Multiplex communications – Wide area network – Packet switching
Patent
1989-05-31
1994-02-22
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 24, 370 32, 375 58, 455 69, 455127, H04B 138, H04B 320, H04L 514
Patent
active
052894592
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to duplex transmission of digital data.
BACKGROUND AND SUMMARY OF THE INVENTION
Duplex transmission of signals within the same frequency band over a link such as a telephone line can be subject to problems due to echo effects, in particular so-called "talker echo" (referred to below simply as echo) where reception by a station of echo versions of its own transmitted signals interferes with its reception function. Echo cancellers are commonly used to alleviate this problem. For the purposes of this description, echo effects will be understood to include any effect caused by a station receiving residues of its own transmission as for example residues due to impedance mismatch, inadequate filtering or, in a radio system, antenna misalignment.
According to the present invention there is provided, a duplex data transmission system comprising first and second stations connected via a transmission path, in which each station has assessment means for assessing received signal quality, at least one station has means for transmitting results of the assessment to the other station, the or each other station has control means operable to estimate a transmitted power adjustment to be made by one station, and at least one station includes means responsive thereto to effect an adjustment to its transmitting power, characterized in that the control means is operable in response to the results of its own assessment and that made by and communicated to it by the other station such that the signal-to-noise ratios of the signals received by the two stations shall be substantially in a predetermined relationship.
Other aspects of the invention are as recited in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:
FIG. 1 is a block diagram of one embodiment of a transmission system according to the invention; and
FIG. 2 is a timing chart illustrating the operation of the system of FIG. 1 .
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to FIG. 1 two stations, 1, 2 serve for duplex transmission of digital data via a telephone line 3. The station 1 has a modem comprising a modulator 11 and demodulator decision circuit 12 and a hybrid circuit 13 for connection to the line 3. The hybrid and other components in the telephone link give rise to echo and an echo canceller 14 is also included, as well as receiver processing circuitry 15 which is assumed to include an adaptive equalizer. Similar components are provided in the other station 2.
The residual echo after cancellation may in some circumstances give rise to problems and the arrangements to be described are aimed at alleviating this problem by adjusting the transmitted power levels. If, when both stations are transmitting at full power, one station is suffering reception errors due to echo, performance may be improved by reducing the transmitted power at that station by an amount which reduces or eliminates the echo problem yet does not result in impaired reception at the other station.
Normally in a line transmission system there will be constraints on the maximum power at which a station can transmit, and it will be desirable that one station should transmit at maximum power. Accordingly, one station will transmit at maximum power, and the other will adjust by reducing its power. It will of course be understood that where a station may increase its power (as for example when a radio channel has been subject to fading) then the adjustment may be an increase in transmission power. References to reducing power in the following will be understood merely as illustrations of power adjustments.
Assessment of the appropriate reduction is made during a training sequence at the commencement of a period of communication ("a call"). Each station has a measurement device (16, 26) which, during transmission by that station (the other station remaining idle) can measure the residual talker echo, and durin
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British Telecommunications PLC
Hsu Alpus H.
Olms Douglas W.
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