Testing TD-LTE with real world test technologies: Page 3 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.
increase capacity. An example is 64QAM (Quadrature Amplitude Modulation) that carries six bits per symbol per OFDM subcarrier. In addition, techniques like OFDMA improve the operation of the system and allow for scalable capacity. These techniques, coupled with multiple antenna technologies, MIMO, ultimately result in system operation that can provide scalable, reliable capacity to mobile stations with aggregate downstream data rates in excess of 100 Mbps and upstream data rates in excess of 50 Mbps.

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

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