No transmit/receive filter will be defined
•In UMTS a transmit/receive filter was defined
•Root raised cosine α= 0.22
•This filter was also used to make EVM measurements
•Deviations from the ideal filter increased the measured EVM
•In LTE with OFDMA/SC-FDMA no filter is defined
•The lack of a filter creates opportunities and problems:
•Signal generation can be optimized to meet in-channel and out of channel requirements
•Signal reception and measurement have no standard reference
•It is expected that real receivers will use the downlink reference signals (pilots) to correct for frequency and phase
•But no standard for how to do this will be specified
•So what corrections should be applied to measurements?
The role of EVM measurements
•The role of EVM measurements should be to provide a reference for signal quality which can be usefully used in the transmit/receive link budget to assess system performance
•The measurement definition should ideally match what is used in real receivers
•Receiver equalizer algorithms are considered proprietary so it has been necessary to define an independent equalizer for measurements
•The EVM equalizer should apply a reasonable level of correction consistent with what is possible with a real-time receiver
•It is counter-productive to use metrology-grade processing to minimize EVM e.g. by using knowledge of the transmitted data or iterative non real-time optimizations
•Minimizing EVM might look good for the transmitter but does nothing to improve the link budget or the performance of the receiver
Downlink EVM equalizer definition
For the downlink, the EVM equalizer has been constrained
Rather than use all the RS data to correct the received signal a moving average is performed in the frequency domain across the channel which limits the rate of change of correction
There is no limit on the dB amount of correction
[802.16e WiMAXTM has +2/-4 dB limits at the channel edge, ±2 dB in the central 50% and 0.1 dB between subcarriers]
Uplink EVM equalizer definition
•This has not yet been fully defined
•The current proposal is to use a similar approach to WiMAX
•Unconstrained equalizer
•Define amplitude flatness across the channel
•In addition it may be necessary to constrain the phase variation as well since this is equally important as a source of demodulation errors
EVM requirements
•EVM requirements are still to be finalized
•There will not just be one uplink EVM requirement
•EVM will likely be defined by
•Modulation depth
•Position within the channel bandwidth
•Power level
•The current requirements for signals above -40 dBm are:
•It is not expected that 64QAM will be allocated at the edge of the signal
EVM vs. time –impact on CP reduction
•The lack of a defined transmit filter means that trade-offs can be made between in-channel performance and out of channel performance (ACLR, Spectrum emission mask)
•But applying too aggressive filtering can introduce delays to the signal which appear like multipath and reduce the effective length of the CP
•For this reason EVM is defined across a window at two points in time either side of the nominal symbol centre
EVM vs. time –impact on CP reduction
•The window lengths for the uplink are not yet defined but here are the downlink values from 36.804:
•The EVM is measured at two points in time –W/2 and + W/2
•The narrowest bandwidths allow the most relaxation with nearly one quarter of the CP being used up by transmit filter distortion
