System and method of wireless communication using large-scale antenna networks
View Patent ↗A communication method uses a distributed cell-free network of spatially independent service antennas. According to the method, pilot sequences are allocated to a user population of access terminals by an allocation procedure that imposes local relative orthogonality of pilot sequences. Channel coefficients for access terminals are determined by measuring allocated pilot sequences as received by each of the service antennas. In embodiments of the invention, the determination of channel coefficients is performed independently and the resulting channel coefficients are locally stored at the respective service antennas. At each service antenna, a processor independently uses locally stored channel coefficients to precode forward link signals or to at least partially decode reverse link signals.
1. A communication method using a distributed cell-free network of service antennas, comprising:
allocating pilot sequences to a user population of access terminals by an allocation procedure that imposes local relative orthogonality of pilot sequences, wherein the allocating comprises:
generating a plurality of unitary matrices each associated with non-overlapping areas of a two-dimensional geographical space using a random field generating mechanism that imposes a controlled amount of spatial correlation for access terminals disposed in near proximity to each other,
designating a row or column of each particular unitary matrix to a respective non-overlapping area that the particular unitary matrix is associated with, and
allocating, to each particular access terminal, the row or column designated to a non-overlapping area where the particular access terminal is located;
determining one or more channel coefficients for each said access terminal by measuring allocated pilot sequences as received by each service antenna in an active subpopulation of the service antennas; and
at each service antenna of the active subpopulation, using the channel coefficients to precode forward link signals or to partially decode reverse link signals.
2. The method of claim 1 , wherein:
the determination of channel coefficients is performed independently by each of the active service antennas and the resulting channel coefficients are locally stored at the respective service antennas; and
each service antenna of the active subpopulation independently uses locally stored channel coefficients to precode forward link signals or to partially decode reverse link signals.
3. The method of claim 2 , wherein the entire active subpopulation of service antennas synchronously transmits a plurality of messages, in which each message is destined for a respective access terminal and the same messages are transmitted from each of said active service antennas.
4. The method of claim 3 , wherein prior to transmitting the plurality of messages, each service antenna precodes each message with one or more weight factors derived from the channel coefficients, and then sums the precoded messages.
5. The method of claim 2 , further comprising:
concurrently receiving a plurality of messages at each service antenna of the active population of service antennas, wherein each message is received on a reverse link from a respective one of the access terminals; and
at each said service antenna, partially decoding each respective message of the plurality using one or more weight factors derived from the channel coefficients.
6. The method of claim 5 , further comprising, at a network node communicatively connected to the distributed, cell-free network of service antennas:
receiving the respective, partially decoded messages from the active service antenna subpopulation; and
summing the partially decoded versions of each respective message as received from the respective active service antennas, thereby to recover a decoded version of each respective said message.
7. The method of claim 5 , wherein, for each active service antenna after a first active service antenna, the partial decoding is carried out by:
receiving a partially decoded version of each respective message from a preceding service antenna; and
adding, to the received version, a further partially decoded version of the same message, wherein the further partially decoded version is formed using weight factors derived from locally stored channel coefficients.
8. The method of claim 1 , wherein the allocation of pilot sequences comprises: providing a set of pilot sequences to be allocated;
ordering the access terminals in a sequence that includes a first and subsequent access terminals; and
allocating the pilot sequences in the ordered sequence, whereby there are a first and subsequent allocated pilot sequences; wherein:
each pilot sequence allocated after the first is selected from those pilot sequences that remain in the provided set; and
the selection is made so as to minimize a measure of contamination with those pilot sequences that have already been allocated.
9. The method of claim 8 , wherein the measure of contamination is derived from slow-fading components of the channel coefficients between the active service antennas and the access terminals.
10. The method of claim 1 , wherein each channel coefficient relates to a pair consisting of an access terminal and a service antenna, and the method further comprises pruning the pairs so as to define one or more active subpopulations of service antennas, wherein each active subpopulation is mutually active with a respective user population of access terminals.
11. The method of claim 10 , wherein the pruning is based on geographical locations of service antennas and access terminals.
12. The method of claim 10 , wherein the pruning is based on slow-fading components of the channel coefficients between service antennas and access terminals.
13. The method of claim 1 , wherein at least some of the service antennas are implemented on wireless user terminals.
14. A network node communicatively connected to a distributed cell-free network of service antennas, the network node comprising a processor, wherein:
the processor is configured, using the service antennas, to allocate pilot sequences to a user population of access terminals by an allocation procedure that imposes local relative orthogonality of pilot sequences, wherein the allocating comprises:
generating a plurality of unitary matrices each associated with non-overlapping areas of a two-dimensional geographical space using a random field generating mechanism that imposes a controlled amount of spatial correlation for access terminals disposed in near proximity to each other,
designating a row or column of each particular unitary matrix to a respective non-overlapping area that the particular unitary matrix is associated with, and
allocating, to each particular access terminal, the row or column designated to a non-overlapping area where the particular access terminal is located;
the processor is configured to deliver the same plurality of messages to each service antenna of an active subpopulation of the network of service antennas for synchronized transmission by the active subpopulation, wherein each message is destined for a respective access terminal;
the processor is configured to receive, from at least one service antenna of an active subpopulation of the network of service antennas, a partially decoded version of a message from each access terminal in a user population of access terminals; and
the processor is configured to recover a decoded version of each respective said message.
15. The network node of claim 14 , wherein:
the processor is configured to receive, from each service antenna of an active subpopulation of the network of service antennas, a partially decoded version of a message from each access terminal in a user population of access terminals; and
the processor is configured to sum the partially decoded versions of each respective message as received from the respective active service antennas, thereby to recover a decoded version of each respective said message.