Decision feedback equalizers, methods, and computer program...

Details Decision feedback equalizers, methods, and computer program... Decision feedback equalizers, methods, and computer program...

1999-03-08

2002-01-22

Decady, Albert (Department: 2133)

Error detection/correction and fault detection/recovery

Pulse or data error handling

Error count or rate

C714S736000, C375S233000, C375S348000

Reexamination Certificate

active

06341360

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the field of modems, and, more particularly, to improving the stability of decision feedback equalizers under severe error event conditions.
BACKGROUND OF THE INVENTION
The demand for remote access to information sources and data retrieval, as evidenced by the success of services such as the World Wide Web, is a driving force for high-speed network access technologies. Today's telephone network offers standard voice services over a 4 kHz bandwidth. Traditional analog modem standards generally assume that both ends of a modem communication session have an analog connection to the public switched telephone network (PSTN). Because data signals are typically converted from digital to analog when transmitted towards the PSTN and then from analog to digital when received from the PSTN, data rates may be limited to 33.6 kbps as defined in the V.34 transmission recommendation developed by the International Telecommunications Union (ITU).
The need for an analog modem can be eliminated, however, by using the basic rate interface (BRI) of the Integrated Services Digital Network (ISDN). A BRI offers end-to-end digital connectivity at an aggregate data rate of 160 kbps, which is comprised of two 64 kbps B channels, a 16 kbps D channel, and a separate maintenance channel. The ISDN offers comfortable data rates for Internet access, telecommuting, remote education services, and some forms of video conferencing. ISDN deployment, however, has been very slow due to the substantial investment required of network providers for new equipment. Because the ISDN is not very pervasive in the PSTN, the network providers have typically tarriffed ISDN services at relatively high rates, which may be ultimately passed on to the ISDN subscribers. In addition to the high service costs, subscribers must generally purchase or lease network termination equipment to access the ISDN.
While most subscribers do not enjoy end-to-end digital connectivity through the PSTN, the PSTN is nevertheless mostly digital. Typically, the only analog portion of the PSTN is the phone line or local loop that connects a subscriber or client modem (e.g., an individual subscriber in a home, office, or hotel) to the telephone company's central office (CO). In recent years, local telephone companies have been replacing portions of their original analog networks with digital switching equipment. Nevertheless, the connection between the home and the CO has been the slowest to change to digital as discussed in the foregoing with respect to ISDN BRI service. A recent data transmission recommendation issued by the ITU, known as V.90, takes advantage of the digital conversions that have been made in the PSTN. By viewing the PSTN as a digital network, V.90 technology is able to accelerate data downstream from the Internet or other information source to a subscriber's computer at data rates of up to 56 kbps, even when the subscriber is connected to the PSTN via an analog local loop.
To understand how the V.90 recommendation achieves this higher data rate, it may be helpful to briefly review the operation of V.34 analog modems. V.34 modems are optimized for the situation where both ends of a communication session are connected to the PSTN by analog lines. Even though most of the PSTN is digital, V.34 modems treat the network as if it were entirely analog. Moreover, the V.34 recommendation assumes that both ends of the communication session suffer impairment due to quantization noise introduced by analog-to-digital converters. That is, the analog signals transmitted from the V.34 modems are sampled at 8000 times per second by a codec upon reaching the PSTN with each sample being represented or quantized by an eight-bit pulse code modulation (PCM) codeword. The codec uses 256, non-uniformly spaced, PCM quantization levels defined according to either the &mgr;-law or A-law companding standard.
Because the analog waveforms are continuous and the binary PCM codewords are discrete, the digits that are sent across the PSTN can only approximate the original analog waveform. The difference between the original analog waveform and the reconstructed quantized waveform is called quantization noise, which limits the modem data rate.
While quantization noise may limit a V.34 communication session to 33.6 kbps, it nevertheless affects only analog-to-digital conversions. The V.90 standard relies on the lack of analog-to-digital conversions outside of the conversion made at the subscriber's modem to enable transmission at 56 kbps.
The general environment for which the V.90 standard was developed is depicted in FIG.
1
. An Internet Service Provider (ISP)
22
is connected to a subscriber's computer
24
via a V.90 digital server modem
26
, through the PSTN
28
via digital trunks (e.g., T1, E1, or ISDN Primary Rate Interface (PRI) connections), through a central office switch
32
, and finally through an analog loop to the client's modem
34
. The central office switch
32
is drawn outside of the PSTN
28
to better illustrate the connection of the subscriber's computer
24
and modem
34
into the PSTN
28
. It should be understood that the central office
32
is, in fact, a part of the PSTN
28
. The operation of a communication session between the subscriber
24
and an ISP
22
is best described with reference to the more detailed block diagram of FIG.
2
.
Transmission from the server modem
26
to the client modem
34
will be described first. The information to be transmitted is first encoded using only the 256 PCM codewords used by the digital switching and transmission equipment in the PSTN
28
. The PCM codewords are modulated using a technique known as pulse amplitude modulation (PAM) in which discrete analog voltage levels are used to represent each of the 256 PCM codewords. These PAM signals are transmitted towards the PSTN by the PAM transmitter
36
where they are received by a network codec. No information is lost in converting the PAM signals back to PCM because the codec is designed to interpret the various voltage levels as corresponding to particular PCM codewords without sampling the PAM signals. The PCM data is then transmitted through the PSTN
28
until reaching the central office
32
to which the client modem
34
is connected. Before transmitting the PCM data to the client modem
34
, the data is converted from its current form as either &mgr;-law or A-law companded PCM codewords to PAM voltages by the codec expander (digital-to-analog (D/A) converter)
38
. These PAM voltages are processed by a central office hybrid
42
where the unidirectional signal received from the codec expander
38
is transmitted towards the client modem
34
as part of a bidirectional signal. A second hybrid
44
at the subscriber's analog telephone connection converts the bidirectional signal back into a pair of unidirectional signals. Finally, the analog signal from the hybrid
44
is converted into digital PAM samples by an analog-to-digital (A/D) converter
46
, which are received and decoded by the PAM receiver
48
. Note that for transmission to succeed effectively at 56 kbps, there must be only a single digital-to-analog conversion and subsequent analog-to-digital conversion between the server modem
26
and the client modem
34
. Recall that analog-to-digital conversions in the PSTN
28
can introduce quantization noise, which may limit the data rate as discussed hereinbefore. Moreover, the PAM receiver
48
needs to be in synchronization with the 8 kHz network clock to properly decode the digital PAM samples.
Transmission from the client modem
34
to the server modem
26
follows the V.34 data transmission standard. That is, the client modem
34
includes a V.34 transmitter
52
and a D/A converter
54
that encode and modulate the digital data to be sent using techniques such as quadrature amplitude modulation (QAM). The hybrid
44
converts the unidirectional signal from the digital-to-analog converter
54
into a bidirectional signal that

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