Energy efficient communications

1. A transceiver for use in a point-to-point packet-based communication link with packet erasures, the transceiver comprising:

a data source to provide bits of data for transmission to a remote device via a channel;

a processor to map the bits of data to symbols in a constellation according to a modulation scheme and to provide the symbols on one or more subcarriers; and

multiple antenna elements, the multiple antenna elements comprising co-located orthogonally polarized elements or spatially separated elements or both,

wherein the symbols are transmitted to the remote device in packets via one or more of the multiple antenna elements, and

wherein the processor is further configured to determine a metric of average transmit energy per bit successfully decoded, and not erased, at the remote device and to adjust transmission powers at the one or more antenna elements to reduce the average transmit energy per successfully decoded and unerased bit.

2. The transceiver of claim 1 , wherein the processor is further configured to form one or more weighted combinations of data that are transmitted via multiple ones of the antenna elements.

3. The transceiver of claim 2 , wherein the one or more weighted combinations of data comprise eigenmodes.

4. The transceiver of claim 2 , wherein the processor selects the one or more weighted combinations of data and determines the transmit power for the selected ones based on channel state information.

5. The transceiver of claim 4 , wherein the channel state information is received from the remote device.

6. The transceiver of claim 4 , wherein the channel state information is determined by the processor from preamble or pilot signals transmitted by the remote device based on an assumption of a reciprocal channel between the transceiver and the remote device.

7. The transceiver of claim 4 , wherein the processor selects a subset of linearly independent weighted combinations which can be transmitted using the multiple antenna elements.

8. The transceiver of claim 1 , wherein the processor selects the one or more subcarriers based on interference information provided by the remote device.

9. The transceiver of claim 8 , wherein the processor selects one or more subcarriers which exhibit a lesser degree of interference at the remote device.

10. The transceiver of claim 1 , wherein the metric of average transmit energy per bit accounts for packet erasures at the remote device.

11. The transceiver of claim 1 , wherein the processor determines the metric of average transmit energy per successfully decoded and unerased bit for a selected packet payload size by estimating a number of unerased packets relative to a total amount of energy required to deliver those unerased packets, and forming the metric of a ratio of the total energy to the estimate of the number of total bits in all unerased packets.

12. The transceiver of claim 11 , wherein the metric is computed using packet error rate, energy per bit to interference-plus-noise power spectral density ratio, packet length, packet length excluding a header and preamble, the number of subcarriers, transmit power and receiver power, the number of symbols delivered per block, the number of symbols in a packet preamble per block used for synchronization and channel estimation, the symbol duration, the number of eigenmodes and the specific selected eigenmodes, the number of downlink symbols, or the number of guard symbols.

13. The transceiver of claim 1 , wherein the processor reduces the average transmit energy per successfully decoded and not erased bit by reducing the packet erasure rate at the remote device.

14. The transceiver of claim 13 , wherein the processor reduces the packet erasure rate by selecting the size of each transmitted packet.

15. The transceiver of claim 1 , wherein the processor reduces the average transmit energy per successfully decoded and unerased bit by:

selecting a data rate, including the modulation scheme and a number of eigenmodes that can be supported by the channel;

determining a weighted combination to be used in transmission for one or more data streams and for the one or more subcarriers to be utilized in the transmission;

determining a maximum transmit energy efficiency and a corresponding optimal signal plus interference to noise ratio (SINR) for each of the one or more subcarriers; and

determining a corresponding transmit power for each of the one or more subcarriers using at least the optimal SINR values.

16. The transceiver of claim 1 , wherein the one or more subcarriers are orthogonal subcarriers.

17. The transceiver of claim 1 , wherein the transceiver is a battery powered unit.

18. The transceiver of claim 1 , wherein the transceiver reduces the average transmit energy per successfully decoded and unerased bit by jointly considering the number of transmitted eigenmodes and the specific selection of eigenmodes, packet length, modulation constellation, the one or more subcarriers, and the transmit power.

19. A method for use in a point-to-point packet-based communication link with packet erasures, the method comprising:

providing bits of data for transmission to a remote device via a channel;

mapping the bits of data to symbols in a constellation according to a modulation scheme;

providing the symbols on one or more subcarriers;

transmitting the symbols to the remote device in packets via one or more of multiple antenna elements, the multiple antenna elements comprising co-located orthogonally polarized elements or spatially separated elements or both; and

determining a metric of average transmit energy per bit successfully decoded, and not erased, at the remote device and adjusting transmission powers at the one or more antenna elements to reduce the average transmit energy per successfully decoded and unerased bit.