Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-09-24
2003-12-02
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
C375S149000, C375S148000, C375S147000, C375S145000, C375S137000, C375S246000, C375S253000, C375S325000, C375S326000, C375S347000
Reexamination Certificate
active
06658042
ABSTRACT:
TECHNICAL FIELD
The present claimed invention relates to the field of digital communication. Specifically, the present claimed invention relates to an apparatus and a method for providing time tracking of a signal using hardware and software.
BACKGROUND ART
Wireless telephony, e.g. cellular phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technology. Because of increasing demand and limited resources, a need arises to improve their capacity, fidelity, and performance.
Referring to prior art
FIG. 1A
, a conventional base station
104
and a mobile unit
102
, e.g. a cell phone, are shown. A CDMA system uses a common bandwidth to transmit the pilot signal and a data signal
106
between a base station
104
and a mobile unit
102
, for multiple users. Hence, the bandwidth is occupied by an amalgam of many signals. In order to extract an individual signal from the amalgam of signals, the individual signal is modulated at the transmitter and demodulated at the receiver.
Referring now to prior art
FIG. 1B
, two signals having an exemplary PN sequence
100
b
are shown. The modulation code used for CDMA is a pseudonoise (PN), e.g. an apparently random sequence of
1
's
120
and
0
's
118
having a given length, e.g.
110
a
. A “long” PN sequence is used to scramble user data, while a “short” PN sequence is used to spread quadrature components for the forward and reverse link wave forms.
The short PN sequence is used for a pilot signal. A base station broadcasts a pilot signal to which a mobile receiver may receive and synchronize, for time tracking purposes. Thus, as shown in prior art
FIG. 1B
, a first signal
114
a
of a conventional PN sequence, e.g.
110
a
, is repeatedly generated, e.g.
110
b
, by a first device such as a mobile unit
102
of prior art FIG.
1
A. Similarly, the same conventional PN sequence, e.g.
112
a
, is repeatedly generated, e.g.
112
b
, by a second device such as a base station
104
of prior art FIG.
1
A.
The mobile unit and base station can communicate to each other when they properly align the starting points of the known PN sequence. Proper alignment is utterly critical, in-applications such as CDMA, because it is necessary for accurately applying the PN sequence to demodulate a signal out of the amalgam of signals that occupy the bandwidth. While a signal may be correctly aligned at the beginning of the signal, e.g.
116
, it may not be properly aligned throughout the entire signal, e.g. offset error
118
, as shown in prior art FIG.
1
A. The misalignment can arise from a difference in the frequency V
cxo
of the PN sequence generated at each unit. This is referred to as a frequency offset between a base station and a mobile unit. Hence a need arises for time tracking a signal generated by one device with a signal received from another device to ensure proper alignment of the signals for modulation and demodulation.
Determining the alignment of the signals can be accomplished by performing digital signal processing (DSP), such as a correlation operation, on the signals. However, imperfections in transmitting, receiving, modulating, and demodulating a signal, inherently integrate noise into the signal. The noise hampers, and sometimes prevents, the DSP operation to demodulate a signal and receive the data contained therein. To improve the time tracking operation, a need arises to remove the noise from the correlation data used for time tracking.
Referring now to prior art
FIG. 1C
, a conventional time tracking block diagram is shown. Conventional time tracking uses hardware
254
to perform the time tracking operations and provide input to system timing
256
. Conventionally, the time tracking hardware removes noise from the signal by passing the signal through a hardware filter
254
a
. However, by using hardware, the conventional filter
254
a
has a fixed configuration. That is, the conventional filter has a single configuration that cannot be changed, unless the hardware is redesigned and replaced. But different operating circumstances and resources for a communication device or a communication system may allow or demand higher or lower performance filters. To optimize performance and resources, a need arises to provide a flexible filter for removing noise from data used for time tracking.
Once a signal is correlated and normalized, it can be compared to a threshold. Conventional methods divide correlation data by a normalizing value. However, a division operation is more difficult to implement in an electronic component. Consequently, a need arises to simplify the normalization operation of a threshold value used for time tracking.
Conventionally, a filter used for correlation data is not reset after a timing correction is made. Consequently, the filter retains and evaluates some obsolete data for a subsequent timing correction. This practice can provide incorrect timing corrections and possibly lead to timing oscillation. Overall, the conventional practice degrades system performance. Hence, a need arises for a method to avoid filtering obsolete data once a timing correction has been made.
In summary, an apparatus and a method is needed to improve the capacity, fidelity, and performance of digital communication. More specifically, a need arises for time tracking a signal generated within a mobile device to a signal received from a base station to ensure proper alignment of the signals. To improve the time tracking, a need arises to remove the noise from the correlation data used for time tracking. To optimize performance and resources, a need arises to provide a flexible filter for removing noise from pilot channel used for time tracking. Also, a need arises for a method to avoid filtering obsolete data once a timing correction has been made. Additionally, a need arises to simplify the normalization operation of a threshold value used for time tracking.
DISCLOSURE OF THE INVENTION
The present invention provides a method and apparatus for improving the capacity, fidelity, and performance of digital communication. More specifically, the present invention provides improved time tracking between a signal from a base station and a signal generated within a mobile device. The present invention improves time tracking by remove noise from the correlation data used for time tracking. To optimize performance and resources, the present invention provides a flexible filter for removing noise from correlation data used in time tracking. Additionally, the present invention provides a method to avoid filtering obsolete data once a timing correction has been made, thereby improving system performance. Additionally, the present invention provides a more efficient method of normalizing a threshold value used for time tracking.
In one embodiment, the present invention recites a method comprising several steps. In one step, a communication device, such as a cell phone, generates a first signal and receives a second signal from an outside source, such as a base station. Both signals are designed to have the same finite and repeating, but not necessarily aligned, pseudonoise (PN) sequence that can be used to synchronize the timing between the two devices. Next, correlation data between a first signal, at a plurality of timing conditions, and a second signal is generated by hardware in the communication device. The plurality of timing conditions allows an on-time condition, an early condition, and a late condition of the first signal to be evaluated for alignment with the second signal. By evaluating several timing conditions, the direction for optimal correlation between the signals can be determined. The correlation data is filtered by software or firmware at the plurality of timing-conditions to remove noise and thereby provide more accurate correlation data for subsequent evaluation. By implementing the filtering function using software or firmware, the present invention provides a ver
McDonough John G.
Tran Howard (Hau) Thien
Chin Stephen
Koninklijke Philips Electronics , N.V.
Muńoz Guillermo
Zawilski Peter
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