Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
2001-03-12
2004-03-09
Tran, Congvan (Department: 2683)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S246100, C455S191300, C455S192100, C455S255000
Reexamination Certificate
active
06704555
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the field of wireless communication systems. More particularly, the present invention relates to a novel and improved apparatus and method for calibrating the crystal oscillator in a receiver to derive more accurate timing.
2. Related Art
In wireless communication systems, such as code division multiple access (“CDMA”) systems, the transmitter and receiver are time synchronized for use in signal demodulation and decoding. In CDMA systems each user uniquely encodes its message signal into a transmission signal in order to separate the signal from those of other users. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the message.
Transmitter and receiver time synchronization can be achieved by transmitting a pilot signal from a base station to a mobile unit. The mobile unit can use the pilot signal to correct a local oscillator which the mobile unit uses as a timing reference. For example, in IS-95 and cdma2000 systems, by tracking the pilot signal, the mobile unit may obtain an accurate timing reference from the base station timing, which is derived from GPS.
The local oscillator used as a timing reference by the mobile unit is typically a crystal oscillator. The crystal oscillator may be a voltage controlled oscillator (“VCO”), for example, which uses a reference or tuning voltage to control, i.e. to change, the frequency of the oscillator. Other types of frequency control are also possible. For example, an oscillator could be a current controlled oscillator.
The oscillation frequency of a crystal oscillator may be affected by changes in the ambient temperature. A temperature compensated oscillator is designed to reduce or compensate for the change of oscillation frequency due to changes in temperature.
The oscillation frequency of the crystal oscillator is usually specified as a nominal frequency within a range of tolerance. For example, an oscillator may be specified as a 19.68 MHz (Mega-Hertz or million cycles per second) frequency oscillator rated at +/−5 parts per million (“ppm”) error. Then, for example, if the 19.68 MHz oscillator is used to synthesize an 800 MHz carrier for application to an RF mixer, the synthesized carrier frequency applied to the RF mixer may be expected to be accurate to within 4,000 Hz error (5 parts per million×800 million Hz or 5×800 Hz). The frequency error may be corrected or compensated for by using, for example, an automatic frequency control loop composed of a frequency error detector, a loop filter, and a voltage controlled oscillator.
The frequency error detector, which is often referred to as a discriminator, computes a measure of the difference between the received carrier frequency and the synthesized carrier frequency, referred to as an “error measure”. This error measure is filtered to produce a “digital” control signal that is converted into an analog tuning signal that is fed to the voltage controlled oscillator. Tuning the voltage controlled oscillator modifies the frequency of the synthesized carrier. In matching the received carrier frequency with the synthesized carrier frequency, this closed loop feedback corrects the timing of the local oscillator.
The frequency error of an oscillator may be affected by several factors. These factors may include (i) temperature, as noted above, (ii) aging of the crystal and other components, (iii) differences in operating voltages within and between mobile units, and (iv) differences in components from one mobile unit to another.
The multitude of different factors and their variability over time cause considerable difficulty in calibrating the digital control signal that is applied to correct the timing of the local oscillator. For this reason, calibration is typically not performed in current wireless mobile systems. Instead, frequency tracking using the feedback of an automatic frequency control loop is relied upon to push the digital control signal in the correct direction. In other words, the digital control signal has the correct arithmetic sign, positive or negative, but the magnitude of the digital control signal is not sufficiently exact for certain purposes in which it is desirable to translate a known, or predetermined, frequency error directly into a digital control signal to correct the timing of the local oscillator.
FIG. 1
illustrates a previous approach using frequency tracking in an automatic frequency control loop in a CDMA wireless communication system. Frequency tracking system
100
shown in
FIG. 1
might, for example, constitute part of a receiver in a CDMA mobile unit. Frequency tracking system
100
may communicate, for example, via radio frequency (“RF”) signal propagation between a base station transmit antenna (not shown) and receive antenna
102
connected to RF front end
104
. RF front end
104
typically uses frequency synthesizers, which match the frequency of the RF carrier, to convert the RF signal to a baseband frequency signal, i.e. the encoded message signal before it was modulated onto the RF carrier for transmission which is more concisely referred to as a “baseband signal”.
The frequency synthesizers used by RF front end
104
receive timing reference
103
from local oscillator
106
, which is a voltage controlled oscillator (“VCO”) in the present example. As seen in
FIG. 1
, the digital baseband signal has an in-phase component, referred to as I component
105
, denoted “I” in
FIG. 1
, and a quadrature component, referred to as Q component
107
, denoted “Q” in FIG.
1
.
Continuing with
FIG. 1
, I component
105
and Q component
107
of the digital baseband signal are fed as an input signal to frequency error discriminator
110
. Pilot demodulation module
112
demodulates the input signal as a sequence of symbols, also referred to as a “sequence of pilot symbols”. Each pilot symbol has an I component
113
and a Q component
115
so that it can be represented as a vector in a 2-dimensional IQ plane, where the I component lies on the horizontal axis, and the Q component lies on the vertical axis. A frequency error, i.e. a mismatch between the carrier frequency synthesized to demodulate the incoming signal in RF front end
104
and the incoming carrier frequency, in local oscillator
106
causes the received sequence of baseband pilot symbols to rotate around the 2-dimensional IQ plane.
Each pilot symbol composed of I and Q components
113
and
115
is fed to unit delay elements
114
and
116
, respectively, and also to phase rotation measure module
118
. Unit delay elements
114
and
116
make previous pilot symbol composed of I and Q components
117
and
119
, respectively, available to phase rotation measure module
118
at the same time as current pilot symbol composed of I and Q components
113
and
115
, so that phase rotation measure module
118
can compute the phase rotation between successive pilot symbols. Each pilot symbol is represented as a vector in a 2-dimensional IQ plane, where the I component is mapped to the horizontal axis, and the Q component is mapped to the vertical axis.
Any 2-dimensional vector (x,y) can be represented in polar coordinates as (r, &thgr;), where r={square root over (x
2
+y
2
)} and
θ
=
tan
-
1
(
y
x
)
.
If, for example, the current pilot symbol composed of I component
113
and Q component
115
is represented in polar coordinates as (r
1
, &thgr;
1
), and the previous pilot symbol composed of I component
117
and Q component
119
is represented in polar coordinates as (r
0
, &thgr;
0
), then phase rotation measure module
118
outputs error measure
121
represented as the phase difference (&thgr;
1
−&thgr;
0
) between successive pilot symbols. Thus, error measure
121
is directly proportional to the frequency error in local oscillator
106
. Phase rotation measure module
118
outputs error measure
121
to gain &agr; filter
122
.
Continuing with
FIG. 1
, error measure
121
, which may
Challa Raghu
Sih Gilbert C.
Brown Charles D.
Qualcomm Incorporated
Seo Howard H.
Tran Congvan
Wadsworth Philip R.
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