Multi-site MIMO cooperation in cellular networks
A method of serving a given data stream to a target mobile terminal, in a cellular communications network that includes a plurality of transmitting sites wherein each transmitting site including at least one antenna, is provided. The method includes designating at least two of the plurality of transmitting sites as cooperating sites; assigning tones to each transmitting site from a sub-band associated with the cooperating sites; dividing the data stream into at least two sub-data streams, each of the sub-data streams for transmission over selected tones; and interlacing tones of the cooperating sites in accordance with a selected one of a time switching and a frequency switching transmit diversity technique. Other techniques for multi-site MIMO cooperation are also provided.
1. In a cellular communications network comprising a plurality of transmitting sites, a method of serving a data stream to a target mobile terminal, comprising:
designating a first transmitting site and a second transmitting site as cooperating sites;
receiving, at the first transmitting site, a precoder reported by said target mobile terminal, the precoder being a member of a set of precoders in a constant amplitude codebook, the precoder indicating a preferred phase shift between a first antenna port associated with the first transmitting site and a second antenna port associated with the second transmitting site, wherein the constant amplitude codebook comprises two-element entries {[1,1] T , [1,j] T , [1,−1] T , [1−j] T }, and the precoder is selected by the target mobile terminal based on a first transmission between the target mobile terminal and the first transmitting site and a second transmission between the target mobile terminal and the second transmitting site;
at said first transmitting site, transmitting said data stream to said target mobile terminal in accordance with a first beamforming technique;
determining a linear phase shift based on the precoder reported by said target mobile terminal; and
transmitting, at said second transmitting site, said data stream to said target mobile terminal in accordance with a second beamforming technique, wherein the transmitting said data stream at said second transmitting site comprises applying the determined linear phase shift relative to the transmission of said data stream at the first transmitting site.
2. The method of claim 1 wherein one of said first and second transmitting sites comprises an antenna array and wherein said transmitting comprises transmitting the same data stream from each antenna of said antenna array.
3. The method of claim 2 further comprising:
at a primary antenna of said antenna array,
receiving channel quality information (CQI) feedback from said target mobile terminal;
responsive to said CQI feedback, providing an indicator of a linear phase adjustment to each other antenna of said antenna array; and
phase shifting beams transmitted by each of said other antennas by said indicated linear phase adjustment to thereby result in constructive addition of beams arriving at said target mobile terminal from said antenna array.
4. The method of claim 3 wherein said phase shifting comprises codebook phase correction.
5. The method of claim 4 wherein said applying said phase shifting comprises, for nomadic target mobile terminals, opportunistic phase correction.
6. The method of claim 5 wherein said opportunistic phase correction comprises applying random phase sequences at each antenna and based on received CQI feedback, selecting a phase combination.
7. The method of claim 1 wherein one of said first and second transmitting sites is a MIMO site.
8. The method of claim 3 wherein said indicator comprises an indicator of a timing mismatch of arriving beams at said target mobile terminal.
9. The method of claim 8 wherein said indicator of said timing mismatch is generated using a selected one of arrival time estimation and channel estimation techniques.
10. The method of claim 1 , wherein the first beamforming technique and the second beamforming technique are the same.
11. A cellular communications system comprising:
a first transmitter located at a first transmitting site, wherein the first transmitter is configured to transmit a data stream to a mobile terminal in accordance with a first beamforming technique; and
a second transmitter located at a second transmitting site, wherein the second transmitter is configured to transmit the data stream to the mobile terminal in accordance with a second beamforming technique with a phase shift relative to the first transmitting site based on a precoder reported by the mobile terminal, the first transmitting site and the second transmitting site are cooperating sites, the precoder being a member of a set of precoders in a constant amplitude codebook, the precoder indicating a preferred phase shift between a first antenna port associated with the first transmitting site and a second antenna port associated with the second transmitting site, wherein the constant amplitude codebook comprises two-element entries {[1,1] T , [1,j] T , [1,−1] T , [1,−1] T }, and the precoder is selected by the mobile terminal based on a first transmission between the mobile terminal and the first transmitting site and a second transmission between the mobile terminal and the second transmitting site.
12. The system of claim 11 , wherein at least one of the first transmitter or the second transmitter comprises an antenna array, and the data stream is transmitted from each antenna of the antenna array.
13. The system of claim 12 , wherein the antenna array comprises a primary antenna, and wherein the primary antenna is configured to:
receive channel quality information (CQI) feedback from the mobile terminal;
responsive to the CQI feedback, provide an indicator of a linear phase adjustment to each other antenna of the antenna array; and
phase shift beams transmitted by each of the other antennas by an indicated amount to result in constructive addition of beams arriving at the mobile terminal from the antenna array.
14. The system of claim 13 , wherein the phase shifting comprises a codebook phase correction.
15. The system of claim 14 , wherein the beams are phase shifted using, for nomadic mobile terminals, opportunistic phase correction.
16. The system of claim 15 , wherein using the opportunistic phase correction comprises applying random phase sequences at each antenna and based on received CQI feedback, selecting a best phase combination.
17. The system of claim 11 , wherein at least one of the first or the second transmitters is a MIMO transmitter.