Transmitter/receiver for GMSK and Offset-QAM

• US 20010001008A1
• Filed: 12/06/2000
• Published: 05/10/2001
• Est. Priority Date: 06/12/1996
• Status: Active Grant

• Current Assignee: Unwired Planet LLC (JP Morgan Chase & Co)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Dent, Paul W.
• Granted Patent: US 6,711,218 B2
• Time in Patent Office: Days
• US Class Current: 375/271
• CPC Class Codes: H04L 1/0054 Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
• CPC Class Codes: H04L 25/0226 sounding signals per se
• CPC Class Codes: H04L 25/03197 methods of calculation involving metrics
• CPC Class Codes: H04L 25/03292 with channel estimation circuitry
• CPC Class Codes: H04L 27/0008 arrangements for allowing a transmitter or receiver to use more than one type of modulation negotiating modulation type for two-way transmission paths H04L5/1453
• CPC Class Codes: H04L 27/2017 in which the phase changes are non-linear, e.g. generalized and Gaussian minimum shift keying, tamed frequency modulation H04L27/201 takes precedence
• CPC Class Codes: H04L 27/3488 Multiresolution systems
• CPC Class Codes: H04L 27/362 Modulation using more than one carrier, e.g. with quadrature carriers, separately amplitude modulated H04L27/366 takes precedence

Abstract

A transmitter encodes a number 2N data bits by using N data bits to select one of 2^N levels of a cosine wave and the other N data bits to select one of 2^N levels of a sine wave. The modulation attains the cosine wave levels at instants offset by half an N-bit symbol interval with respect to the sine wave levels, and is called Offset QAM (OQAM). A received OQAM signal is amplified, filtered and digitized at a sampling rate of preferably only one sample per N-bit half-symbol interval. Successive N-bit half-symbols comprise information modulated alternately on a cosine and a sine carrier wave. The receiver may remove this successive rotation by applying successive derotations of successive digitized samples by like amounts. The derotated samples are then correlated with known sync half-symbols. The sync correlations determine a set of channel coefficients describing the dependence of each digitized sample on one or more unknown half-symbols. The computed channel estimates are then used to predict the expected received sample values for all possible sequences of successive unknown half-symbols that are to be decoded. Received samples are then compared with all possible expected values and a measure of disagreement or error metric computed. That sequence with the lowest cumulative error metric is used to determine the decoded symbol output. In other aspects of the invention, dual mode transmitters and receivers are disclosed that enable different types of modulation to be alternatively utilized in a same apparatus.

Claims

1. A communications method for transmitting information bits from a transmitter to a receiver comprising the steps of:

assembling the information bits into N-bit groups of bits to form half-symbols;

encoding even numbered ones of said half-symbols into one of two to the power N possible amplitude levels of a cosine wave at even instants of a half-symbol clock and odd numbered ones of said half-symbols into one of the same number of amplitude levels of a sine wave at odd instants of said half-symbol clock;

using said cosine and sine waves together to form complex symbols for transmission, each complex symbol carrying 2N information bits;

transmitting said complex symbols as a transmission signal on a designated frequency channel to a receiver;

receiving said transmission signal on said designated frequency channel and converting said received transmission signal to representative complex numerical values at a sampling rate of one complex sample per half-symbol interval; and

forming a set of prerotated samples from said representative complex numerical values by rotating alternate ones of said representative complex numerical values by plus or minus 90 degrees in phase angle relative to the representative complex numerical values in between the alternate ones of the representative complex numerical values;

processing said prerotated samples to recover said information bits.

2. The method of

claim 1

, further comprising the step of forming the transmission signal by using filtering to smooth transitions between said amplitude levels of the cosine and sine waves.

3. A method for transmitting information bits from a transmitter to a receiver including the steps of:

assembling said information bits into N-bit groups of bits to form half-symbols;

encoding even numbered ones of the half-symbols into one of two to the clock and odd numbered ones of said half-symbols into one of the same number of amplitude levels of a sine wave at odd instants of said half-symbol clock;

using said cosine and sine waves together to form complex symbols for transmission, each complex symbol carrying 2N information bits; and

transmitting said complex symbols to the receiver in a transmission signal that conveys the half-symbols interleaved with other half-symbols that are known to the receiver in advance.

4. The method of

claim 3

, further comprising the step of forming the transmission signal by using filtering to smooth transitions between said amplitude levels of the cosine and sine waves.

5. The method of

claim 3

in which said known half-symbols are encoded into only two of the two to the power N possible amplitude levels of said cosine and sine waves.

6. The method of

claim 5

in which said known half-symbols are encoded into only the greatest positive or the greatest negative amplitude levels of said cosine and sine waves.

7. A method of transmitting information bits to a receiver at either a first information rate or a second information rate, comprising the steps of:

assembling said information bits into groups of unknown half-symbols, each containing a first number N1 of information bits when transmission at said first information rate is desired, and alternatively containing a second number N2 of information bits when transmission at said second information rate is desired;

encoding even numbered ones of the unknown half-symbols into one of two to the power N possible amplitude levels of a cosine wave at even instants of a half-symbol clock and odd numbered ones of said unknown half-symbols into one of the same number of amplitude levels of a sine wave at odd instants of said half-symbol clock, wherein N=N1 when transmission at said first information rate is desired, and N=N2 when transmission at said second information rate is desired;

using said cosine and sine waves together to form complex symbols for transmission, each complex symbol carrying 2N information bits; and

transmitting said complex symbols to the receiver in a transmission signal that conveys the unknown half-symbols interleaved with other half-symbols that are known to the receiver in advance, wherein the other half-symbols each comprise a number N3 of information bits known to the receiver, wherein further N3 may alternatively be equal to N1 or N2 independently of whether transmission at said first or second information rate is desired.

8. The method of

claim 7

, further comprising the step of forming the transmission signal by using filtering to smooth transitions between said amplitude levels of the cosine and sine waves.

9. The method of

claim 7

, wherein the other half-symbols that are known to the receiver in advance each comprise N1 information bits known to the receiver, while the unknown half-symbols each comprise N2 information bits.

10. The method of

claim 9

wherein said first number N1 equals one.

11. The method of

claim 10

, wherein said second number N2 equals two.

12. The method of

claim 9

, wherein said second number N2 equals two.

13. The method of

claim 7

, wherein said first number N1 equals one.

14. The method of

claim 13

, wherein said second Number N2 equals two.

15. The method of

claim 7

, wherein said second number N2 equals two.

16. A method of transmitting information bits to a receiver at either a first information rate or a second information rate, comprising the steps of:

applying the same information bits to a first and a second modulator when transmission at said first information rate is desired, and alternatively applying half of the information bits to said first modulator and the other half to said second modulator when transmission at said second information rate is desired, thereby producing a first and a second modulated signal; and

combining said first and said second modulated signals using respective first and second weighting factors to produce a signal for transmission.

17. The method of

claim 16

, wherein said first information rate is selected during transmission of bits known in advance to said receiver, and said second information rate is used during transmission of bits unknown in advance to said receiver.

18. The method of

claim 16

, wherein said first and second modulators each modulate the applied information bits using Gaussian Minimum Shift Keying (GMSK).

19. The method of

claim 16

, wherein said weighting factors are in a 2:

1 ratio relative to each other.

20. The method of

claim 17

, wherein said known symbols form an equalizer training sequence for adapting said receiver to compensate for inter-symbol interference.

21. The method of

claim 17

, wherein said known symbols alternatively comprise a first known symbol pattern when said first information rate is used, and a second known symbol pattern when said second information rate is used.

22. The method of

claim 21

, wherein said receiver detects which of said first or said second known symbol patterns was transmitted and adapts itself to decode information at said first information rate or said second information rate accordingly.

23. A receiver for decoding unknown information symbols interspersed with known information symbols transmitted using a signal modulated either with Gaussian Minimum Shift Keying (GMSK) or alternatively with Offset 16-Quadrature Amplitude Modulation (O-16QAM) and compensating for inter-symbol interference caused by a multipath propagation channel, comprising:

state memory means for storing a plurality of states, each state comprising a decoded value-string and an associated path metric, each of said decoded value-strings corresponding to different hypotheses of information contained in signal samples already processed, and each of said associated path metrics being indicative of a likelihood that the corresponding hypothesis was a correct hypothesis;

channel estimation means for estimating channel coefficients indicative of the phase and amplitude of each path of said multipath propagation based on received signal samples that correspond to said known information symbols;

detection means for detecting whether said unknown information symbols were transmitted using said GMSK or said O-16QAM modulation and providing an equalizer mode indication signal; and

Viterbi processing means for controlling the processing of each successive signal sample to update each state of said state memory by extending the decoded value-string and updating the associated path metric, each updated state being derived from one of four previous states when said equalizer mode indication signal is indicative of O-16QAM, and alternatively derived from one of two previous states when said equalizer mode indication signal is indicative of GMSK.

24. The receiver of

claim 23

, wherein each value in said decoded value-string is indicative of either a binary 1 or a binary zero when said equalizer mode indication signal is indicative of GMSK, and alternatively each said value is indicative of a pair of binary bits when said equalizer mode indication signal is indicative of O-16QAM.

25. The receiver of

claim 23

, wherein each value in said decoded value-string is indicative both of a decoded information symbol and of a likelihood that the decoded information symbol is correct.

26. The receiver of

claim 23

, wherein said known information symbols are transmitted using said GMSK modulation and said unknown information symbols are transmitted using said O-16QAM modulation.

27. The receiver of

claim 23

, wherein said known information symbols are transmitted using only those two symbols of said O-16QAM that attain the greatest positive or negative signal amplitude.

28. A receiver for decoding unknown information symbols interspersed with known information symbols transmitted using a signal modulated either with Offset Quadrature Phase Shift Keying (OQPSK) or alternatively with Offset 16-Quadrature Amplitude Modulation (O-16QAM) and compensating for inter-symbol interference caused by a multipath propagation channel, comprising:

state memory means for storing a plurality of states, each state comprising a decoded value-string and an associated path metric, each of said decoded value-strings corresponding to different hypotheses of information contained in signal samples already processed, and each of said associated path metrics being indicative of a likelihood that the corresponding hypothesis was a correct hypothesis;

channel estimation means for estimating channel coefficients indicative of the phase and amplitude of each path of said multipath propagation based on received signal samples that correspond to said known information symbols;

detection means for detecting whether said unknown information symbols were transmitted using said OQPSK or said O-16QAM modulation and providing an equalizer mode indication signal;

Viterbi processing means for controlling the processing of each successive signal sample to update each state of said state memory by extending the decoded value-string and updating the associated path metric, each updated state being derived from one of four previous states when said equalizer mode indication signal is indicative of O-16QAM, and alternatively derived from one of two previous states when said equalizer mode indication signal is indicative of OQPSK.

29. The receiver of

claim 28

, wherein each value in said decoded value-string is indicative of either a binary 1 or a binary zero when said equalizer mode indication signal is indicative of OQPSK, and alternatively each said value is indicative of a pair of binary bits when said equalizer mode indication signal is indicative of O-16QAM.

30. The receiver of

claim 28

, wherein each value in said decoded value-string is indicative of a decoded information symbol and of a likelihood that the decoded information symbol is correct.

31. The receiver of

claim 28

, wherein said known information symbols are transmitted using said OQPSK modulation and said unknown information symbols are transmitted using said O-16QAM modulation.

32. The receiver of

claim 28

, wherein said known information symbols are transmitted using only those two symbols of said O-16QAM that attain the greatest positive or negative signal amplitude.

33. A transmitter for transmitting information alternatively using either Gaussian Minimum Shift Keying (GMSK) modulation or Offset Quadrature Amplitude Modulation (OQAM), comprising:

at least two GMSK modulation means having inputs for associated binary information bit streams and providing corresponding modulated output signals;

amplifying and combining means for amplifying each of said modulated output signals and combining the amplified signals in predetermined amplitude ratios;

control means for selecting said associated binary information bitstreams supplied to said at least two GMSK modulation means to all be identical to one another when said GMSK modulation is desired, and alternatively for selecting at least two of said binary information bitstreams supplied to said at least two GMSK modulation means to be different from one another when said OQAM modulation is desired.

34. The transmitter of

claim 33

, wherein said amplifying means are saturated power amplifiers.

35. The transmitter of

claim 33

, wherein said combining means is a directional coupler.