APPARATUS FOR AND METHOD OF MINIMIZING BACKOFF FOR ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING TRANSMISSION

Abstract

A novel and useful mechanism for reducing the required backoff and the peak to average power ratio (PAPR) needed for an OFDM transmitter whiles still meeting spectral mask and EVM specifications. The mechanism searches, for each packet to be transmitted, for several possible scrambler and encryption sequences that would yield the best spectral mask and EVM with lowest PAPR. The search can be performed using the existing transmitter and receiver PHY circuit chain to modulate and demodulate the candidate hypotheses. Once the scrambler sequence and/or encryption sequence is selected, the packet is transmitted using the selected scrambler and encryption sequences. In addition, the invention exploits the fact that even for very low backoff margins, a reduced number of candidate hypotheses may be tested while still yielding a high probability of meet the spectral mask and EVM specifications.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating the definition of backoff for an example OFDM transmission;

FIG. 2 is a diagram illustrating an example of a spectral mask;

FIG. 3 is a block diagram illustrating an example communication device in more detail incorporating the backoff mechanism of the present invention;

FIG. 4 is a block diagram illustrating an example transmitter incorporating the reduced backoff mechanism of the present invention;

FIGS. 5A, 5B and 5C are a flow diagram illustrating a first reduced backoff method of the present invention;

FIG. 6 is a flow diagram illustrating a second reduced backoff method of the present invention;

FIG. 7 is a diagram illustrating the cumulative distribution function of the mask margin; and

FIG. 8 is a diagram illustrating the cumulative distribution function of the number of hypotheses required to be scanned to meet the spectral mask requirements.

Claims

1. A method of minimizing backoff for use in a transmitter, said method comprising the steps of: scanning a plurality of sequence hypotheses;for each sequence hypothesis, calculating a transmit circuit output signal and peak to average value thereof; andselecting the sequence that yields a minimum peak to average value.

2. The method according to claim 1, wherein said step of calculating comprises calculating a transmit output signal by applying said sequence hypothesis to a transmit circuit.

3. The method according to claim 2, wherein said transmit circuit comprises a conventional orthogonal frequency division multiplexing (OFDM) transmit PHY circuit.

4. The method according to claim 1, wherein said sequence comprises a scrambler seed sequence.

5. The method according to claim 1, wherein said sequence comprises an encryption seed sequence.

6. The method according to claim 1, wherein said step of scanning comprises the step of scanning a subset of possible sequence hypotheses.

7. The method according to claim 1, wherein said step of scanning comprises the step of scanning a subset of possible sequence hypotheses randomly chosen from all possible sequence hypotheses.

8. A method of minimizing backoff for use in a transceiver incorporating a transmitter and receiver, said method comprising the steps of: first scanning a plurality of sequence hypotheses;for each sequence hypothesis, first calculating a transmit circuit output signal and peak to average value thereof;selecting the sequence that yields a minimum peak to average value;for the selected sequence, second scanning all possible gain value hypotheses;for each gain hypothesis, second calculating a receiver output signal and measuring the spectral mask and error vector magnitude (EVM) thereof; andselecting the maximum gain that does not violate spectral mask and EVM requirements.

9. The method according to claim 8, wherein said step of first calculating comprises calculating a transmit output signal by applying said sequence hypothesis to a transmit circuit.

10. The method according to claim 9, wherein said transmit circuit comprises a conventional orthogonal frequency division multiplexing (OFDM) transmit PHY circuit.

11. The method according to claim 8, wherein said step of second calculating comprises the steps of: multiplying said transmit circuit output with a gain hypothesis to generate a gain adjusted output signal therefrom;passing said gain adjusted output signal through a model of a power amplifier coupled to said transmitter to yield adjusted symbols therefrom; andpassing said adjusted symbols through said receiver to yield said receiver output signal therefrom.

12. The method according to claim 8, wherein said sequence comprises a scrambler seed sequence.

13. The method according to claim 8, wherein said sequence comprises an encryption seed sequence.

14. The method according to claim 8, wherein said step of first scanning comprises the step of scanning a subset of possible sequence hypotheses.

15. The method according to claim 8, wherein said step of first scanning comprises the step of scanning a subset of possible sequence hypotheses randomly chosen from all possible sequence hypotheses.

16. A method of minimizing backoff for use in a transceiver incorporating a transmitter and receiver, said method comprising the steps of: first scanning a plurality of sequence hypotheses;second scanning a plurality of gain value hypotheses;for each gain hypothesis, first calculating a receiver output signal and measuring the spectral mask and error vector magnitude (EVM) thereof;selecting the maximum gain that does not violate spectral mask and EVM requirements; andselecting the sequence that yields said maximum gain.

17. An apparatus for minimizing backoff in a transmitter, comprising: a coder operative to generate a bitstream from an input data packet in accordance with a sequence input thereto;a transmit circuit coupled to said coder;a peak to average calculator operative to calculate a peak to average value of the output of said transmit circuit; andmeans for scanning a plurality of sequences through said coder and selecting the sequence that yields a minimum peak to average value.

18. The apparatus according to claim 16, wherein said transmit circuit comprises an orthogonal frequency division multiplexing (OFDM) transmit PHY circuit.

19. The apparatus according to claim 16, wherein said sequence comprises a scrambler seed sequence.

20. The apparatus according to claim 16, wherein said sequence comprises an encryption seed sequence.

21. The apparatus according to claim 16, wherein said means for scanning comprises means for scanning a subset of possible sequence hypotheses.

22. The apparatus according to claim 16, wherein said means for scanning comprises means for scanning a subset of possible sequence hypotheses randomly chosen from all possible sequence hypotheses.

23. An apparatus for minimizing backoff in a transceiver having a transmitter and a receiver, comprising: a coder operative to generate a bitstream from an input data packet in accordance with a sequence input thereto, said bitstream input to said transmitter;a peak to average calculator operative to calculate a peak to average value of the output of said transmitter;a gain multiplier coupled to the output of said transmitter and operative to generate a gain adjusted signal in accordance with a gain value applied thereto;a table operative to generate a table output from said gain adjusted signal in accordance a model of the input/output characteristics of a power amplifier portion of said transmitter, said table output input to said receiver;means for measuring a spectral mask and error vector magnitude (EVM) of the output of said receiver; andmeans for scanning a plurality of sequences through said coder and a plurality of gain values through said gain multiplier and selecting the sequence and maximum value that yields acceptable spectral mask and EVM.

24. The apparatus according to claim 23, wherein said transmitter comprises an orthogonal frequency division multiplexing (OFDM) transmit PHY circuit.

25. The apparatus according to claim 23, wherein said receiver comprises an orthogonal frequency division multiplexing (OFDM) receive PHY circuit.

26. The apparatus according to claim 23, wherein said sequence comprises a scrambler seed sequence.

27. The apparatus according to claim 23, wherein said sequence comprises an encryption seed sequence.

28. A mobile communications device, comprising: a cellular radio;an orthogonal frequency division multiplexing (OFDM) transmitter comprising a scrambler;an OFDM receiver;a processor coupled to said transmitter and said receiver, said processor operative to: scan a plurality of scrambler sequence hypotheses;for each scrambler sequence hypothesis, calculate a transmit circuit output signal and peak to average value thereof;select the scrambler sequence that yields a minimum peak to average value;for the selected scrambler sequence, scan all possible gain value hypotheses;for each gain hypothesis, calculate a receiver output signal and measure the spectral mask and error vector magnitude (EVM) thereof;select the maximum gain that yields acceptable spectral mask and EVM; andmeans for transmitting packets via said transmitter utilizing said selected scrambler sequence and selected gain value.