Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-12-13
2004-11-09
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S316000, C375S344000
Reexamination Certificate
active
06816540
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an AFC control apparatus and method in a mobile communication system and mobile communication equipment using these apparatus and method and, more particularly, to an AFC control apparatus and method in CDMA mobile communication equipment, which receive a spread signal obtained by phase-modulating a baseband signal, multiply this reception signal with a local signal to obtain a baseband signal, and despread this baseband signal.
2. Description of the Prior Art
In recent years, a CDMA (Code Division Multiple Access) communication scheme resistant to radio interference and invasion attempts have received a great deal of attention as a communication scheme used in a mobile communication system. In CDMA, the transmitting side spreads a user signal using a spreading code and transmits a spread signal. The receiving side despreads the reception signal using the same spreading code as that on the transmitting side to obtain an original user signal.
The CDMA communication scheme does not allow despreading reading on the receiving side unless the phase of the spreading code sequence on the transmitting side is locked with that on the receiving side. For this purpose, a mobile station uses a reference oscillator TCXO (Temperature Controlled Xtal Oscillator) for generating a reference frequency signal used for demodulating a signal received from base station. At the same time, the mobile station performs AFC (Automatic Frequency Control) control for matching the frequency of the reference frequency signal with that of the reference frequency signal of the transmitting base station. This AFC control is performed with reference to a pilot symbol contained in data transmitted on the base station.
The signal format of a downstream channel serving as a channel from the base station to the mobile station will be described with reference to FIG.
1
. Transmission data from the base station is constituted by a plurality of radio frames having intervals of 10 ms. Each radio frame consists of 16 time slots. Each time slot is used to simultaneously output an audio channel and common pilot channel. These channels are spread using different spreading codes (both are known). In the audio channel, audio data is formed into a data symbol and transmitted together with a plurality of pilot symbols (e.g., two symbols). In the pilot channel, only pilot symbols containing a variety of control information is transmitted. For example, the pilot channel has 10 pilot symbols.
The pilot symbols of the audio channels are different in different time slots, but have a predetermined pattern. For this reason, the mobile station can anticipate a pilot symbol before it is received. The data symbol is used for information such as audio information. The mobile station can measure any frequency error using this pilot symbol with respect to the frequency of the base station.
This frequency error will be described with reference to FIG.
2
. The CDMA communication scheme uses QPSK (Quadrature Phase Shift Keying) as a linear modulation scheme before spreading and modulation. Each symbol is 2-bit data which takes any one of (0,0), (0,1), (1,0), and (1,1). These values are plotted on the vector diagram shown in FIG.
2
. In other words,
FIG. 2
shows the phase shift amounts of pilot symbols used in frequency error measurements for AFC control.
In
FIG. 2
, the magnitude of the in-phase (I) component is plotted along the abscissa, while the magnitude of the quadrature component (Q) is plotted along the ordinate. The vector of transmission data from the base station takes one of (0,0), (0,1), (1,0), and (1,1). If the vector of transmission data is predetermined like a pilot symbol, all pilot symbols can be plotted on the vector diagram by directly plotting (0,0) and rotating (0,1), (1,0), and (1,1), respectively, through +90 degrees, −90 degrees, and 180 degrees.
If a frequency error &thgr; is present in the mobile station, the actually measured data of the pilot symbol located on (0,0) seems to be phase-shifted from the vector of the first pilot symbol to the vector of the second pilot symbol, as shown in FIG.
2
. This frequency error &thgr; is converted into a voltage, and the control voltage of the TCXO is controlled.
AFC control will be described with reference to FIG.
3
. An antenna
1
receives a signal transmitted from the base station. A mixer
2
removes a carrier wave from the reception signal to obtain a baseband signal. An A/D converter
3
converts this baseband signal into a digital signal. A depreading section
4
multiplies the digital signal with a PN code (despreading code: C
1
to C
256
for a spread rate of 256) to obtain a despread signal. An integrator
5
obtains the integral of one symbol of the despread output, thereby generating one-symbol data.
A frequency error measuring section
7
calculates a frequency error value using the pilot symbols of the symbol data. An AFC control section
8
converts the calculated frequency error value into a control voltage for a TCXO
9
to control the frequency of the TCXO
9
. The phase shift angle between the adjacent symbols is measured using the pilot symbols of the symbol data obtained from the integrator
5
. The phase shift angle is converted into a frequency error value, and the TCXO is controlled with the control voltage.
Exemplifying the relationship between the phase rotation amount (phase shift amount) and frequency error between adjacent symbols, when the symbol rate and the phase rotation amount between the first and second pilot symbols are 15 ksps and 90 degrees, respectively, the frequency error is given by
Frequency Error &thgr;=15 ksps×(90 degrees/360 degrees)=3.75 kHz
If the frequency of the carrier wave is 2 GHz, a shift of 3.75 kHz becomes a frequency error value of 3.75 kHz/2 GHz=1.875 ppm.
Assume that the frequency error value of the TCXO reference frequency in initial setting is large, e.g., 5 ppm (when an inexpensive TCXO is used to reduce the unit cost of a mobile station, the initial frequency error value is about 5 ppm). In this case, as shown in
FIG. 4A
, the phase shift angle between the adjacent symbols is +225 degrees, exceeding +180 degrees. The actually measured phase shift amount is calculated not as +225 degrees, but as −135 degrees in the erroneous symbol moving direction, resulting in AFC control error.
Japanese Unexamined Patent Publication No. 9-331307 proposes a frequency control technique of dividing one symbol into a plurality of portions, e.g., first and second half portions in place of using an intersymbol phase shift amount, and obtaining a phase shift amount between the symbol data of the first and second half portions.
FIG. 5
is a block diagram showing AFC control disclosed in this reference. The same reference numerals as in
FIG. 3
denote the same parts in FIG.
5
. Only parts different from
FIG. 3
will be described with reference to FIG.
5
. An integrator
5
comprises two integrators 5.1 and 5.2 for integral data of the first half portion (0 to T/2; T is a symbol period) and the second half portion of one symbol. A frequency error measuring section
7
calculates a phase shift amount between outputs from these two integrators, thereby obtaining a frequency error.
The detailed arrangement of a despreading section
4
and the integrator
5
in
FIG. 5
is shown in FIG.
6
. Output data from an A/D converter
3
is sequentially latched by cascade-connected F/Fs (flip-flops)
41
at chip clock timings. Multipliers
42
multiply these latch outputs with despreading codes C
1
to C
256
(for a spread rate of 256) to obtain product signals. Adders
51
to
53
constructing the integrator
5
add these product signals. At this time, the adder
52
outputs the sum data (integral) of the first half portion (first product output to 129th product output) of 0 to T/2 of one symbol. The adder
53
outputs the sum data of the second half portion (130th product output to 256th pro
Chin Stephen
Perilla Jason M.
Whitham Curtis & Christofferson, PC
LandOfFree
AFC control apparatus and method in mobile communication... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with AFC control apparatus and method in mobile communication..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and AFC control apparatus and method in mobile communication... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3362277