LTE and Wifi standards define Transmit EVM but not for the "user side". User side EVM usually depends on receiver specs (noise floor, etc) but what are the standard acceptable values of OFDM after wireless transmission using blind equalization or for spatial diversity or multiplexing?
Dear Usman,
welcom,
Error vector magnitude is a metric used to measure how far the communication link is faithful in transporting the the modulated symbols.
So, of we plot the initial ideal constellation diagram at the output of the mapper of the bits on the symbols to produce the QAM symbols together with the those normalized symbols received at the input of the demapper in the receiver, then we observe that a difference between the opriginal constellation and the received symbol constellation.
The difference between every symbol vector from its ideal value is called the error vector magnitude.
The deviation or distortion can have different causes:
- The nonlinearity of the power amplifier of the transmitter,
- The amplitude and phase distortion in the channel because of multipath fading,
- The white noise generated in the receiver.
- Timing and synchronization errors,
All these error have their effect on the error vector magnitude. As the distortion or the noise increase the error vector magnitude increases and the bit error rate decreases.
The communication system performance parameters is defined by the allowed bit error rate for a given quality of service. What is given in the standard is the metrics of the quality of service rather than intermediate parameter like the evm.
A quantity equivalent to the error vector modulation is the signal to noise ratio. They may be an alternative quantities which limits the bit error rate.
For more information about the equivalence between the EVM and the S/N ratio please refer to the paper in the link:https://pdfs.semanticscholar.org/1dac/68d836b67fe3bda9a0382180ef68114a0f1f.pdf
Best wishes
Thank you so much Prof. Abdelhalim Zekry for your reply. I understand that EVM is the intermediate performance metric but EVM, BER and SNR are interchangeable, then why the standardization bodies only provide minimum requirement of EVM for the transmitter? and not after the wireless channel and user end.
Should I look for the standards defined for BER and SNR for the user side and convert them in EVM? Actually I work on system level and the EVM results that I get (after the wireless channel and user receiver equipment) needs to compare with the minimum acceptable EVM levels defined by the standards. But I haven't been able to find the EVM requirements for receiver ends. Can you please comment on it.
The link that you sent for a paper isn't valid. Can you please share the name of the document or alternate link? I'll be grateful
Dear Usman,
It is so that the power amplifier in the transmitter has some nonlinearities leading to error vector magnitude of the QAM symbols. Ideally the power amplifier must not introduce any distortion in the symbols.
So, if it is given in the standard, it is some form of design help as at the end effect it is the delivered signal quality at the destination which is assessed by the quality of service. The bit error rate is the most appropriate quality of service as it gives directly the end quality of the data delivered at the receiver end.
As i hinted above, all the effects mentioned in the first post contribute to the resultant EVM,
If all the effects are random with an equivalent error standard deviation sigma, then the overall standard deviation sigmat^2= summation sigmai62 where i is a running index for all deviations.
Since sigmat is known and sigma transmitter are known, one can get the residual allowed error vector magnitudes.
The title of the paper in the link given in the first post is: On the Extended Relationships Among EVM, BER and SNR as Performance Metrics
Best wishes
Hi Usman,
At first, I would like say, I cannot agree with Prof. Abdelhalim abdelnaby Zekry more on his answers, which are excellent.
Nevertheless, I am also curious. Is there any particular reason for you to ask about 16QAM only, it seems?
BTW, the NodeB transmission minimum requirement is 12.5% if I remember this correctly.
To answer your question, I guess there are at least three reasons below. Correct me please if I missed anything.
Reason 1. Nowadays the UE receivers are increasingly implemented with joint and iterative demodulation and decoding. A sole requirement on demodulation EVM may not be necessary. Instead it may be harmful or put too much burden to UE makers.
Reason 2. It is hard to verify or test EVM directly on final phone products. In general when a standardization body asserts an implementation requirement, it is usually assumed that this requirement will be easy for a third-part lab or certification body to verify it without breaking the device.
Reason 3. In general, there are trade-offs between minimum performance requirement and implementation freedom. In addition, there are also balance between minimum implementation on the base station side and interoperability.
In other words, too many requirements may be restrictive on promoting innovative receiver or transmitter implementations. On the other hand, some requirements are important for interoperability or improving implementation quality. As such, there are many balances. ... Indeed, this may answer another related question, why it is defined on the NodeB side.
Cheers
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