Bidirectionality and phase balance
A time division duplexed signal poses a unique challenge to test equipment design. The signal path through test equipment used for FDD protocols isn’t required to support phase balance when conducting bidirectional testing since the uplink and downlink both work in different spectrum and follow their own pilot.
A TD protocol however requires that both uplink and downlink paths be balanced in order to correctly emulate a bidirectional connection. This is especially important as the base station is able to use information from the uplink to control the downlink transmission.
Fading and noise floor
A fading channel emulator is employed to provide realistic fast fading conditions; ideally, the emulated fading will match that observed by a subscriber using the devices on the service provider’s network.
With an OFDM signal, as used in 3GPP TD-LTE, certain subcarriers may be faded, or momentarily reduced in amplitude by 20 dB or more due to the frequency selective fading. As each subcarrier is a modulated signal, with modulation up to 64QAM, this momentary drop in signal amplitude must be considered relative to the noise floor of the channel emulator equipment.
For example, if a signal with an average output power of -40 dBm is momentarily reduced by 20 dB due to fading, the amplitude will be -60 dBm. To maintain an adequate SNR for 64QAM at 25 dB, the test equipment noise floor should be no