doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 1 Use of EVM to Measure Rx Output Signal Quality Brian Hart, Phil Ryan, David Skellern (Cisco Systems) TGk May 2003

doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 2 Why EVM? Strongly support the use of a measurement to quantify the quality of the signal that is able to be recovered by a particular receiver –for the many good reasons presented in Joe Kwak’s presentation in doc: IEEE /218r2 (file r2-K-PSNI_Measurement_V2.ppt) EVM is the appropriate measurement basis for this purpose –It works for all digital modulations –It has the desired properties specified on slides 9 & 10 of doc: IEEE /218r2 –It can provide a direct indication of observed S/(N+I) considering all channel impairments and implementation losses when measured at the demodulator –It is defined already in the standard as a signal quality measure –Commercial EVM measurement equipment is available. Allowing flexibility in the basis of the measurements, as is possible in the PSNI proposal, will reduce the usefulness of the PSNI concept (just as flexibility in RSSI has led to the need for something like RCPI) A derived quantity representing observed analog S/(N+I) is unnecessary We should just use EVM rather than invent a new measure

doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 3 Constellation Diagram Pre-eminent tool in debugging digital communication systems Captures all the degradations present on a communications link –in proportion to the impact they have –at a point where the cumulative degradation is easy to estimate –includes: quantisation, non-ideal converters, clock frequency offsets and phase noise, carrier frequency offsets and phase noise, power- amplifier distortions, multipath, co-channel interference, adjacent channel interference, other interference, thermal noise, and poor receiver algorithms. Is used for numerous purposes, eg –can indicate whether synchronisation and equalisation are operating correctly –can provide an indication of the C/N of the link

doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 4 EVM in standard Using the constellation diagram in this way leads to a quantity called Error Vector Magnitude or EVM The a standard defines the EVM for a multi-packet average in section The EVM of a single packet is determined from the received complex constellation points as follows: –(the sum of the squared errors between the received and transmitted constellation points) normalised by (the ensemble average of the sum of the squared transmitted constellation points). The b standard defines a related quantity for DSSS and CCK in section These EVM definitions are used to test transmitter accuracy, and therefore assume an ideal channel and high-quality receiver.

doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 5 If real-world channels and receivers are substituted then the EVM definition still applies. EVM now applies to the whole communications link Experience shows that EVM or an equivalent measure is useful for many purposes In environments, the MAC receives DSSS, CCK and OFDM packets. It is most convenient if the three modulation schemes report the same EVM-related measure when operating over an AWGN channel at the same C/N. Standardization ensures this and accordingly we recommend it for k Extending the Use of EVM

doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 6 EVM Proposal Calculate EVM as: 10 log10 ( "the ensemble average of the sum of the squared transmitted constellation points" / ("the sum of the squared errors between the received and estimated transmitted constellation points" ) For a signals, the constellation is obtained as per section a)- g), except (i) that there is no requirement that the receiver has any particular accuracy, and (ii) it should be permissible to calculate the EVM over the data subcarriers only (i.e. omit the pilot subcarriers) and/or over the Signal field only. For b signals, process the signal by an actual receiver until samples of synchronised and equalised DSSS/CCK chips are obtained, then compare them to their estimated transmitted chip values (i.e. for 11b we have BPSK or QPSK chip constellations). [Minor problem: this measure gets more positive for better channels, and is strictly an *Inverse* EVM - but it could be argued that it is more natural for a larger number to indicate better performance]

doc.: IEEE /457 Submission May 2003 Hart/Ryan/Skellern CiscoSlide 7 EVM Encoding Encode as 8 signed bits in two's complement with 0.5 dB per step. Range represented is thus to dB. –Negative EVMs arise with 1 Mbit/s DSSS. –EVMs of 32 dB and above are not unlikely for a and therefore a positive limit of 63.5 dB is justified. A resolution of 0.5 dB is adequate for most purposes An accuracy of ±1 dB in reporting the EVM is allowed