OFDM
In a wireless communication system, such, as the 3rd generation (3G) wireless and its long term evolution (LTE), it is desired to concurrently support multiple services and multiple data rates for multiple users in a fixed bandwidth channel. One scheme adaptively modulates and codes symbols before transmission based on current channel conditions. Another option available in LTE which uses orthogonal frequency division multiplexed access (OFDMA), is to exploit multi-user frequency diversity by assigning different sub-carriers or groups of sub-carriers to different users.
MIMO
In order to further increase the capacity of a wireless communication system in fading channel environments, multiple-input-multiple-output (MIMO) antenna technology can be used to increase the capacity of the system without an increase in bandwidth. Because the channels for different antennas can be quite different, MIMO increases robustness to fading and also enables multiple data streams to be transmitted concurrently.
While MIMO systems perform well, they also can increase the hardware and signal processing complexity, power consumption, and component size in transceivers. This is due in part to the fact that each receive antenna requires a receive radio frequency (RF) chain, which typically comprises a low noise amplifier, a frequency down-converter, and an analog to digital converter. Similarly, each transmit antenna element requires an RF chain that comprises a digital to analog converter, a frequency up-converter, and a power amplifier.
Moreover, processing the signals received in spatial multiplexing schemes or with space-time trellis codes requires receivers where the complexity can increase exponentially as a function of the number of antenna.
Antennas Selection
Antennas are relatively simple and cheap, while RF chains are considerably more complex and expensive. Antenna selection reduces some of the complexity drawbacks associated with MI MO systems. Antenna selection reduces the hardware complexity of transmitters and receivers by using fewer RF chains than the number of antennas. In antenna selection, a subset of the available antennas is adaptively selected by a switch, and only signals for the selected subset of antennas are processed by the available RF chains. As used herein, a subset, in all cases, means one or more of all the available antennas in the set of antennas. Note, that invention also allows multiple subsets to be used. For example, there can be four antennas and one RF chain, or eight antennas and two RF chains, which includes four subsets.
In order to select the ‘best’ subset of antennas, all channels corresponding to all possible transmitter and receive antenna pairs need to be estimated, even though only a selected subset of the antennas is eventually used for transmission,
Pilot Tones
Antenna selection can use repetitive pilot tones. Let Nt denote the number of transmit antennas, Nt the number of receive antennas, and let Rt=Nt/Lt and Rt=Nt/Lt be integers. Then, the available transmit (receive) antenna elements can be partitioned into Rt(Rt) disjoint subsets. The pilot repetition approach repeats, for Rt·Rt times, a training sequence that is suitable for an Lt×Lt MIMO system. During each repetition of the training sequence, the transmit (receive) RF chains are connected to different subsets of antennas. Thus, at the end of the Rt·Rt repetitions, the complete channel is estimated at the receiver.
in case of transmit antenna selection in frequency division duplex (FDD) systems, in which the forward and reverse links are not identical, the receiver feeds back the optimal subset of the selected antennas to the transmitter. In reciprocal time division duplex (TDD) systems, the transmitter can perform the selection by itself.
For indoor LAN applications with slowly varying channels, antenna, selection can be performed using a media access (MAC) layer protocol, see IEEE 802.1.1n wireless LAN draft specification, I. P802.11n/D1.0, “Draft amendment to Wireless LAN media access control (MAC) and physical layer (PHY) specifications; Enhancements for higher throughput,” Tech. Rep., March 2006.
Instead of extending the physical (PHY) layer preamble to include the extra training fields (repetitions) for the additional antenna elements, antenna selection training is done by the MAC layer by issuing commands to the physical layer to transmit and receive packets by different antenna subsets. The training information; which is a single standard training sequence for a Lt×Lt. MIMO system, is embedded in the MAC header field.
OFDMA Structure in LTE
The basic uplink transmission scheme is described in 3GPP TR 25.814, v1.2.2 “Physical Layer Aspects for Evolved UTRA.” The scheme is a single-carrier transmission (SC-OFDMA) with cyclic prefix (CP) to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver side.