Modem with data compression selected constellation

• US 4,937,844 A
• Filed: 11/03/1988
• Issued: 06/26/1990
• Est. Priority Date: 11/03/1988
• Status: Expired due to Fees

• Current Assignee: Racal-Datacom, Inc. (Thales SA)
• Original Assignee: Racal Data Communications Incorporated
• Inventors: Kao, Ming-Luh
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Bocure, Tesfaldet
• Application Number:
  US07/269,812
• Time in Patent Office: 600 Days
• Field of Search: 375/39, 375/122, 375/42, 375/58, 375/8, 371/43-45, 370/84, 370/118
• US Class Current: 375/240
• CPC Class Codes: H04L 25/05 Electric or magnetic storage of signals before transmitting or retransmitting for changing the transmission rate
• CPC Class Codes: H04L 27/3405 Modifications of the signal space to increase the efficiency of transmission, e.g. reduction of the bit error rate, bandwidth, or average power

Abstract

A data modem includes a data compression circuit which compresses incoming data prior to transmission. The compression ratio obtained from the compression process is used to select a constellation for transmission of the data. When higher compression rates are achieved, fewer constellation points (symbols) representing fewer bits per point are transmitted. In this manner, incoming data rate can be held constant while utilizing a more robust constellation which has greater immunity to transmission line impairments. This reduces errors and retransmissions which can ultimately lead to reduction in effective throughput in conventional systems.

Claims

1. A transmitter/modulator, comprising in combination:

transmitting means for transmitting information over a transmission line, said information being represented by points in one of a plurality of selectable constellations, each said selectable constellation having a different number of points;

comprissing means for compressing information to be transmitted by said transmitting means; and

selecting means, responsive to said compressing means, for selecting one of said plurality of constellations based upon a level of compression obtained by said compressing means.

2. The apparatus of claim 1, wherein the number of points in said selected constellation in inversely to a compression ratio obtained by said compression means so that a constellation with fewer points is selected when higher levels of compression are obtained.

3. The apparatus of claim 1, wherein said plurality of selectable constellations have varying degrees of noise immunity, and wherein said selecting means selects a constellation whose noise immunity is related to the level of compression so that a constellation with higher noise immunity is selected when higher levels of compression are obtained.

4. The apparatus of claim 1, wherein saaid constellation is selected by said selecting means based upon the following table:

space="preserve" listing-type="tabular">______________________________________ Average Compression Ratio No. Constellation Points ______________________________________ 1;

1 2.sup.N N;

N-1 2.sup.N-1 N;

N-2 2.sup.N-2 . . . . . . N;

N-i 2.sup.N-i ______________________________________

where N is a positive integer greater than a positive integer i.

5. The apparatus of claim 1, wherein said compressing means includes means for compressing said information using a Ziv Lempel algorithm.

6. The apparatus of claim 1, further comprising inband signaling means for transmitting a signal indicative of a change in constellation.

7. The apparatus of claim 6, wherein said inband signaling means includes means for transmitting a predetermined symbol which does not form a part of any of said constellations used for transmission of data to signal said change in constellations.

8. The apparatus of claim 1, further comprising means for measuring a level of compression achieved by said compressing means.

9. The apparatus of claim 1, wherein said selecting means includes buffering means for buffering data compressed by said compressing means and monitoring means for detecting the amount of compressed data in said buffering means as a measure of said level of comrpession.

10. A method of enhancing the error performance of a data transmitter/modulator, comprising the steps of:

receiving data to be transmitted by said modem;

compressing said data;

determining whether said data is being compressed at a compression level greater than a predetermined threshold; and

changing a constellation used for transmission to a constellation having fewer points in the event said compression level exceeds said predetermined threshold.

11. The method of claim 10, further comprising the step of transmitting and inband signal to effect change of said constellation.

12. The method of claim 10, further comprising the step of selecting a constellation having a greater number of points in the event said compression level falls below predetermined compression limit.

13. In a data transmitter/modulatr, a method of selecting between a plurality of transmission modes, comprising the steps of:

receiving data to be transmitted by said transmitter/modulator, said transmitter/modulator being capable of tramsmitting said data at a predetermined data rate;

compressing said data;

determining a compression level obtained in said compressing step; and

selecting a transmission mode which utilizes the bandwidth made available by said compressing step to enhance the error performance while maintaining a data throughput which is at least as great as said predetermined data rate.

14. The method of claim 13, wherein said transmission modes are characterized by utilizing a predetermined number of QAM constellation points.

15. The method of claim 13, further comprising selecting one of said plurality of transmission modes by comparing said compression level with a predetermined threshold.

16. A method of enhacing the error performance of a transmiter/modulator, comprising the steps of:

freeing bandwidth by compressing information to be transmitted by said transmitter/modulator; and

utilizing said free bandwidth to enhance the reliability of transmission by said transmitter/modulator by utilizing a more reliable transmission method, said transmission method being selected depending upon the degree of compression obtained.

17. The method of claim 16, wherein said reliability is enhanced by transmitting a constellation having fewer points than would be needed to transmit the same information without compression at the same rate.

18. A receiver/demodulator, comprising in combination:

receiving means for receiving a plurality of synchronous transmitted data symbols, said transmitted data symbols each being part of one of a plurality of constellations of symbols;

a slicer for determining which of said transmitted data symbols from a particular one of said plurality of constellations are received;

detecting means for detecting and embedded constellation change symbol within said plurality of transmitted data symbols;

control means responsive to said detecting means for modifying operation of said slicer so that received data symbols are sliced according to a different one of said plurality of constellations;

decoding means for decoding each of said data symbols into compressed data symbols; and

adaptive data expanding means for expanding said compressed data symbols.