Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2000-01-18
2004-08-03
Duong, Frank (Department: 2666)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S350000, C370S508000
Reexamination Certificate
active
06771629
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods of in-band signaling for measurement of system latency in wireless and wire line communications and, in particular, to the use of latency measurements for time synchronization and synchronization error measurement between a reference clock and a remote clock in communication over a wireless and/or wire line voice communication network.
BACKGROUND OF THE INVENTION
Numerous methods of signaling are known for accurately synchronizing a slave oscillator with a distant master oscillator. One such known method uses SPS signals that are transmitted from master oscillator-bearing earth satellites of a satellite positioning system (SPS) such as the Global Positioning System (GPS) or GLONASS. A slave oscillator is synchronized to a SPS master oscillator in a normal SPS signal receiving mode called “lock.” In a mobile unit including an SPS positioning receiver, the amount of synchronization error between the SPS master oscillator and a slave oscillator of the SPS positioning receiver impacts the ability of the SPS positioning receiver to accurately determine its position from the SPS signals using satellite ephemeris data. For example, the synchronization error of a slave oscillator of a GPS receiver must be less than about +/−500 microseconds (&mgr;sec) from a GPS satellite master oscillator in order to obtain a location fix from a cold start in less than 30 seconds. In lock mode, the slave oscillator is typically synchronized to within +/−10 &mgr;sec of the GPS satellite master oscillator. When SPS signals are not available, for example because SPS satellites are out of view, or when the mobile unit has not acquired an SPS satellite signal, the mobile unit must be re-synchronized due to drift of the slave oscillator over time. Re-synchronization requires a significant amount of time if SPS signals must be used. SPS synchronization from a cold start is also time consuming. Synchronization processing times of up to one minute or more from cold start are not uncommon.
Other types of electronic equipment such as computer networking equipment, instruments, control systems, and ranging devices also rely upon accurately synchronized internal clocks. U.S. Pat. No. 5,510,797 of Abraham et al. describes the use of an SPS receiver in connection with computers and time-controlled instruments to synchronize their internal clocks.
U.S. Pat. No. 4,368,987 of Waters describes a synchronization method for satellites in which a master pulse is transmitted by a master-clock station to a slave station where a slave pulse having conjugate phase with respect to the received master pulse is retransmitted for receipt by the master station. A measurement at the master station of a time difference between the master pulse and the received slave pulse is used to calculate a time phase difference between the master clock and the slave clock. The time phase difference is then used to synchronize the clocks. Waters requires cooperation between the satellite-based master station and the satellite-based slave station in order to determine phase difference and for clock synchronization. Thus, the method described by Waters is not a substitute for re-synchronization of an SPS-enabled mobile unit. SPS satellites, which were originally developed for military use, will not retransmit a slave pulse in response to a master pulse received from the mobile unit. Nor will the SPS satellites, conversely, receive a conjugate slave pulse generated by the mobile unit or calculate a phase time difference.
For calls originating from wire line telephones, Automatic Number Identification (ANI) service allows a call receiving station, such as a Public Safety Answering Point (PSAP), to quickly lookup the name and address of the caller (registered telephone owner) in an owner database. The portable nature of wireless communications devices eliminates the viability of such a lookup scheme in wireless networks. Wireless mobile telephone units incorporating SPS receivers have been contemplated as a way to generate location data that can then be transmitted to a call receiving station. In theory, the generation and transmission of location data in this manner would be especially useful for locating a wireless caller that dials 911 to report an emergency, but who is unable to verbally provide location information to a PSAP operator.
While SPS-enabled wireless telephones may provide the capability to accurately determine and transmit location data, numerous practical realities present obstacles to the timely and efficient generation and transmission of location data to a call receiving station. For example, the SPS receiver of the SPS-enabled wireless telephone may need to synchronize to SPS time before it can generate useable location data. In an emergency situation involving a call to a PSAP, the amount of time required to synchronize the SPS receiver using SPS satellite signals can cost lives.
FIG. 1
shows a diagram of a prior art voice communications network
10
including a wireless communications network
12
coupled to a wire line communications network (POTS network)
14
. With reference to
FIG. 1
, wireless communications network
12
includes one or more cellular base stations
16
each having an associated base station antenna
18
and a mobile switching center
20
. Mobile switching center
20
couples cellular base station
16
to POTS network
14
to allow a wire line call taker
22
, such as a PSAP, to communicate with a mobile unit
24
of wireless communications network
12
. In operation, mobile unit
24
transmits and receives signals that are respectively received and transmitted by cellular base station
16
over two transmission channels
26
. These transmission channels
26
include a voice channel
27
(which is also known as the call path, the voice call path, the voice call connection, the audio call path, the audio traffic channel, and the traffic channel) for transmitting radio-frequency signals representative of voice, and a control channel
28
(also known as an overhead channel and the non-call path) for transmitting call initiation and control signals. In digital wireless communications networks, transmissions over control channel
28
consist of packetized digital data. Protocols for control channel
28
and the type of data that can be carried on control channel
28
are determined by the type of control channel communications protocol in use by wireless communications network. Because each type of wireless network uses its own protocol, control signals must be decoded at cellular base station
16
.
Other inherent limitations of the prior art will become apparent upon a review of the following summary of the invention and detailed description of preferred embodiments.
SUMMARY OF THE INVENTION
Wireline and wireless communications systems have some system latency, typically less than 500 milliseconds (ms), due to propagation and processing of signals traveling in the call path. In wireless communications networks, differences in air interface protocols, base stations, handset manufacturers, and transmission distances make the system latency variable.
The present invention provides methods for determining a system latency of a voice communication network for signals transmitted between a reference station and a remote unit over an audio call path of the voice communications network. The system latency is then taken into account during synchronization of the remote unit with a reference oscillator of the reference station. Measurement of system latency is accomplished by a signaling sequence including transmitting a reference signal over the audio call path from the reference station to the remote unit, where a reply signal is generated and transmitted back to the reference station over the call path after a preselected reply delay interval. The reference signal and the reply signal are transmitted for respective predetermined reference and reply durations, which may be dictated by signal attenuation characteristics of the voice communicatio
Preston Dan A.
Preston Joseph
Proctor Rod L.
Airbiquity Inc.
Duong Frank
Stoel Rives LLP
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