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