Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-12-27
2002-09-03
Vo, Don N. (Department: 2631)
Pulse or digital communications
Transceivers
Modems
C379S093090
Reexamination Certificate
active
06445731
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the reduction of the required amount of signal processing in a modulator/demodulator (modem) which is transferring packet-based data or other information which is intermittent in nature on a communication channel.
2. Related Art
Modern data networks commonly use complex digital signal processing (DSP) devices called modems to transport data over communication channels. Data is typically transported via an analog transmission signal which is representative of a synchronous, constant rate bit stream. This form of communication channel is suitable for the transmission of real-time information such as voice or video. However, it is increasingly common to use modems for the transmission of packet-based information. For example, packet-based information is used to access the Internet and the World Wide Web. However, packet-based information is typically bursty in nature, with an average data rate which is often much less than the available peak data transfer rate of the communication channel.
FIG. 1
is a block diagram of a transmitter circuit
100
of a conventional modem. Transmitter circuit
100
includes packet queue
101
, framer
102
, channel coding circuit
103
, output shaper
104
, modulator
105
and digital-to-analog (D/A) converter
106
. In accordance with conventional modem protocols, transmitter circuit
100
transforms source data received by packet queue
101
into a continuous time analog transmit signal, which is provided at the output terminal of D/A converter
106
.
More specifically, within transmitter circuit
100
, the source data is grouped into packets and stored in packet queue
101
. These packets are not synchronous with respect to the modem bit clock, but arrive at packet queue
101
at random times. Framer
102
receives the packets from packet queue
101
, and in response, composes a continuous bit stream which is synchronous with respect to the modem bit clock. To create such a synchronous bit stream in response to the asynchronous packets, framer
102
generates idle information (i.e., nulls or a marking tone) when no packets are available, and generates packet data when packets are available. The packet data and idle information are delineated in such a way that a receiver circuit of a modem (see, e.g.,
FIG. 2
) can determine where the packet boundaries lie.
The synchronous bit stream generated by framer
102
is then coded by channel coding circuit
103
. Channel coding circuit
103
is used to compensate for noise and distortion in the communication channel. Channel coding circuit
103
provides redundant information (e.g., convolutional encoding) to allow for error correction. Channel coding circuit
103
further performs a scrambling function, as well as mapping the coded bit stream onto symbol values. The stream of symbol values generated by channel coding circuit
103
is provided to output shaper
104
.
Output shaper
104
digitally filters the stream of symbol values received from channel coding circuit
103
. Output shaper circuit
104
limits the frequency bandwidth of these symbol values within a predetermined range and may also be adjusted to help compensate for channel distortion. The filtered sample stream provided by output shaper
104
is provided to modulator
105
, which modulates a carrier signal by the filtered sample stream. The output of modulator
105
is provided to D/A converter
106
, which generates an analog TRANSMIT signal for transmission on the communication channel (i.e., telephone line).
Transmitter circuit
100
exhibits three distinct disadvantages. First, because transmitter circuit
100
transmits constantly (either packet data or idle information), a modem can be functionally connected to only one telephone line at any given time. Moreover, only a small percentage of the total information carrying capacity of the communication channel is used to transmit data, while a large percentage of this capacity is used to transmit idle information. Additionally, transmitter circuit
100
is unsuited to multi-drop operation on a single communication channel. The first disadvantage mentioned above is particularly deleterious where a number of xDSL modems are collected together in a central office to provide data communications to a number of remote locations. In this case, each remote location requires a dedicated xDSL modem in the central office.
The analog TRANSMIT signal is transmitted over the telephone line to the telephone company central office. Within the central office, an analog to digital converter converts the analog TRANSMIT signal into a digital signal. This digital signal is multiplexed onto a digital backbone circuit and routed to a second central office location. The digital signal is demultiplexed within the second central office location and routed over a digital trunk to a digital server which performs additional processing on the digital signal.
FIG. 2
is a block diagram of a receiver circuit
200
of a conventional modem. Receiver circuit
200
includes analog-to-digital (A/D) converter
201
, resampler
202
, equalizer
203
, carrier recovery circuit
204
, symbol decision circuit
205
, channel decoding circuit
206
, framer
207
, packet queue
208
, echo canceler
209
, timing update circuit
210
, equalizer update circuit
211
and carrier update circuit
212
. Carrier recovery circuit
204
and symbol decision circuit
205
are sometimes referred to as a demodulator circuit. A/D converter
201
is coupled to the telephone line to receive the analog signal from the telephone company central office. A/D converter
201
samples this analog signal, thereby converting the analog signal into a digital signal.
The modem which includes receiver circuit
200
also includes a transmitter circuit (i.e., a near end transmitter circuit, not shown) which is similar to transmitter circuit
100
. During full duplex operation, this near end transmitter circuit may be generating a TRANSMIT signal at the same time that receiver circuit
200
is attempting to receive the analog signal from the remote (or far end) transmitter circuit
100
. Under these conditions, receiver circuit
200
may receive an echo of the TRANSMIT signal. Echo canceler
209
generates a signal which is a replica of this echo. The signal generated by echo canceler
209
is then subtracted from the output signal provided by A/D converter
201
.
Resampler
202
adjusts the raw input samples received from A/D converter
201
to match the symbol rate of the transmitter circuit
100
. Timing update circuit
211
extracts timing information which is used to control resampler
202
. Equalizer
203
compensates for linear distortions introduced by the communication channel (e.g., the telephone line). Carrier recovery circuit
204
extracts the carrier signal from the received signal and provides rough symbols (or a soft symbol decision) to symbol decision circuit
205
. Symbol decision circuit
205
quantizes the rough symbols and makes hard decisions as to the identity of the received symbols. Equalizer update circuit
211
and carrier update circuit
212
receive the symbols provided by symbol decision circuit
205
. In response, equalizer update circuit
211
and carrier update circuit
212
determine quantizer error. In response to this quantizer error, equalizer update circuit
211
and carrier update circuit
212
adjust the coefficients used by equalizer
203
and carrier recovery circuit
204
, respectively, thereby improving the accuracy of subsequent hard symbol decisions.
Channel decoding circuit
206
uses redundant information present in the received analog signal to correct for quantizer errors. Channel decoding circuit
206
typically implements a maximum likelihood sequence estimator (MLSE) circuit (such as a Viterbi decoder or other form of error correction. Channel decoding circuit
206
provides a decoded bit stream to framer
207
. Finally, framer
207
decodes the bit stream into packet data, discarding the idle information, and loading the
Corry Alan G.
Forrest Craig S.
Frank Edward H.
Holloway John T.
Mallory Tracy D.
Broadcom Homenetworking, Inc.
Christie Parker & Hale LLP
Vo Don N.
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