Express bit swapping in a multicarrier transmission system

Details Express bit swapping in a multicarrier transmission system Express bit swapping in a multicarrier transmission system

2000-02-24

2004-12-07

Ghebretinsae, Temesghen (Department: 2631)

Pulse or digital communications

Systems using alternating or pulsating current

Plural channels for transmission of a single pulse train

C375S295000, C375S222000, C375S358000

Reexamination Certificate

active

06829307

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to high speed data transmission systems that use multi-carrier modulation. More particularly, an express parameter changing command and protocol suitable for use in multi-carrier transmission systems is disclosed.
In recent years, there has been increased interest in the use of multi-carrier modulation in high speed modems. By way of example, the Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has promulgated a discrete multi-tone based standard for the transmission of digital data over Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting data over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard and is incorporated herein by reference. Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to at least 6 million bits per second (i.e., 6+ Mbit/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 “tones” or “sub-channels” that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from a central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user).
Although the ADSL standard has been widely accepted, there are ongoing efforts to both improve the T1.413 ADSL standard and to provide ADSL or other standards for communications at other data rates. By way of example, there are currently ongoing efforts to define a simplified version of the standard which, among other things uses just 128 tones. This effort is being undertaken by T1.413 and is commonly referred to as the G.lite standardization effort. There is also an effort to define a standard for significantly higher data, which is referred to as the VDSL (Very High Rate Digital Subscriber Line) standard. The VDSL standard is intended to facilitate transmission rates of at least 25.96 Mbit/s and preferably at least 51.92 Mbit/s in the downstream direction. To achieve these rates, the transmission distance over twisted pair phone lines must generally be shorter than the lengths permitted using ADSL. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the “curb” to the customer premise is standard unshielded twisted pair (UTP) telephone lines.
One issue that is inherent in high speed DSL modems that use multi-carrier modulation is how to handle variations in line conditions. For example, in the T1.413 standard and other proposed DMT based systems, the communicating modems go through a brief training period before data communications begin. During the training period test signals are transmitted to effectively test the quality of the line at various frequencies. Generally the line quality is determined by measuring the signal-to-noise ratio (SNR) on each of the tones. The number of “bits” allocated to each tone are then determined based in large part on the detected training signals. However, after the training period, the transmission line often encounters changes that may affect its ability to transmit information at the allocated rates on some of the tones. The transmission line changes may result from a variety of causes including: a customer taking a phone off its hook or hanging up; temperature induced line changes; changes in cross talk noise due to adjacent lines becoming active or inactive, and the increase of AM radio signals at night.
If the line quality deteriorates over time, then more errors are likely to occur and something must be done to adjust the assigned bit allocation. One way to adjust the bit allocation is to simply retrain the modems. However, retraining has the drawback of taking a relatively large amount of time which causes a brief service interruption. The other way to adjust the bit allocation defined in the T1.413 standard is a procedure referred to as “bit swapping”. The bit swapping protocol contemplates that when errors are detected on a particular tone, the amount of information transmitted on that particular tone will be reduced by some number of bits. If another tone is believed to have additional SNR, the amount of information transmitted is increased by a corresponding amount.
In the T1.413 standard, the bit swapping protocol is specifically defined. More specifically, when a particular receiver determines that a bit swap needs to be made, it send a bit swap request over an overhead channel (typically referred to as the AOC-ADSL Overhead Channel). The bit swap request has a designated format which is illustrated in FIG.
1
. As seen therein, the first byte of the bit swap request is a message header
12
. Message header
12
consists of all ones, which identifies the command as a bit-swap request. The message header
12
is followed by a message
14
of eight (or twelve) bytes. The message
14
is divided into 4 (or six) segments, each segment is called a message field
16
. Each message field
16
contains a one byte command
18
followed by a one byte tone index
20
which identifies the tone to which the command is to be applied. The one byte command includes the functions: add a bit, delete a bit, increase power by 1, 2, 3 dB, decrease power by 1, 2, 3 dB, do nothing, and proprietary commands
The T1.413 further requires that a 3-byte bit-swap acknowledge command be sent back to the unit requesting the bit swap to confirm receipt of the bit swap request. The bit-swap acknowledge command specifies a specific symbol count on which the swap will be implemented. The acknowledgement command is used to simplify detection of the implementation of a new bit distribution, but the acknowledgement, slows swapping speed and still may cause a failure if the acknowledgement is not received.
The T1.413 protocol further mandates that the bit-swap request command be transmitted five successive times and that the receiving unit only acknowledge the bit swap command if it receives a majority of those five transmissions. Thus, it takes 45 bytes to request to move one bit plus 15 bytes to acknowledge the request (which also must be repeated 5 times). Ignoring latencies, the minimum time for a swap is thus on the order of 30 ms (60 bytes at 16 kbytes/sec). However, current standards actually mandate that swapping occurs no more often than once every 800 ms, which allows for transceiver simplification, but further slows the swapping process in situations where more then four tones are to be changed. Thus, the standard ADSL bit-swapping mechanisms permit slow variations of the transmitter. However, with the advent of splitterless ADSL and the increasing popularity of DSL in general, it has become apparent that DSL lines will be subject to abrupt changes that require significant changes in the bit distribution. Given the slowness of the standardized bit swapping protocol and the drawbacks of requiring a retraining event, it has become apparent that a more efficient mechanism is needed to reduce the time it takes to implement bit redistribution in multi-carrier transmission systems.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects of the invention methods and devices for adaptively changing a parameter (such as sub-carrier gain or bit allocation) of a communication signal in a multi-carrier based transmission system are described. In one aspect of the invention, a unit that determines a need for a change sends a change request to a second unit. The change request identifies one or more specific sub-carriers to be altered and a desired value for a parameter associated with each identified sub-carrier. The requesting unit then monitors the communicatio

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