Discrete multi-tone data transmission system using an overhead bus for synchronizing multiple remote units

• US 5,625,651 A
• Filed: 06/02/1994
• Issued: 04/29/1997
• Est. Priority Date: 06/02/1994
• Status: Expired due to Term

• Current Assignee: Amati Communications Corp. (Texas Instruments, Inc.)
• Original Assignee: Amati Communications Corp. (Texas Instruments, Inc.)
• Inventors: Cioffi, John M.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Le, Amanda T.
• Application Number:
  US08/252,829
• Time in Patent Office: 1,062 Days
• Field of Search: 370/71, 370/124, 370/185.1, 370/100.1, 370/103, 370/105, 370/105.1, 370/105.2, 370/108, 370/118, 375/354, 375/356-358, 375/362, 375/371, 375/373, 375/219-222, 375/260, 375/340, 455/51.1, 455/51.2, 455/53.1, 455/54.1, 455/56.1, 455/69, 455/70, 455/54.2, 340/825.14, 340/825.2, 340/825.21
• US Class Current: 375/354
• CPC Class Codes: H04L 1/0003 by switching between different modulation schemes
• CPC Class Codes: H04L 1/0025 Transmission of mode-switching indication
• CPC Class Codes: H04L 1/206 for modulated signals
• CPC Class Codes: H04L 2001/0093 Point-to-multipoint
• CPC Class Codes: H04L 27/2601 Multicarrier modulation systems
• CPC Class Codes: H04L 5/0044 allocation of payload
• CPC Class Codes: H04L 5/023 Multiplexing of multicarrier modulation signals multicarrier modulation H04L27/2601

Abstract

A bi-directional data transmission system that facilitates communications between a central unit and a plurality of remote units using a frame based discrete multi-tone (DMT) transmission scheme is disclosed. The discrete multi-tone data transmission system has a multiplicity of discrete subchannels including an overhead bus. In a method aspect of the invention, frames transmitted from the plurality of remote units are synchronized at the central unit. When a selected remote desires to initiate communications, it loop times it own clock to the clock of the central unit and transmits a remote initiated synchronization signal to the central unit over a dedicated overhead subchannel in the overhead bus. The central unit responds with a centrally initiated synchronization signal that contains information indicative of a frame boundary phase shift required to synchronize the selected first remote unit with other remote units that are currently communicating with the central unit. The remote responds by shifting the phase of the frames it outputs by an amount indicated by the centrally initiated synchronization signal. This synchronizes the frame boundaries of the frames outputted by the selected remote unit with frame boundaries of frames output by the other remote units that are currently communicating with the central unit. The synchronization is arranged to occur such that the frame boundaries from the various remotes substantially coincide when they are received at the central unit. Specific central and remote modem designs suitable for implementing such a system are also described.

Claims

1. In a bi-directional data transmission system that facilitates communications between a central unit and a plurality of remote units using a frame based discrete multi-tone transmission scheme that has a multiplicity of discrete subchannels, a method of synchronizing frames transmitted from a selected first remote unit to the central unit, the method comprising the steps of:

(a) when the selected first remote unit desires to establish communications with the central unit, receiving a first signal from the central unit and loop timing a clock at the selected first remote unit with a clock signal carried in the first signal;

(b) transmitting a remote initiated synchronization signal from the loop timed selected first remote unit to the central unit over an overhead subchannel when the selected first remote unit desires to establish communications with the central unit;

(c) transmitting a centrally initiated synchronization signal from the central unit to the selected first remote unit when the central unit receives the remote initiated synchronization signal, wherein the centrally initiated synchronization signal contains information indicative of a frame boundary phase shift required to better synchronize the selected first remote unit with other remote units that are currently communicating with the central unit; and

(d) shifting the phase of the frames outputted by the selected first remote unit in response to the centrally initiated synchronization signal to adjust the frame boundaries of the frames outputted by the selected first remote unit relative to the frame boundaries of frames output by the other remote units that are currently communicating with the central unit;

whereby the synchronization is arranged such that when fully synchronized, the frame boundaries from the various remotes will substantially coincide when they are received at the central unit.

2. A method as recited in claim 1 wherein the frame based discrete multi-tone transmission scheme includes two dedicated overhead subchannels, and the remote initiated synchronization signal and the centrally initiated synchronization signal are transmitted over different overhead subchannels.

3. A method as recited in claim 2 wherein all of the remote units send their associated remote initiated synchronization signals over a single subchannel.

4. A method as recited in claim 1 wherein a single dedicated overhead subchannel is provided and the remote initiated synchronization signal and the centrally initiated synchronization signal are both transmitted over the single dedicated overhead subchannel.

5. A method as recited in claim 1 wherein the bi-directional data transmission system is an Asymmetric Digital Subscriber Line system that includes the transmission of signals over twisted pair telephone lines and wherein a modulator modulates encoded digital information onto subcarriers that correspond to subchannels that each have a bandwidth that is approximately 4.3125 kHz wide.

6. A method as recited in claim 1 wherein the bi-directional data transmission system is a cable system that includes the transmission of signals over a coaxial cable.

7. A method as recited in claim 1 wherein the bi-directional data transmission system is a digital cellular television system that includes the transmission of radio signals.

8. A method as recited in claim 1 wherein when two of the remote units transmit their associated remote initiated synchronization signals at substantially the same time, the remote units each resend an associated remote initiated synchronization signal.

9. A method as recited in claim 1 wherein the number of subchannels available to the selected remote unit for transmission of data to the central unit are dynamically allocated.

10. A method as recited in claim 1 wherein steps b, c and d are repeated until the selected remote unit is fully synchronized and further comprising the step of initiating normal communications from the selected remote unit to the central unit.

11. A discrete multi-tone central modem unit for use in a bi-directional data transmission system that facilitates communications between the central modem unit and a plurality of remote modem units, the central modem unit comprising:

an encoder for encoding digital information;

a monitor for monitoring a communication line to determine line quality parameters indicative of noise levels at each of a multiplicity of subchannels, each subchannel corresponding in frequency to an associated subcarrier;

a modulator for modulating the encoded digital information onto a multiplicity of subcarriers in a frame based discrete multi-tone signal, each subcarrier corresponding to an associated tone and an associated subchannel, the modulation being arranged to take into consideration at least the detected line quality parameters and a permissible power mask parameter, and wherein the modulator is capable of dynamically updating both the subchannels used and the amount of data transmitted on each subchannel during transmission in order to accommodate real time changes in specific parameters;

an adder for appending a cyclic prefix to the discrete multi-tone signal before it is applied to a transmission line; and

a synchronizer for monitoring signals received over a dedicated overhead subchannel, identifying a remote initiated synchronization signal that is received on the overhead subchannel, determining the phase shift between a frame boundary of the remote initiated synchronization signal and a frame boundary of a frame in said discrete multi-tone signal and generating a centrally initiated synchronization signal for transmission to the remote modem units that is indicative of a frame boundary phase shift required to synchronize a selected remote modem that initiated the remote initiated synchronization signal with other remote units that are currently communicating with the central modem unit.

12. A discrete multi-tone remote modem unit for use in a bi-directional data transmission system that facilitates communications between a central modem unit and a plurality of said remote modem units, the remote modem unit comprising:

a demodulator for demodulating a first discrete multi-tone signal indicative of a first set of digital information, the demodulator being arranged to receive modulation information as part of the discrete multi-tone signal, wherein the demodulator is capable of dynamic updating during reception in response to changed modulation information in order to accommodate real time changes in the modulation scheme, the demodulator being arranged to strip a cyclic prefix from the discrete multi-tone signal;

a decoder for decoding the demodulatcd digital information in real time;

an encoder for encoding a second set of digital information;

a modulator for modulating the encoded second set of digital information onto a multiplicity of subcarriers in a second discrete multi-tone signal, each subcarrier in the second discrete multi-tone signal corresponding to an associated tone and an associated subchannel; and

a synchronizer for generating a first synchronization signal that is applied to an overhead subcarrier when the discrete multi-tone remote modem desires to initiate communications to the central modem, receiving a second synchronization signal from the central modem that is indicative of a frame boundary phase shift required to synchronize the remote modem with other remote units that are currently communicating with the central modem unit, and shifting the phase of the second discrete multi-tone signal so that it is synchronized at the central modem with multi-tone signals sent by said other remote units.

13. A remote modem unit as recited in claim 12 wherein the demodulator further includes a time domain equalizer.

14. A remote modem unit as recited in claim 12 wherein the demodulator and the decoder are part of a receiver, and the remote unit further comprises an analog notch filter arranged to filter the first discrete multi-tone signal before it is passed to the receiver to reduce the energy level of the signals handled by the receiver.

15. In a bi-directional data transmission system that facilitates communications between a central unit and a plurality of remote units using a frame based discrete multi-tone transmission scheme that has a multiplicity of discrete subchannels, a method of synchronizing frames transmitted from a selected remote unit to the central unit, the method comprising the steps of:

when the selected first remote unit desires to establish communications with the central unit, receiving a first signal from the central unit and loop timing a clock at the selected first remote unit with a clock signal carried in the first signal;

transmitting a remote initiated synchronization signal from the selected first remote unit to the central unit over a dedicated overhead subchannel;

receiving a centrally initiated synchronization signal transmitted from the central unit in response to the remote initiated synchronization signal, wherein the centrally initiated synchronization signal contains information indicative of a frame boundary phase shift required to synchronize the selected first remote unit with other remote units that are currently communicating with the central unit; and

shifting the phase of the frames outputted by the selected first remote unit in response to the centrally initiated synchronization signal to better adjust the frame boundaries of the frames outputted by the selected first remote unit relative to the frame boundaries of frames output by the other remote units that are currently communicating with the central unit, the synchronization being arranged to occur such that the frame boundaries from the various remotes are arranged to substantially coincide when they are received at the central unit.