Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1998-06-04
2001-08-07
Legree, Tracy (Department: 2681)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S445000, C370S352000, C375S242000
Reexamination Certificate
active
06272358
ABSTRACT:
TECHNICAL FIELD
This invention relates to method and apparatus for improving the quality of mobile to mobile telephone connections using Vocoders.
Problem
Conserving bandwidth in the mobile radio spectrum has always been a major objective. Recent break-throughs in digital mobile telephony feature both time division multiple access pulse code modulation, (TDMA), and code division multiple access (CDMA), are two of the major digital systems in use in mobile telephony to achieve this conservation of spectrum. Digitized speech, while being transmitted within the public switched telephone network in PCM form at a 64 kilobit (Kb) per second rate, can be readily compressed using a number of techniques. One of the best bit rate compression techniques is the use of Vocoders which allows fairly high quality speech to be transmitted at rates of 13 Kb per second and even less. However, all Vocoders introduce quantitization errors during the voice encoding and decoding processes.
In a typical mobile to mobile connection, illustrated in
FIG. 1
, mobile station
1
generating a voice signal, has that voice signal go through a first Vocoder
11
in the mobile station on its way to a central office switch
50
, containing a speech handler
13
. The speech handler
13
contains a second Vocoder
21
for converting the vocoded signal to a pulse code modulation signal for transmission via link
30
between the switch
50
serving mobile station
1
, and the switch
51
, serving the destination mobile station
2
. In that switch
51
, another speech handler
14
accepts the PCM signal received from link
30
, and converts that signal to a vocoded signal. This vocoded signal is transmitted by radio to mobile station
2
, where a vocoder
12
decodes the vocoded signal, and generates an analogue signal for driving the speaker of mobile station
2
. It can be seen that in this connection that 4 Vocoders are used. The use of 4 Vocoders reduces the voice quality of the speech from mobile station
1
to mobile station
2
because each pair of back to back encoders and decoders introduces substantial quantitization error. An additional problem is that 4 Vocoders in tandem introduce extra delay in the transmission of speech signals.
Solution
It is therefore desirable to eliminate the two middle Vocoders, and various solutions for performing that function, called Vocoder by-pass, have been proposed. Applicants have recognized that all of these proposed solutions have one or more problems, including the following:
The insertion or removal of the two middle Vocoders must be synchronized because if it is staggered, no speech can pass between the two end stations during the stagger interval. These actions may occur in the middle of a call. A connection which starts out as a mobile to mobile connection may subsequently be changed to a mobile to PSTN connection if three-way calling is invoked. Similarly, a mobile to PSTN connection which, of course, cannot use Vocoder by-pass can be transformed during the course of a call into a mob-ile to mobile connection frequently by first having a three-way connection, and then having the land based station drop off the call.
Second, the transmission of vocoded speech packets over 64 Kb PCM facilities, required if the two middle Vocoders are to be by-passed, can only be accomplished seamlessly if the packets so transmitted remain intact, i.e., retain their bit integrity. There are a number of facilities in PCM voice switching systems which interfere with such clear transmission of packetized signals, including provisions for echo cancellation, and bit robbing for signaling purposes. While there are certain specialized facilities for transmitting pure 64 Kb data, these specialized facilities are segregated from the normal voice facilities, and to require mobile to mobile calls to use one of these segregated facilities, would create major problems of administration and cost.
Applicants have recognized that a problem of the prior art is that there is no truly satisfactory arrangement for Vocoder by-pass.
The above problem is solved and an advance is made over the prior art in accordance with our invention wherein when a mobile to mobile call is detected, a connection is established between the switches serving the two mobiles over a high speed data network. In one of the Applicants' preferred embodiments, this high speed data network is an asynchronous transfer mode (ATM) network, and a link of that network is established between the two switches. In alternative embodiments, other data networks can be used, preferably, those already deployed for other uses. Because mobile to mobile calls frequently are transformed into calls involving a land based telephone connected to the public switched telephone network (PSTN) a separate PCM connection between the two switches is established simultaneously. The conversation is sent over the PCM facility, and, in the Vocoder by-pass connection state, also over the high speed data network path of these facilities. (Alternatively, signals are transmitted over the PCM path only when the high speed data network path is not being used). If one of the switches deactivates Vocoder by-pass, that switch will simply transmit PCM signals over the PCM path after having decoded the vocoded speech signals received from its connected mobile station. The other switch will then recognize that no signals are coming in over the high speed data network path, but that signals are coming over the PCM path, and will simply take the received PCM signals and now route these signals through its own Vocoder. Effectively, the switch from the two Vocoder connection to the four Vocoder connection is accomplished in the two directions of transmission at slightly different times, but this is not a problem since the transition is smooth for each of the two directions. Advantageously, this arrangement allows Vocoder by-pass to be invoked without special administration of transmission facilities, and avoids problems of trying to synchronize the switch between using and not using Vocoder by-pass.
In accordance with one preferred embodiment of Applicants' invention, vocoded voice frames are received in a frame selector of a speech handler within a switch. The frame selector selects between frames received from different cell sites in order to select the frame received with the strongest signal. The frame selector also performs the function of passing a frame either to a Vocoder, for conversion to PCM, or to a by-pass function for transmission of the frame over the high speed data network. Advantageously, existing equipment can be used in a slightly enhanced functionality to perform the bypass for Vocoder selection.
In accordance with one embodiment of Applicants' invention, frames received over the by-pass path are delayed by passing through a buffer in order that at the time of a switch between the by-pass mode, and the non by-pass mode, the delay in the speech is equalized.
REFERENCES:
patent: 5608779 (1997-03-01), Lev et al.
patent: 5729536 (1998-03-01), Doshi et al.
patent: 5793810 (1998-08-01), Han et al.
patent: 5953331 (1999-09-01), Duncan et al.
patent: 5978676 (1999-11-01), Guridi et al.
patent: 5995923 (1999-11-01), Mermelstein et al.
patent: 6034994 (2000-03-01), Yoon
patent: 6070089 (2000-05-01), Brophy et al.
Brent Kenneth Wayne
Chambers Michael Dwayne
Lee Anne Yin-Fee
Lee Chinmei Chen
Legree Tracy
Lucent Technologies - Inc.
Ulrich Werner
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