Methods and apparatus for signal conditioning in OFDMA systems
View Patent ↗Methods and systems for conditioning an orthogonal frequency division multiplex (OFDM) signal are disclosed. The OFDM signal may be conditioned prior to transmission by a transmitter in an orthogonal frequency division multiple access (OFDMA) system operating on a channel with a plurality of subcarriers grouped into subchannels. The OFDM signal may be clipped based on a desired peak-to-average-power ratio (PAPR) to produce a clipped-off portion of the OFDM signal. The clipped-off portion of the OFDM signal may be transformed into the frequency domain to produce a frequency-domain signal. An in-band spectral shaping mask may be applied to subcarriers or subchannels of the frequency-domain signal within the channel to control the levels of distortion on the individual subcarriers or subchannels. The shaped frequency-domain signal is transformed into the time domain to produce a time-domain signal. A conditioned signal is produced for transmission by subtracting the time-domain signal from the OFDM signal.
1. A method for conditioning an orthogonal frequency division multiplex (OFDM) signal prior to transmission by a transmitter in an orthogonal frequency division multiple access (OFDMA) system operating on a channel with a plurality of subcarriers grouped into subchannels, the method comprising:
clipping said OFDM signal in the time domain to produce a clipped-off portion of said OFDM signal, wherein a time sample of the clipped-off portion of the said OFDM signal is set to zero if a corresponding time sample of the OFDM signal is not greater than a threshold that is based on a desired peak-to-average-power ratio (PAPR);
transforming said clipped-off portion of said OFDM signal to produce a frequency-domain signal;
applying an in-band spectral shaping mask to individual subcarriers or subchannels of the frequency-domain signal within the channel to control levels of in-band distortion on the individual subcarriers or subchannels, wherein a different in-band distortion level is allowed for a different subcarrier or subchannel;
transforming the shaped frequency-domain signal to produce a time-domain signal; and
producing a conditioned signal for transmission by subtracting the time-domain signal from said OFDM signal.
2. The method of claim 1 , further comprising determining if an additional iteration of conditioning is required for the conditioned signal.
3. The method of claim 1 , wherein the allowable in-band distortion level on a subcarrier or subchannel is determined based on attributes associated with the said subcarrier or subchannel, the attributes comprising:
a modulation and coding scheme (MCS) of said subcarrier or subchannel;
a transmission power level of said subcarrier or subchannel
a channel condition of said subcarrier or subchannel
an antenna scheme for transmission or reception on said subcarrier or subchannel;
information carried by said subcarrier or subchannel;
whether said subcarrier or subchannel is used by the transmitter; or
a combination thereof.
4. The method of claim 3 , wherein the allowable in-band distortion level is proportional to a robust level of MCS on the subcarrier or subchannel.
5. The method of claim 3 , wherein the allowable in-band distortion level is correlated with the transmission power level of the subcarrier or subchannel.
6. The method of claim 3 , wherein the allowable in-band distortion level on a subcarrier or subchannel carrying control a signal is lower than the allowable level of in-band distortion on a subcarrier or subchannel carrying data.
7. The method of claim 3 , wherein the allowable level of in-band distortion on a subcarrier or subchannel used by the transmitter to transmit data is lower than the allowable level of in-band distortion on a subcarrier or subchannel not used by the transmitter.
8. The method of claim 1 , wherein the in-band spectral shaping mask varies from OFDM symbol to OFDM symbol, from time slot to time slot, or time frame to time frame.
9. The method of claim 1 , further comprising applying a transmission mask to out-of-band frequency components of said frequency domain signal.
10. A signal generator for generating an orthogonal frequency division multiplex (OFDM) signal at a transmitter in an orthogonal frequency division multiple access (OFDMA) system operating on a channel with a plurality of subcarriers grouped into subchannels, the signal generator comprising:
means for generating an OFDM symbol in the time domain;
means for clipping said OFDM symbol to produce a clipped-off portion of said OFDM symbol, wherein a time sample of the clipped-off portion of the said OFDM signal is set to zero if a corresponding time sample of the OFDM signal is not greater than a threshold, the threshold based on a desired peak-to-average-power ratio (PAPR);
means for transforming said clipped-off portion of said OFDM symbol to produce a frequency-domain signal;
means for applying spectral shaping mask to individual subcarriers or subchannels of the frequency-domain signal within the channel to control levels of in-band distortion on the individual subcarriers or subchannels, wherein a different distortion level is allowed for a different subcarrier or subchannel;
means for transforming the shaped frequency-domain signal to produce a time-domain; and
means for producing a conditioned signal for transmission by subtracting the time-domain signal from said OFDM symbol.
11. The signal generator of claim 10 , further comprising means for determining if an additional iteration of conditioning is required for the conditioned signal.
12. The signal generator of claim 10 , further comprising means for buffering a signal.
13. The signal generator of claim 10 , further comprising means for switching a signal.
14. The signal generator of claim 10 , wherein the allowable in-band distortion level on a subcarrier or subchannel is determined based on attributes associated with said subcarrier or subchannel, the attributes comprising:
a modulation and coding scheme (MCS) of said subcarrier or subchannel;
a transmission power level of said subcarrier or subchannel
a channel condition of said subcarrier or subchannel
an antenna scheme for transmission or reception on said subcarrier or subchannel;
information carried by said subcarrier or subchannel;
whether said subcarrier or subchannel is used by the transmitter; or
a combination thereof.
15. The signal generator of claim 14 , wherein the allowable in-band distortion level is proportional to a robust level of MCS on a subcarrier or subchannel.
16. The signal generator of claim 14 , wherein the allowable in-band distortion level is correlated with the transmission power level for the subcarrier or subchannel.
17. The signal generator of claim 14 , wherein the allowable in-band distortion level on a subcarrier or subchannel carrying control a signal is lower than the allowable level of in-band distortion on a subcarrier or subchannel carrying data.
18. The signal generator of claim 14 , wherein the allowable in-band distortion level on a subcarrier or subchannel used by the transmitter to transmit data is lower than the allowable level of in-band distortion on a subcarrier or subchannel not used by the transmitter.
19. The signal generator of claim 10 , wherein the in-band spectral shaping mask varies from OFDM symbol to OFDM symbol, from time slot to time slot, or time frame to time frame.
20. The signal generator of claim 10 , further comprising means for applying a transmission mask to out-of-band frequency components of said frequency domain signal.
21. A non-transitory computer readable storage medium storing thereon computer executable instructions for conditioning an orthogonal frequency division multiplex (OFDM) signal prior to transmission by a transmitter in an orthogonal frequency division multiple access (OFDMA) system operating on a channel with a plurality of subcarriers grouped into subchannels, the computer readable storage medium comprising:
instructions for clipping said OFDM signal in a time domain to produce a clipped-off portion of said OFDM signal, wherein a time sample of the clipped-off portion of the said OFDM signal is set to zero if a corresponding time sample of the OFDM signal is not greater than a threshold based on a desired peak-to-average-power ratio (PAPR);
instructions for transforming said clipped-off portion of said OFDM signal to produce a frequency-domain signal;
instructions for applying an in-band spectral shaping mask to individual subcarriers or subchannels of the frequency-domain signal within the channel to control levels of in-band distortion on the individual subcarriers or subchannels, wherein a different distortion level is allowed for a different subcarrier or subchannel;
instructions for transforming the shaped frequency-domain signal to produce a time-domain signal; and
instructions for producing a conditioned signal for transmission by subtracting the time-domain signal from said OFDM signal.
22. The computer readable storage medium of claim 21 , wherein an allowable in-band distortion level on a subcarrier or subchannel is determined based on attributes associated with the said subcarrier or subchannel, the attributes comprising:
a modulation and coding scheme (MCS) of said subcarrier or subchannel;
a transmission power level of said subcarrier or subchannel
a channel condition of said subcarrier or subchannel
an antenna scheme for transmission or reception on said subcarrier or subchannel;
information carried by said subcarrier or subchannel;
whether said subcarrier or subchannel is used by the transmitter; or
a combination thereof.
23. The computer readable storage medium of claim 21 , wherein the in-band spectral shaping mask varies from OFDM symbol to OFDM symbol, from time slot to time slot, or time frame to time frame.