Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
1999-11-23
2003-11-18
Vo, Nguyen T. (Department: 2686)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S426100
Reexamination Certificate
active
06650874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to cordless telephones. In particular, this invention relates to correction of a local oscillator of a remote handset in a cordless telephone.
2. Background of Related Art
Cordless telephones have gained in popularity over the years, and can now be found in many if not most homes or businesses. A cordless telephone is one in which the handset is not wired to its base unit, but instead uses wireless communication techniques between a remote handset and its base unit, typically allowing the remote handset to be used up to 1000 feet or more away from its base unit.
FIG. 8A
illustrates a typical remote handset
800
of a digital cordless telephone.
The remote handset
800
includes a controller
805
, a coder-decoder (CODEC)
810
, a speaker
815
, a microphone
820
, a radio frequency (RF) transceiver
825
, a local oscillator
830
, an EEPROM
835
, a keypad
840
, a timing recovery circuit
845
and a program ROM
837
.
In the transmit direction, the microphone
820
outputs an analog signal to the CODEC
810
, which converts the microphone input signal to a digital microphone signal. As part of the conversion process, a clock signal is provided from the local oscillator
830
for the CODEC
810
to sample the microphone signal. The digital microphone signal is then passed to the RF transceiver
825
for encoding into a radio frequency (RF) signal for transmission to a complementary base unit. The controller
805
also retrieves frequency control information from the EEPROM
835
to select the frequency that the RF transceiver
825
transmits. The program ROM
837
also provides a storage medium for the software that operates the remote handset
100
and for a security word.
In the receive direction, the RF transceiver
825
receives a RF signal from the complementary base unit. The RF transceiver
825
converts the RF signal to a digital signal that is passed to the CODEC
810
for decoding. The timing recovery circuit
845
provides correction information to the controller
805
to adjust the local oscillator
830
for the decoding of the digital signal. The output of the CODEC
810
is an analog signal for output by the speaker
815
.
FIG. 8B
illustrates a base unit
850
of the digital cordless telephone. The base unit
850
contains circuitry which is complementary to that contained in the remote digital handset
800
, i.e., a complementary RF transceiver
870
, a controller
855
, a CODEC
860
, an EEPROM
880
, a program ROM
882
, a timing recovery circuit
885
and a local oscillator
875
. The base unit
850
also includes a telephone line interface
865
to interface with a public switched telephone network and a ring detect circuit
890
to detect the ring signal corresponding to an incoming telephone call.
For optimum performance between the remote handset
800
and the base unit
850
, both local oscillators,
830
and
875
, typically need to be frequency aligned. Preferably, the handset's local oscillator
830
typically needs to be frequency aligned with the base unit's local oscillator
875
to within 1 part per million (ppm) for reliable and noise-free communication.
A local oscillator may drift for a variety of reasons. A temperature change, a voltage change, or a tolerance variation in the components used in the digital cordless telephone may contribute to local oscillator drift.
There are several ways to correct for local oscillator drift. One method is called a coarse frequency search. A remote handset of a cordless telephone in the coarse frequency search will adjust the remote handset's oscillator to within a range of 5 ppm from as far off as 300 ppm. The coarse frequency search may be performed at any time, but its purpose is to achieve frequency alignment to within about 5 ppm at best. A coarse frequency search is very time-consuming, e.g., 1-2 sec., and will drain the remote handset's battery if done while the cordless telephone is off-hook.
Another method to correct for local oscillator drift is to use a synchronization bit(s) or frame. In a typical cordless telephone, a remote handset and a base unit communicate over the RF link using packets or frames. As part of the frame, several bits are reserved as synchronization bits.
FIG. 9
illustrates a typical frame
900
used in communication between a remote handset and a base unit including a synchronization field.
As shown in
FIG. 9
, the frame
900
includes a data field
910
, error correction code (“ECC”) field
920
and a synchronization field
920
. Each respective field includes a number of bits. The number of bits per field is dependent on the functionality of the field.
The data field
910
of the frame
900
typically contains the encoded voice signals.
The ECC field
920
of the frame
900
typically contains the error correction code for the data field
910
. As the voice signals are encoded, typically, an error correction code is included in the frame
900
to ensure that the voice signals are properly transmitted and received.
The synchronization field
930
provides a method for a remote handset and base unit to frequency align by using the synchronization field to correct the receiving local oscillator or to derive a clock signal.
Although this method is effective, the synchronization field technique requires time for the receiving remote handset or base unit to frequency align. Moreover, this synchronization time may introduce unwanted delays in the communications between the base unit and the remote handset.
There is a need for an improved method and/or apparatus to frequency align a remote handset's local oscillator with a base unit's local oscillator to a high degree, e.g., to within 1 ppm for reliable and noise free communication.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, a method to frequency align a remote handset with a base unit of a digital cordless telephone is disclosed. The method comprises exchanging a series of commands between the remote handset and the base unit. The apparatus frequency aligns a local oscillator located within the remote handset during the exchanging of commands.
Another aspect of the present invention is an apparatus to frequency align a remote handset with a base unit of a digital cordless telephone. The apparatus comprises a local oscillator located within the remote handset and a controller adjusting the local oscillator when a timing slip is greater than a predetermined threshold.
Another embodiment of the present invention is a method to frequency align a remote handset with a base unit of a digital cordless telephone. The method comprises reviving the remote handset from a standby mode and initiating a normal link verification with the base unit by the remote handset. The method subsequently continues by attempting to establish a link between the remote handset and the base unit. A local oscillator is frequency aligned within the remote handset during the attempted establishment of a link.
Yet another aspect of the present invention is an apparatus to frequency align a remote handset with a base unit of a digital cordless telephone. The apparatus comprises a local oscillator and a controller waking from a standby mode to initiate a normal link verification to align the local oscillator of the remote handset with the local oscillator of the base unit.
REFERENCES:
patent: 4726051 (1988-02-01), Schuermann
patent: 5249305 (1993-09-01), Wieczorek et al.
patent: 5307370 (1994-04-01), Eness
patent: 5436937 (1995-07-01), Brown et al.
patent: 5764689 (1998-06-01), Walley
patent: 5949291 (1999-09-01), Newland
patent: 6125139 (2000-09-01), Hendrickson et al.
patent: 6470057 (2002-10-01), Hui et al.
Banik Somnath
Cannon Joseph M.
Diethorn Eric J.
Grundvig Jeffrey P.
Johanson James A.
Agere Systems Inc.
Bollman William H.
Ly Nghi H.
Vo Nguyen T.
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