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LTEin400 PMR/PAMR impact on TETRA systems

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[member was deleted] 31/03/17 17:19

This Topic will contain posts related to LTEin400 PMR/PAMR impact on TETRA systems

Steffen Ring 06/07/17 10:57

The text relating to intermodulation and the proposal to arbitrarily add an attenuation of 15dB to input signals in a SEAMCAT intermodulation analysis in sections 4.4.1 to 4.4.5 of the draft report is flawed and should be removed from the report. The analysis makes assumptions about receiver selectivity from blocking specifications, which is not a valid link to make. The input parameters to any simulation must be based on specifications for the parameters concerned, and therefore for intermodulation it should be based on the intermodulation characteristics of the victim receivers (TETRA, Tetrapol, analogue etc) as specified in the relevant published harmonised standard. This is the only way of providing results based on a factual foundation. We propose deleting these four sections of the report.

It is valid to provide additional simulations based on agreed external filtering as this can be a practical solution to coordination, especially for base stations, and we would have no objection to additional simulation results showing the effects of additional filtering, provided the filtering can be practically realised, as this can be used to show sensitivity analysis in addition to simulations based on standardised specifications.

Steffen Ring 14/09/17 16:01

Dear Spectrum Engineering Colleagues

At the recent SE7 in Copenhagen we agreed jointly to develop new text/modified text to the section on Intermodulation in prparation for the forthcoming SEAMCAT simulation on selected IM scenarios. Here is our first proposal enclosed.

 

Best regars

Steffen Ring

[member was deleted] 21/09/17 00:09

Dear all,

 

We've got a request to the SE-community and especially to Motorola.

 

For the further analysis of non-linaer effects within the TETRA MS receiver we would like to get a better understanding of TETRA receiver design. Would vendors of TETRA devices that implemented the superhet receiver architecture share with us the information on the tuning range and the used intermediate frequency(ies)?

It would be great if we could have such information shared.

 

Best regards,

Daan Beaufort

Steffen Ring 09/10/17 09:37



On 21/09/17 00:09 Daan Beaufort wrote:
"

Dear all,

 

We've got a request to the SE-community and especially to Motorola.

 

For the further analysis of non-linaer effects within the TETRA MS receiver we would like to get a better understanding of TETRA receiver design. Would vendors of TETRA devices that implemented the superhet receiver architecture share with us the information on the tuning range and the used intermediate frequency(ies)?

It would be great if we could have such information shared.

 

Best regards,

Daan Beaufort

".

Steffen Ring 09/10/17 09:40

Dear SE7 participants

 

So far, we (Motorola Solutions) are awaiting some feedback from other industry members which we hope to have for the meeting commencing 16th October.  However the three platforms initially investigated have no selectivity within the wanted receiver tuning range, which is typically 400-470MHz or 380-470MHz.  Only band limiting filters are employed at the band edges together with high IFs (e.g. the order of 100MHz).

 

Warm Regards

Steffen Ring

Dave Chater-Lea

Jeppe Jepsen

 

[member was deleted] 11/12/17 13:28

Dear Petteri, SE7-colleagues,

Indeed the Executive Summary should make mention of the discussion on the validity of the IMD Plugin. In addition, it's worthwhile to summarize the findings. Therefore we propose the following text:

The simulations show the effect of LTE transmitters which could cause interference to narrowband PMR receivers in adjacent frequency spectrum. It can be seen that the interference probabilities for low to medium BS and MS densities are generally 1% or even less, although a 25 dB duplex filtering at the BS may be required to keep the interference from the LTE BS into the PMR BS at these low levels. Even lower interference probabilities are expected if the bursty nature of the traffic is included in the models. Hence, compatibility of LTE with other PMR technologies in the 400 MHz is easily achieved.

The effects are greatest when the victim receiver is closest to the LTE transmitter, and when the victim receiver frequency is closest to the edge of the LTE transmitter bandwidth.

The studies regarding the intermodulation distortion currently show diverging results because of different assumptions regarding non-linear behaviour of the narrowband PMR receiver at higher frequency offsets as specified by standards, further assumptions based on the receiver design practice and outstanding validation of used simulations tools.

Best regards,

Daan Beaufort

Steffen Ring 11/12/17 22:30

Dear Daan, SE7 Colleagues

 

MSI would like agree on your proposed text, with these few changes:

 

The simulations show the effect of LTE transmitters which could cause interference to narrowband PMR receivers in adjacent frequency spectrum. It can be seen that the interference probabilities for low to medium BS and MS densities are generally can be 1% or even less, although a 25 dB duplex filtering at the BS may be required to keep the interference from the LTE BS into the PMR BS at these low levels. Simulations in higher density LTE BS environments show higher probability of interference of greater than 1%. Even lLower interference probabilities are expected if the bursty nature of the traffic is included in the models. Hence, compatibility of LTE with other PMR technologies in the 400 MHz is easily achieved.

The effects are greatest when the victim receiver is closest to the LTE transmitter, and when the victim receiver frequency is closest to the edge of the LTE transmitter bandwidth and can exceed 10% probability within 500m of an LTE BS.

The studies regarding the intermodulation distortion currently show diverging results because of different assumptions regarding non-linear behaviour of the narrowband PMR receiver at higher frequency offsets as specified by standards, further assumptions based on the receiver design practice and outstanding validation of used simulations tools. Simulations based on published standards and mass market receiver design show that although LTE BS emissions are the dominant effect with no duplexer attenuation on the BS transmitter, receiver intermodulation distortion becomes the dominant effect when the transmitter duplex filter attenuation exceeds 25dB.

 

Warm Regards

Steffen Ring

Steffen Ring 12/12/17 11:13

Dear Chairman and SE7 Colleagues,

Motorola Solutions has proposals below to:

ANNEX 2 (On Receiver Performance)

Comments:

We have been considering if this Annex 2 could be reused as a selectivity analysis as the idea of showing that improved selectivity in the receiver can improve IMD rejection is indeed valid.

However as soon as Annex 2 is moving into modelling, it starts making statements such as "IMD can be neglected if the input to the receiver is less than -40dBm", which is not correct, we had to conclude, that nothing else of this Annex really can be salvaged, other than:

A 2.1 Non-linear Model of the Receiver

 

In order to model the victim receiver’s behaviour in the presence of blockers, spurious and interferers, whether wideband or narrowband, certain assumptions about its nonlinearity, selectivity and dynamic range have to be made. An obvious way is to derive the receiver’s characteristics from the standards the victim receiver has to comply to. This typically contains: blocking test, intermodulation rejection test, spurious response rejection test and nominal error rate (NER) test.

However, the standards represent minimum performance requirements. Practical designs typically reflect much higher performance, which accounts for tolerances and for the fact that equipment vendors want to give their customers a good experience also in challenging signal scenarios.

Some examples:

  1. The standard reflects a test with a single blocker whereas in practical usage scenarios there might be multiple ones.
  2. The standard specifies two interferers, which are placed at certain frequency offsets so that the intermodulation product falls exactly together with the weak wanted signal. In practice however there might be a multitude of interferers and thus more mixing products with larger strength and at higher frequency offsets.
  3. The spurious response rejection test only allows for 5% exceptions. The receiver design approach must take this into account.

Modern PMR receiver design has to take these factors into account whilst allowing mass market low cost production techniques, and allowing receivers to have high switching bandwidths to minimise the number of models produced, and maximise the flexibility afforded to system implementers.

A 2.2 Trade-off between selectivity and linearity in receiver design

 

In order to increase robustness of receivers to challenging signal scenarios, one can either go for highest analogue selectivity by very narrow tuneable preselectors as the first stage in receiver or by ensuring that the IP3 of the overall receiver is adequate to meet the demands of type approval specifications and practical system implementation.

Very high selectivity through tuneable preselectors can solve nearly all unforeseeable blocking, spurious and intermodulation problems, however it would imply very high Q filters. These are practical in base stations, but not practical in mobile stations as they would prevent large switching bandwidth and would increase cost and size.

A receiver designer therefore balances out both strategies by implementing moderate selectivity in a tuneable analogue preselector by using resonators with moderate Q factors and by using active devices with moderate IIP3.

To meet the requirements of high switching bandwidth, front end filtering in receivers is limited to ensuring that image frequency rejection using a high intermediate frequency can be achieved. The designer then ensures that the IP3 performance of the entire receiver chain is adequate to meet the non linearity performance needed to meet intermodulation requirements, and other specifications including spurious response. The TETRA standard can be met with a receiver IP3 performance of ‑9.5dBm.

 

 

Steffen Ring 12/12/17 11:25

Dear Chairman and SE7 Colleagues,

Motorola Solutions has taken a final stab on 4.6 Conclusions, as followa:

 

4.6          Conclusions

Simulations of interference from LTE transmitters into narrowband PMR receivers in adjacent frequency spectrum show that the interference probabilities based on OOBE and Blocking for low to medium BS and MS densities are can be 1 % or less, although a 25 dB duplex filtering at the BS may be required to keep the interference from the LTE BS into the PMR BS and MS at these low levels.

The effects of LTE BS interference into MS may be significant in the proximity (< 500 m) of the LTE BS and are greatest when the victim MS receiver is closest to the LTE transmitter and simultaneously far from PMR BS, and when the frequency offset of the LTE transmitter is very small referred to the PMR channel. Interference probability can exceed 10% within this 500m radius.

Lower interference probabilities are expected if the bursty nature of the M2M traffic will be included in the calculation models.

Transmitter out of band emissions are the dominant effect and can be mitigated by use of additional filtering such as a duplex filter. However receiver intermodulation distortion in in PMR receivers caused by neighbouring broadband signals also contributes to the interferance effect, and becomes dominant when duplex filtering exceeds 25dB.

The current version of SEAMCAT has an intermodulation plug-in is using a conservative linear approach for IM distortion based on performance in published standards. Performance may be improved if the victim receiver has additional selectivity in its receiver amplifier stages. However this is unlikely to be the case in the frequency offsets of interest in mass market PMR MS design.