TD-LTE: testing times for MIMO beamforming: Page 3 of 4

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.
for testing must now be able to accurately account for a number of parameters that were less critical in earlier technologies. One relatively arcane but significant example is in the phase relationship between uplink and downlink channels. In frequency domain duplex scenarios such as FDD-LTE, where uplink and downlink exist in separate frequency bands, there is no expectation of any particular phase relationship between the uplink and downlink. TD-LTE, however, is different. Uplink and downlink share a single band and are separated only by negligible slices of time. The technology itself relies on channel reciprocity, including nearly identical phase relationships between the two links.

This poses a risk in the receiver testing process. In order for test equipment to provide the proper level of control to the engineer, the equipment used to emulate the radio channel internally disaggregates uplink and downlink channels. Unless the equipment being used has been designed with phase accuracy in mind, there is no reason to expect that an emulated channel will accurately represent the reciprocal TD-LTE channel. To make matters even more complicated, the sheer number of transmitted signals (e.g. eight transmitted RF signals in planned TD-LTE deployments) must be replicated within this phase-controlled environment.

This issue has been recognized and resolved with modern channel emulation equipment. As an example, Spirent’s MB5 MIMO Beamforming Test System implements automated phase calibration specifically for this type of testing. Any system set up intended to properly emulate the TD-LTE channel must not only maintain the proper correlation between radio links in both directions, but it must offer a realistic phase relationship between uplink and downlink in order to maintain the realistically reciprocal properties of a TD-LTE system. The MB5, for example, maintains this relationship through the use of hardware that automatically performs phase calibration on a per-band basis.

Conclusion

There is no question that TD-LTE technology will, over the next several years, become a significant global force in

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