But these increases do not come without some cost. Higher order modulations demand high dynamic range and linearity. A 64QAM signal may need in excess of 20 dB SNR to achieve better than desired maximum block error rate. OFDM systems transmit many small subcarriers which introduces wide changes in instantaneous power level; a peak to average power ratio (PAPR) greater than 10 dB may not be uncommon. And, with frequency selective fading environments typical of mobile communications, certain OFDM subcarriers may be deeply faded while others may not, further increasing the demand on dynamic range. The TD-LTE standard currently implements the uplink with SC-FDMA and was specifically designed to reduce the impact of deeply fades and therefore reduce the power consumption of the UE.
Channel emulator input dynamic range
There are several considerations associated with input power when choosing a channel emulator for use with a 3GPP TD-LTE device. These considerations include input power range, peak power and signal-to-noise margin.
The transmitted signal from the 3GPP LTE device can have a very wide dynamic power range. Although the average power may have some maximum value, when OFDM is employed, the PAPR can be greater than 10 dB, and hence the system must accommodate this maximum. Even with SC-FDMA as employed in transmitters for 3GPP TD-LTE UE devices, PAPR is still present and may be 8 dB or more. Mobile devices also implement transmit power control to vary their output power, typically as a function of