TD-LTE: testing times for MIMO beamforming

May 28, 2012 //By Nigel Wright, Spirent Communications
The time-division flavor of LTE, commonly known as TD-LTE, has some powerful supporters and has been gathering media attention recently. While much initial focus for TD-LTE is on Asia, the technology looks likely to be adopted worldwide by operators with unpaired spectrum, such as incumbent WiMAX and TD-SCDMA licensees, who are looking to get in on the growing LTE ecosystem.

At a high level, TD-LTE is simply LTE with the uplink and downlink multiplexed in the time domain rather than the frequency domain. TD-LTE’s ability to be realized in a single frequency band (as opposed to the paired bands required for FDD-based LTE) makes it an attractive option in an age where the pressure on spectrum is rapidly increasing. In addition, since the downlink in a cellular system typically carries more data than the uplink, TD-LTE has a desirable ability to change uplink and downlink capacity, often dynamically, by altering the number of time slots allocated to each direction.

A key part of making TD-LTE an effective solution for operators is another wireless innovation: Multiple-Input Multiple-Output (MIMO) antenna techniques. These offer faster data rates and better system capacity than Single Input Single Output (SISO) systems, without expending resources in the time or frequency domains. In an m × n MIMO system (one using m transmitting antenna elements and n receiving antenna elements), theoretical maximum data rates are limited by the smaller of {m, n}.

MIMO beamforming

When MIMO antenna techniques are married with TD-LTE, an interesting and useful attribute results: since the uplink and downlink share a single frequency band, the channel can be regarded as reciprocal. As a result, channel estimation of the uplink can be used to make reasonable assumptions regarding downlink channel characteristics. Channel reciprocity in a single uplink/downlink frequency provides lends itself to a way of improving both coverage and system quality: MIMO beamforming.

Beamforming uses multiple antennas to “steer” signals towards areas where subscriber density is high. Two or more antennas will transmit the same signal, but with carefully controlled phase/amplitude characteristics. The result is constructive interference in areas of desired reception and destructive interference where reception is unimportant. While beamforming has other uses (for example, it can be used as with receiving antennas to “null out” a nearby interferer), it is most commonly applied

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