requirements of a CDMA mobile telephone, a reference clock supplied to the transmitter
20
may have a frequency fref different from 9.8304 MHz. For example, fref may be equal to 14.4 MHz.
In this case, fref/fx8=14.4 MHz/9.8304 MHz=375/156=1.468. Thus, in 375 cycles of the reference clock, 256 cycles of the internal clock are produced. Therefore, in order to produce an internal clock signal with 256 cycles, 119 clock cycles have to be removed from the 375 cycle reference clock.
The closest digital division ratio to 1.468 is 1.5. If this division ratio occurs over M cycles of the reference clock, then the number N of internal clock cycles removed over M cycles may be expressed as N=M−(M/1.5)=119. Therefore, M=357. Accordingly, 357 out of 375 reference clock cycles are converted with the division ratio equal to 1.5, and for the remaining 18 cycles, the division ratio is equal to 1.
Thus, during the conversion of the reference clock into the internal clock, the division ratio must be changed, for example, from 1.5 to 1. As a result, jitter in the internal clock occurs.
For example, if the division ratio is changed from 1.5 to 1, the reference time t changes by a half cycle of the reference clock. The reference time change &Dgr;t is equal to ½ of {fraction (1/14.4)} MHz={fraction (1/28.8)} MHz=34.7 nanoseconds.
The clock jitter can be considered as parasitic “ramped phase” modulation illustrated in FIG.
2
. Such modulation causes substantial phase errors at the output of the transmitting circuit
42
. For example, the parasitic phase step &Dgr;&THgr;p−p caused by the reference time change At can be expressed as follows:
&Dgr;&THgr;p−p=(&Dgr;t/Tx8)×2&pgr;,
where Tx8 is the period of the internal clock.
Accordingly, &Dgr;&THgr;p−p=(9.8304/28.8)×2 &pgr;=2.143 radian=34% of 1 cycle. In the above-illustrated example, the root-mean-square phase error &Dgr;&THgr;rms produced as a result of the clockjitter can be expressed as &Dgr;&THgr;rms=&Dgr;&THgr;/{square root over (12)}=0.6186 rad.
The phase error due to clock jitter causes an error voltage at outputs of the D/A converters. As a result, the transmitter
20
has a high level of spurious emissions radiated at frequencies outside the assigned CDMA channel. As shown in
FIG. 3
, which illustrates a simulated spectrum of a transmitted RF signal in a conventional CDMA reverse link, the transmitted RF signal has poor adjacent channel power ratio (ACPR), which characterizes spurious emissions outside the assigned CDMA channel. Spurious emissions and the method of their measurement are defined in the TIA/EIA/IS-98-A Interim Standard entit
Chin Stephen
Jiang Lenny
Philips Electric North America Corporation
Vodopia John F.
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