Pre-coding in multi-user MIMO
In a multi-user communication system, a pre-coder in a transmitter comprises a Discrete Fourier Transform (DFT) spreader configured to spread data symbols with Fourier coefficients to generate DFT-spread data symbols. An OFDM modulator, such as an inverse-DFT, modulates the DFT-spread data symbols onto OFDM subcarriers to produce a pre-coded OFDM transmission signal. The DFT spreader is configured to reduce the transmission signal's peak to average power.
1. An OFDM transmitter, comprising:
an OFDM spreader configured to spread a plurality of data symbols with Fourier coefficients to generate a discrete Fourier Transform (DFT)-spread data signal;
a mapper configured to map the DFT-spread data signal to a plurality of OFDM subcarriers; and
an OFDM modulator configured to modulate the DFT-spread data signal onto the plurality of OFDM subcarriers to produce an OFDM transmission signal comprising a superposition of the OFDM subcarriers, wherein the OFDM spreader is configured to provide the superposition with a reduced peak-to-average power ratio.
2. The OFDM transmitter recited in claim 1 , wherein the OFDM spreader comprises an N-point DFT and the OFDM modulator comprises an M-point inverse discrete Fourier Transform, wherein M>N.
3. The OFDM transmitter recited in claim 1 , wherein the OFDM modulator comprises an inverse fast Fourier transform.
4. The OFDM transmitter recited in claim 1 , wherein the data symbols comprise reference-signal symbols, which comprise at least one of known training symbols and synchronization symbols.
5. The OFDM transmitter recited in claim 1 , wherein the plurality of OFDM subcarriers comprise at least one of contiguous OFDM subcarriers and equally spaced non-contiguous OFDM subcarriers.
6. The OFDM transmitter recited in claim 1 , further comprising a pulse-shaping module configured to provide the OFDM transmission signal with a predetermined pulse shape.
7. The OFDM transmitter recited in claim 1 , further comprising a cyclic prefix appender configured to append at least one of a cyclic prefix, a postfix, and a guard interval to the OFDM transmission signal.
8. The OFDM transmitter recited in claim 1 , wherein the OFDM spreader is configured to provide channel precoding.
9. The OFDM transmitter recited in claim 1 , wherein the plurality of data symbols are at least one of time-multiplexed with reference-signal symbols, frequency-multiplexed with reference-signal symbols, and code-multiplexed with reference-signal symbols.
10. The OFDM transmitter recited in claim 1 , further comprising a Cyclic Delay Diversity (CDD) module configured to provide CDD to the OFDM transmission signal.
11. A method, comprising:
multiplying a plurality of data symbols by a discrete Fourier Transform (DFT) spreading matrix to produce a plurality of DFT-spread symbols;
mapping the plurality of DFT-spread symbols to a plurality of OFDM subcarriers; and
modulating the plurality of DFT-spread symbols onto a plurality of OFDM subcarriers to produce a DFT-spread OFDM signal to be transmitted into a wireless channel, wherein the DFT-spread OFDM signal comprises a superposition of the OFDM subcarriers, and wherein the DFT spreading matrix is configured to provide the superposition with a reduced peak-to-average power ratio (PAPR).
12. The method recited in claim 11 , wherein the plurality of OFDM subcarriers is a subset of subcarriers, the subset comprising at least one of a block of contiguous subcarriers, and a set of non-contiguous subcarriers.
13. The method recited in claim 11 , wherein the plurality of OFDM subcarriers is selected based on measured channel quality of the subcarriers.
14. The method recited in claim 11 , wherein the mapping is configured to provide the DFT-spread OFDM signal with reduced PAPR.
15. The method recited in claim 11 , wherein at least one of the DFT spreading matrix and the mapping is configured to provide a DFT-spread reference signal comprising a coherent superposition of reference symbol components on different subcarriers.
16. The method recited in claim 11 , wherein the plurality of data symbols comprises at least one reference-signal symbol employed for at least one of synchronization and channel estimation.
17. The method recited in claim 11 , further comprising appending at least one of a cyclic prefix, a postfix, and a guard interval onto the DFT-spread OFDM signal.
18. The method recited in claim 11 , wherein the DFT spreading matrix comprises an N-point DFT, and the modulating comprises employing an M-point inverse-DFT, and wherein N<M.
19. The method recited in claim 11 , further comprising providing Cyclic Delay Diversity to the DFT-spread OFDM signal.
20. The method recited in claim 11 , further comprising pulse shaping to provide the OFDM transmission signal with a predetermined pulse shape.
21. A User Equipment configured to perform the method recited in claim 11 .
22. An apparatus comprising:
a processor; and
a non-transitory memory coupled to the processor, the non-transitory memory including a set of instructions stored therein and executable by the processor to:
for a set of data symbols, perform an N-point discrete Fourier transform (DFT) spreading on the set of data symbols to generate a plurality N of DFT-spread data symbols;
map the N DFT-spread data symbols to at least N subcarriers of a plurality M of Orthogonal Frequency Division Multiplexing (OFDM) subcarriers to generate a set of complex subcarrier amplitudes; and
perform an M-point inverse discrete Fourier transform (IDFT) on the set of complex subcarrier amplitudes to generate a time-domain sequence to be transmitted into a wireless channel, the time-domain sequence comprising a superposition of the OFDM subcarriers, wherein the N-point DFT spreading is configured to provide the superposition with a reduced peak-to-average power ratio.
23. The apparatus recited in claim 22 , wherein the N subcarriers comprise at least one of contiguous OFDM subcarriers and non-contiguous OFDM subcarriers.
24. The apparatus recited in claim 22 , configured to reside on a User Equipment.
25. The apparatus recited in claim 22 , wherein M>N.
26. The apparatus recited in claim 22 , wherein the IDFT comprises an inverse fast Fourier transform.
27. The apparatus recited in claim 22 , wherein the non-transitory memory further comprises instructions to perform pulse-shaping on at least one of a set of signals, the set comprising the plurality of DFT-spread data symbols and the time-domain sequence, in order to provide the superposition with a predetermined pulse shape.
28. The apparatus recited in claim 22 , wherein the non-transitory memory further comprises instructions to append at least one of a cyclic prefix, a postfix, and a guard interval to the time-domain sequence.
29. The apparatus recited in claim 22 , wherein the at least N subcarriers is selected based on a measured channel quality.