Dithering scheme using multiple antennas for OFDM systems
View Patent ↗A ground-or roof-top-based repeater in an OFDM system uses multiple transmission antennas to retransmit satellite signals. By using multiple transmission antennas, multiple identical OFDM signals are transmitted. Dithering is performed by, using a phase of Rayleigh process generator, introducing a slight variable-frequency phase offset to all but one of the multiple identical transmitted OFDM signals, and thus the effective overall channel is more dynamic and provides spatial diversity to minimize long periods of fading in fading subchannels of the OFDM signals when the receiver is in a slow moving or stationary situation. To overcome the additional cancellation problem that can occur when two or more of the transmitting antennas are in a line-of-site position with the receiver and approximately the same distance from the receiver, a delay is deliberately introduced to make the delayed signal(s) appear to be reflected signals. This delay will not negatively impact the OFDM receiver performance as long as the delay is within the guard interval used in the OFDM process.
1. A method for transmitting an information stream over a wireless communication medium to one or more receivers, the method comprising:
(a) applying both orthogonal-frequency division multiplexed (OFDM) modulation and splitting to the information stream to generate a first copy and M−1 other copies of an OFDM signal, where M is the total number of antennas with an integer value greater than 1;
(b) applying M−1 delays and M−1 variable-frequency phase offsets to the M−1 other copies of the OFDM signal to generate M−1 delayed, phase-offset copies, wherein:
(i) the M−1 delays are relative to the first copy;
(ii) the M−1 delays are longer than spatial delays introduced as a result of separation distances between the antennas; and
(iii) the M−1 delays are shorter than a guard interval of the OFDM signal; and
(c) transmitting the first copy and the M−1 delayed, phase-offset copies via M respective antennas over the wireless communication medium.
2. The invention of claim 1 , wherein the M−1 variable-frequency phase offsets are applied in step (b) by multiplying each of the M−1 other copies by a phase generated by a Rayleigh process.
3. The invention of claim 1 , wherein the OFDM modulation is applied to the information stream before the splitting.
4. The invention of claim 3 , wherein the OFDM signal is multiplied by 1/M after the OFDM modulation and prior to the splitting.
5. The invention of claim 1 , wherein the M−1 delays are applied prior to the M−1 variable-frequency phase offsets.
6. The invention of claim 1 , wherein the application of the M−1 variable-frequency phase offsets increases effective channel response between the M antennas and the one or more receivers.
7. An apparatus for transmitting an information stream over a wireless communication medium to one or more receivers, the apparatus comprising: both an orthogonal-frequency division multiplexing (OFDM) modulator and a splitter adapted to apply OFDM modulation and splitting, respectively, to the information stream to generate a first copy and M−1 other copies of an OFDM signal, where M is the total number of antennas with an integer value greater than 1;
both a delay buffer and a first multiplier adapted to apply M−1 delays and M−1 variable-frequency phase offsets, respectively, to the M−1 other copies of the OFDM signal to generate M−1 delayed phase-offset copies, wherein:
the M−1 delays are relative to the first copy:
the M−1 delays are longer than spatial delays introduced as a result of separation distances between the antennas: and
the M−1 delays are shorter than a guard interval of the OFDM signal; and a transmission device adapted to transmit the first copy and the M−1 delayed phase-offset copies via M respective antennas over the wireless communication medium.
8. The invention of claim 7 , wherein the first multiplier applies the M−1 variable-frequency phase offsets by multiplying each of the M−1 copies by a phase generated by a Rayleigh process.
9. The invention of claim 7 , wherein OFDM modulation is applied by the OFDM modulator before splitting is applied by the splitter.
10. The invention of claim 9 , further comprising a second multiplier adapted to multiply the OFDM signal by 1/M before the OFDM signal is split.
11. The invention of claim 7 , wherein the M−1 delays are applied by the delay buffer before the variable-frequency phase offsets are applied by the first multiplier.
12. The invention of claim 7 , wherein the application of the M−1 variable-frequency phase offsets increases effective channel response between the M antennas and the one or more receivers.