Bandwidth efficient QAM on a TDM-FDM system for wireless communications

• US 6,252,910 B1
• Filed: 04/21/1999
• Issued: 06/26/2001
• Est. Priority Date: 11/11/1998
• Status: Active Grant

• Current Assignee: Benhov GmbH, LLC (Intellectual Ventures LLC)
• Original Assignee: Com Space Corporation
• Inventors: Kluesing, Alvin Dale, West, Randall J., McCarty, Robert Joseph Jr.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): JIANG, LENNY R
• Application Number:
  US09/295,660
• Time in Patent Office: 797 Days
• Field of Search: 375/240, 375/259, 375/260, 375/261, 375/295, 375/298, 375/316, 375/324
• US Class Current: 375/261
• CPC Class Codes: H04L 27/36 Modulator circuits; Transmitter circuits
• CPC Class Codes: H04L 5/06 the signals being represented by different frequencies combined with time-division multiplexing H04L5/26
• CPC Class Codes: H04W 52/362 Aspects of the step size
• CPC Class Codes: H04W 52/367 Power values between minimum and maximum limits, e.g. dynamic range

Abstract

Efficient usage of available spectrum is increased by logically dividing government licensed frequency channels into sub-channels, each of which can individually transmit a signal between a base unit and terminal. The sub-channels are each offset from the center of the frequency channel by a unique offset amount to avoid interference. Power control, sub-channel interference cancellation, and frequency control are employed to minimize the effects of out-of-band sub-channel signals on adjacent sub-channels. Any given sub-channel can be dynamically configured to transmit voice or data signals. Further spectral efficiency is realized using time division multiplexing on some or all of the sub-channels.

Claims

1. A method of transmitting N information signals over a single pre-defined frequency channel having a bandwidth of BWc, each of the N information signals being transmitted on a single sub-channel of bandwidth BWsc, comprising:

a. modulating each of N information signals to form N modulated signals, each having a bandwidth equal to or less than (BWc−

BW_GB)/N, wherein BW_GB corresponds to the collective bandwidth of guardbands inserted between adjacent sub-channels and inserted at each end of the bandwidth of the pre-defined frequency channel, each modulated information signal having a center frequency of F_c, wherein said modulating is accomplished using differential modulation, and wherein each of the N modulated signals is comprised of symbols, the value of each symbol being determined by reference to a preceding symbol;

b. off setting the center frequency of each modulated signal by a frequency offset individual to that modulated signal, the frequency offset being different for each modulated signal;

c. combining the modulated signals to form a composite signal; and

d. transmitting the composite signal to a terminal.

2. The method of claim 1 wherein the modulating step comprises mapping groups of bits of each information signal onto a quadrature amplitude modulation constellation and passing the resulting data stream through a Nyquist filter.

3. The method of claim 2 wherein said quadrature amplitude modulation constellation is a differential sixteen point star constellation.

4. The method of claim 1 wherein said modulating step and said offsetting step are effected simultaneously.

5. A method of communicating N information signals over a single pre-defined frequency channel having a bandwidth of BWc, each of the N information signals being transmitted on a single sub-channel of bandwidth BWsc comprising the steps of:

a. modulating each of N information signals to form N modulated signals, each having a bandwidth equal to or less than (BWc−

BW_GB)/N, wherein BW_GB corresponds to the collective bandwidth of guardbands inserted between adjacent sub-channels and inserted at each end of the bandwidth of the pre-defined frequency channel, each modulated information signal having a center frequency of F_c, wherein said modulating is accomplished using differential modulation, and wherein each of the N modulated signals is comprised of symbols, the value of each symbol being determined by reference to a preceding symbol;

b. offsetting the center frequency of each modulated signal by a frequency offset individual to that modulated signal, the frequency offset being different for each modulated signal;

c. combining the modulated signals to form a composite signal;

d. transmitting the composite signal;

e. receiving the composite signal at a terminal;

f. offsetting the center frequency of the received composite signal to re-center to Fc a first selected one of the modulated information signals comprising the composite signal;

g. filtering the composite signal to remove the modulated information signals not re-centered to Fc; and

h. demodulating the first selected one of the modulated information signals.

6. The method of claim 5 further comprising the steps of:

i. offsetting the center frequency of the received composite signal to re-center to Fc a second selected one of the modulated first information signals comprising the composite signal;

j. filtering the received composite signal to remove the modulated information signals not re-centered to Fc; and

k. demodulating the second selected one of the modulated information signals.

7. The method of claim 5 further comprising the steps of:

i. time division multiplexing the information signals prior to the modulating step of step a such that each information signal comprises two or more communications signals, and

j. time division de-multiplexing the demodulated first selected one of the modulated information signals.

8. A transmission apparatus for transmitting N information signals over a pre-defined frequency channel, having a bandwidth of BW_c, comprising:

a. N information signal inputs to receive at least N information signals;

b. N differential modulators, each coupled to a corresponding one of the N information signal inputs wherein each of the information signals is differentially modulated to form a single differentially modulated sub-channel signal, each having a center frequency Fc and a bandwidth BWsc, wherein each of the differentially modulated signals is comprised of symbols, the value of each symbol being determined by reference to a preceding symbol;

c. N sub-channel frequency offset multipliers, each coupled to a corresponding one of the N modulators to receive a differentially modulated sub-channel signal and offset its center frequency from Fc by one of N unique sub-channel offset frequencies;

d. a sub-channel summer coupled to the N sub-channel frequency offset multipliers to receive said N offset differentially modulated sub-channel signals and combine them to form a composite signal; and

e. a transmitter coupled to said summer to receive said composite signal and transmit it over said pre-defined frequency channel.

9. The apparatus of claim 8 wherein the information signal comprises a plurality of time division multiplexed signals.

10. The apparatus of claim 8 wherein the modulators, sub-channel frequency offset multipliers, and sub-channel summer are realized in digital signal processing circuitry.

11. The apparatus of claim 8 wherein the information signals comprise digitized voice signals.

12. The apparatus of claim 8 wherein the information signals comprise control signals directed to a terminal receiving the composite signal.

13. The apparatus of claim 8 wherein each differentially modulated sub-channel signal is offset from its center frequency F_c by an offset frequency F_n calculated as:

14. The transmission apparatus of claim 8 wherein N information signals are transmitted over a first pre-defined frequency channel and K additional information signals are transmitted over a second pre-defined frequency channel.

15. The apparatus of claim 8 wherein said pre-defined frequency channel has a bandwidth of 25 kHz and N equals four.

16. The apparatus of claim 8 wherein said pre-defined frequency channel has a bandwidth of 12.5 kHz and N equals two.

17. The apparatus of claim 8 wherein the N information signals are logically organized as sequential time frames, each time frame comprising:

a. a first time slot containing a first signal intended for a first terminal;

b. a second time slot containing a second signal intended for a second terminal.

18. The apparatus of claim 17 wherein each time frame further comprises a third time slot containing control information, the control information being associated with at least one of the first and second time slots and being directed to a terminal receiving at least one of the N information signals.

19. The apparatus of claim 17 wherein at least one of the first and second time slots contains data and control signals.

20. A receiver apparatus for receiving one of N differentially modulated information signals forming a modulated composite signal and transmitted over a pre-defined frequency channel, each of said N information signals comprising a series of symbols, comprising:

a. a receiver for receiving the composite signal;

b. a sub-channel offset controller coupled to said receiver wherein the composite signal is offset by a sub-channel offset corresponding to a first desired one of the differentially modulated N information signals;

c. a demodulator coupled to the offset controller wherein the first desired one of the differentially modulated N information signals is filtered and converted to a digital data stream, said filtering including determining the value of the symbols comprising the differentially modulated information signal with reference to a preceding symbol.

21. The apparatus of claim 20 wherein:

a. the sub-channel offset controller offsets the composite signal by a second sub-channel offset corresponding to a second desired one of the N information signals; and

b. the demodulator filters and converts to a digital data stream the second one of the desired information signals.

22. The apparatus of claim 20 wherein said information signal comprises a plurality of time division multiplexed communication signals and further comprising:

a time division demultiplexer wherein the time division multiplexed communication signals are logically separated into individual communication signals.

23. A method for transmitting a digital data stream having a data rate R, over N pre-defined frequency channels:

a. dividing the N pre-defined frequency channels into two or more sub-channels, each sub-channel being offset from the center of its associated frequency channel by a unique frequency offset, and each sub-channel having a data capacity equal to C;

b. allocating a sufficient number of sub-channels, Y, to transmit the digital data stream, such that Y*C=R;

c. apportioning the digital data stream over the Y sub-channels; and

d. simultaneously transmitting the digital data stream over the Ysub-channels comprising the N pre-defined frequency channels.

24. The method of claim 23 wherein other sub-channels are allocated for transmission of other digital data streams.

25. The method of claim 23 wherein other sub-channels are allocated for transmission of voice communications.

26. The method of claim 23 wherein the multiple pre-defined frequency channels are contiguous.

27. The method of claim 23 wherein said pre-defined frequency channels are government licensed portions of the RF spectrum.

28. The method of claim 23 wherein said pre-defined frequency channels are 25 kHz, 12.5 kHz, or 6.25 kHz in bandwidth.

29. The method of claim 23 further comprising the steps of:

a. simultaneously receiving the signal transmitted on the N sub-channels; and

b. re-constructing the digital data stream from the N portions received.

30. A system for transmitting N information signals over a single pre-defined frequency channel having a bandwidth of BWc, each of the N information signals being transmitted on a single sub-channel of bandwidth BWsc, comprising:

a base unit comprising;

N signal inputs to receive at least N information signals, at least one of the N information signals being independent from each other of the N information signals;

N differential modulator, each coupled to a corresponding one of the N information signal inputs wherein each of the information signals is differentially modulated to form a single differentially modulated sub-channel signal, each having a center frequency F_c and a bandwidth BWsc, wherein each of the differentially modulated signals is comprised of symbols, the value of each symbol being determined by reference to a preceding symbol;

N sub-channel frequency offset multipliers, each coupled to a corresponding one of the N modulators to receive a differentially modulated sub-channel signal and offset its center frequency from F_c by one of N unique sub-channel offset frequencies;

a sub-channel summer coupled to the N sub-channel frequency offset multipliers to receive said N offset differentially modulated sub-channel signals and combine them to form a composite signal;

a radio transmitter coupled to the sub-channel summer wherein the composite signal is transmitted to one or more mobile units over a wireless medium;

a first mobile unit comprising;

a radio receiver for receiving the composite signal;

a sub-channel offset controller coupled to said radio receiver wherein the composite signal is offset by a first sub-channel offset corresponding to a first one of the at least one independent information signals and each of the other N information signals is filtered out;

a demodulator coupled to the offset controller wherein the first one of the at least one independent information signals is filtered and converted to a digital data stream; and

a second mobile unit comprising;

a second radio receiver for receiving the composite signal;

a sub-channel offset controller coupled to said radio receiver wherein he composite signal is offset by a second sub-channel offset corresponding to a second one of the at least one independent information signals and each of the other N information signals is filtered out;

a demodulator coupled to the offset controller wherein the second one of the at least one independent information signals is filtered and converted to a digital data stream.

31. The system of claim 30 wherein said digital data stream comprises a voice signal.

32. The system of claim 30 wherein the number of sub-channels comprising a pre-defined frequency channel is selectable.

33. The system of claims 30 wherein the at least one independent information signal comprises a separate communication path from each other of the N information signals.