Testing TD-LTE with real world test technologies: Page 4 of 7

July 24, 2012 // By Erik Org, Azimuth Systems
Many wireless data networks were initially designed to offer symmetrical data capacities as the original killer wireless application, voice, required equal capacity for both uplink and downlink. Spectrum blocks were licensed and auctioned as paired spectrum suitable for Frequency Division Duplexing (FDD) protocols, which served carriers and subscribers well when voice was the primary application.
their distance from the eNodeB. TD-LTE transmit power control may result in 63 dB or more of actual power change. Furthermore, when the device is transmitting a higher order modulation, such as 64QAM, an adequate signal to noise ratio (SNR) must be maintained. When interfacing with a 3GPP TD-LTE UE, a channel emulator that can allow for direct connection of devices that transmit from +23 to -40 dBm, and still allows for sufficient PAPR and SNR margin, provides for a robust and efficient test configuration.

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

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