LTE 868MHz SRD Report v2

863MHz to 870MHz short range device (SRD) band exit options. Why? And Where to?

Martyn Gawthorpe, Director, Continental Compliance Ltd.

1 © Continental Compliance Ltd www.rfdesignuk.com 7th June 2011 v2

This document provides information on the impact of the forthcoming broadband data cellular radio networks (LTE, Long Term Evolution) on the low power radio systems operated at 863MHz to 870MHz in Europe. Focus is provided on sensitive narrow band Category 1 applications, for example social alarm systems. Where coexistence between LTE and SRD product is judged impossible, options are discussed. Signal levels are approximated in some cases to assist easy digestion of the issues. Consequently the analysis doesn’t extend to accuracy beyond several dB, but the conclusions presented remain none the less clear and valid with that level of uncertainty. Why might an ‘exit’ be required?

LTE EQUIPMENT INTERFERENCE TO THE 863 - 870MHz SRD BAND

Our web pages provide document downloads and links that explain in some detail the organisation of 800MHz LTE frequency assignments in Europe. Base Stations (BS) operate from 791MHz to 821MHz and User Equipment (UE) from 832MHz to 862MHz. 800MHz LTE in-channel (intentional) power presents an interference issue to those SRD receivers with only modest blocking and intermodulation rejection. More importantly and in common with other cellular radio system regulatory requirements, permissible incidental emissions from LTE BS and UE extend well beyond the confines of their ‘in channel’ frequencies. The permissible level of these sideband emissions as defined by the ‘transmission mask’ and spurious emissions in regulatory standards are highly significant in relation to a sensitive SRD receiver operating in the bands adjacent to LTE channel allocations. The threat to 868MHz band SRD equipment that proves a major issue in many cases, even for the very best and relatively high cost receivers, is the modulation sideband energy radiated from LTE UE that appears inside the SRD operating channel. First let’s take a quick look at the blocking and

intermodulation rejection required by an SRD receiver

operated in the vicinity of LTE equipment.

We will see later, these turn out to be very much

secondary issues.

LTE BS & UE BLOCKING THREAT TO A CATEGORY 1 RECEIVER

The top end of the LTE BS frequency allocation is 821MHz. BS effective radiated power (ERP) approaches +60dBm/10MHz. I’ll use the Friis free space path loss (FSPL) transmission relationship to estimate path loss between LTE energy sources and the victim SRD receiver antenna. In practice the path loss will be modified by the system environment and the SRD antenna configuration. Consideration will be given to this when presenting the conclusions later on. A category 1 receiver at 869MHz is required to demonstrate better than -20dBm (-84dBC on -104dBm sensitivity) blocking rejection at 2MHz and greater offsets. The minimum path loss between LTE BS and the SRD at blocking onset is therefore approximately +60dBm (in 10MHz BW) to -20dBm = 80dB. This equates to approximately 300m FSPL. It’s unlikely a social alarm will be required to operate within 300m of a high power LTE BS antenna. Many Category 1 receivers will provide greater than 84dBC blocking rejection at 821MHz and provide even more favourable (shorter) protection distances. The top end of the LTE UE frequency allocation is 862MHz, just over 7MHz from the social alarm channels. LTE UE maximum radiated power is approximately +20dBm. The permissible path loss is therefore 40dB. This equates to a 3m FSPL. A category 1 receiver that only just achieves the mandatory blocking rejection limit would be in trouble from LTE UE closer than 3m distance. This would be worthy of consideration except as we will see later, this threat is overwhelmingly dominated by other interference mechanisms. Conclusion to LTE signals blocking an 869MHz category 1 receiver: BS low risk. UE some risk. Co-located equipment in dwelling / adjacent dwellings. Protection distance required >3m.




LTE BS & UE INTERMODULATION THREAT TO A CATEGORY 1 RECEIVER

SRD intermodulation rejection performance is no

longer specified by mandatory limits. A category 1

receiver may typically have adjacent channel, 3rd

order

intermodulation (IMD) rejection of -60dBm. The

receiver topology will determine performance at the

greater offsets that LTE BS and UE occupy. Front end

filtering will in most instances reduce the LTE BS

threat to be similar or of a lower order compared to

blocking. The LTE UE threat may be significant where

‘wide’ SAW filters are deployed in the SRD RX.

LTE uses complex modulation schemes and the victim

SRD receiver will respond to the LTE transmissions in

a similarly complex fashion with regards to the receiver

non linearity that is characterised as 3rd

order IMD.

An analysis using CW interferers (a worst case view) at

LTE BS frequencies shows that front end filtering of

>40dB below 821MHz is required to achieve 300m

protection distance for SRD RX with -60dBm 3rd

order

IMD performance very close to channel. This level of

performance will be found in most Category 1

receivers.

An analysis using CW interferers at LTE UE

frequencies shows that front end filtering of >40dB

below 862MHz is required to match the 3m protection

distance for SRD RX with -60dBm 3rd

order IMD

performance very close to channel. This level of

performance will only be found in Category 1 receivers

using the latest generation narrow SAW filters, but 2

filters would likely be necessary.

In practice the LTE signals are not CW and typically an

SRD RX IMD response is likely to be better still. Even

considering the worst case, the threat is

overwhelmingly dominated by other interference

mechanisms discussed later.

Conclusion to LTE signals interfering with an 869MHz Category 1 receiver through IMD:

BS low risk. UE some risk. Protection distance required is estimated 3m to 10m for older generation SAW filters.

SRD RECEIVER SPURIOUS RESPONSES

Intermediate frequency images and local oscillator components give rise to spurious responses. Where these fall inside LTE BS and UE channel allocations, very high rejection is required. SRD receivers need to be reviewed individually to assess the threat. Conclusion: BS very high risk for some receiver configurations. UE very high risk for some receiver configurations.

IMPORTANT!! Excepting SRD receiver spurious responses, the mechanisms reviewed so far turn out to be secondary to the main interference threat, LTE out of channel emissions falling inside the SRD frequency allocations.

LTE BS & UE OUT OF CHANNEL EMISSIONS AND

SRD RECEIVER DE-SENSING

The complex modulation schemes used in LTE BS and UE facilitate high data throughput but produce significant out of channel power. The out of channel power is regulated by performance standards that need to be met for type approval purposes. LTE modulation spectra transmitted into adjacent bands is regulated by transmission mask limits. Energy radiated further away from LTE channel centre by 2.5 times the bandwidth, has spurious emissions limits imposed. The nature of the signals generated by LTE in the 868MHz SRD band are similar to white Gaussian noise. The regulatory performance standards use limits expressed as a mean power in a specified bandwidth. Unfortunately, the permissible limits are above the typical SRD receiver noise floor, when LTE is operated in the vicinity of the SRD. The amount of SRD range performance degradation will depend upon the SRD sensitivity required for satisfactory signal to noise and SRD system modulation characteristics. At 863MHz to 870MHz, incidental energy from LTE BS is regulated with spurious emission limits. In the SRD band the limit for spurious emissions from LTE BS is -36dBm in 100kHz. (ref EN 301 908-1 V5.1.1). Taking the example of a 15kHz bandwidth social alarm receiver, the regulatory limit scales to -44dBm in 15kHz. Assuming a typical receiver noise floor of -125dBm (noise figure 7.5dB), a protection distance of approximately 300m is indicated. It’s unlikely a social alarm will need to operate within 300m of a high power LTE BS antenna. Further, LTE BS will most likely meet the spurious emission limit with some margin at this offset.




LTE UE ADJACENT BAND EMISSIONS

LTE UE emissions attract a regulatory limit of -11.5 dBm in 1MHz bandwidth at 867MHz to 870MHz (ref EN 301 908-13 2011-05). -11.5dBm in 1MHz scales to -29.5dBm in 15kHz bandwidth, to compare with for example a sensitive social alarm receiver. The SRD protection distances for LTE UE immediately appear problematic. With LTE destined to become a mainstream World Wide Web conduit, little imagination is required to visualise the (high) user density of hand portable and fixed browsing hardware that will be in use in all types of environments, a few metres from 868MHz band SRD products. The protection distance estimate for minimal degradation of a -125dBm noise floor SRD receiver using Friis free space path loss (FSPL) is approximately 1,000m. System environment effects will reduce this significantly, but the numbers remaining ‘alarming’, pardon the pun. Considering a typical user environment we need to use a more sophisticated path loss model to estimate performance in buildings. Multipath propagation introduces a large peak to peak variation in the path loss at 800MHz between two arbitrarily positioned equipments inside building structures, for example between dwellings. Our simple protection distance model here will assume a FSPL plus an additional attenuation of 10dB for scattering and material absorption. Multipath effects are discounted so as to present a near worst case. At spacing’s between the LTE UE and the SRD RX of <20m in and around a domestic dwelling this relationship demonstrates good agreement in physical testing. At 10m spacing between LTE UE and the sensitive SRD RX: FSPL 51dB. Plus 10dB weighting = 61dB. Power in SRD RX is -90.5dBm, 34.5dB above the RX floor. 34.5dB noise floor degradation is estimated to reduce the SRD range to just 10% of the interference free performance. 90% of range lost. The interference power received at the SRD will increase with the number of LTE UE operated in the vicinity of the SRD receiver, exacerbating the estimate.

IMPORTANT!! SRD systems with lower sensitivity receivers will suffer less degradation from this mechanism and individual tests are recommended to assess real world performance in your application and user environment.

LTE INTERFERENCE THREAT TO THE 868MHz SRD SUMMARY

It’s impossible to predict the impact on a specific SRD system in a particular location without physical testing. However the underlying news is not good. Because LTE UE emits significant power inside the SRD channels, some noticeable user performance degradation will be experienced on almost all products. ‘Zero’ range products, for example Remote Key Entry (RKE) is the type of application that will suffer least since their path budget typically enjoys a large unused margin. Wireless audio product on one hand may have spare margin in the SRD path budget but are particularly sensitive to time varying modulation characteristics of the LTE signals, since near zero audible degradation is permissible to meet user performance expectations. Worst effected will be high sensitivity receivers typified by social alarms. These products are specified to the user for operation over tens of meters in dwelling. This usually requires the entire available path budget to be utilised. Systems with roof top antennas may suffer from LTE UE some numbers of km away! Recent SRD regulatory performance standards for Class 1 and then Category 1 applications have rightly ensured out of channel rejection is to a high specification. Those benefits have now effectively been ‘let go’ by the regulators by allowing future flooding of the SRD channels by LTE UE spectral splash.

WHERE?

LTE UE interference mitigation options for many 868MHz band SRD applications are limited. Where possible, lower receiver sensitivity and increased SRD transmitter power is a sure fire way to reduce degradation from LTE interference.

For low data bandwidth SRD applications, very narrow receiver bandwidth increases the wanted signal to noise ratio in the presence of LTE interference.

Some SRD modulation schemes can work marginally better than others in the presence of the LTE interferer.

Once an existing SRD system is judged unable to meet the required customer performance at 868MHz, where to go with a new and re-specified product?




2.4GHz ISM (Industrial, Scientific and Medical)

Around year 2000, the new 868MHz SRD band was a very welcome, massively well received opportunity for manufacturers to market to a number of European countries with a single frequency radio product variant. Prior to this, taking social alarms as an example, a plethora of VHF and UHF allocations were used across Europe. A harmonised licence exempt allocation was terrific news and has worked extremely well for the past 10 years. A move from the 868MHz SRD band naturally sees the search for alternative frequency allocations with pan-European availability. Some ISM allocations operate worldwide and one in particular enjoys the availability of a range of low cost TX/RX sub assembly offerings as a result, the 2.4GHz ISM band. The question ‘where to after 868MHz?’ has folks looking first towards 2.4GHz and the available technology. 2.4GHz modules and silicon are available in a variety of RF power outputs and DC consumption classes and these offer an attractive option for a small number of 868MHz SRD products. The pros and cons of 2.4GHz in particular applications are increasingly being reviewed by wireless engineers and I won’t attempt to detail the current state of the art in hardware and appropriateness here. What we can say is tread with extreme caution and plan time for very detailed field testing of your proposed solution in representative environments.

Below is a list of pointers that I think will help the non expert ‘ask the right questions’ and form an initial view on the possibilities.

The whole 2.4GHz ISM band is saturated in many business environments and dense urban areas. Users include telecommunications, heating equipment and sundry ISM applications. High noise floors seen at the RX antenna restrict high sensitivity, longer range (than several metres) applications that ‘must work’ within a short time frame, e.g. minutes.

2.4Ghz hardware typically consumes considerably more power than VHF or UHF equipments achieving comparable range.

The propagation characteristics at 2.4GHz are significantly less favourable to in-building applications compared to sub 1GHz channels. That’s why the LTE guys wanted 800MHz badly.

For a body worn application working in excess of 10m in building, for example a social alarm transmitter or nurse call system, the user body interaction plus the less favourable in-building propagation characteristics reduce reliable

range massively compared to sub 1GHz channels for the same available (conducted) RF power.

In general conclusion body worn product operating over 10m plus distances in-building is going to find 2.4GHz very challenging place to be. The DC power input for a given SRD operational range is greatly disadvantaged compared to 868MHz.

Fixed point equipment with efficient antennas working over several 10s of metres can work (in almost any user environment) providing smart channel use and extensive protocol redundancy are featured.

433MHz SRD band

Looking lower in frequency, the 433MHz SRD band is a candidate to re-visit, having been dropped by many in the rush to the sprawling width of the 868MHz channels around year 2000.

A list of pointers regarding the 433Mhz issues:

The 433.050 to 434.790MHz allocation is pan European and widely available for licence exempt use at powers of up to 10mW ERP with guaranteed <10% duty. Audio and video applications are allowed provided that a digital modulation method is used with a maximum bandwidth of 300kHz. Analogue and digital voice applications are allowed with a max bandwidth ≤ 25kHz. The 434.040 to 434.790MHz allocation is available with up to 25kHz channel spacing with 1mW ERP. Power density limited to -13 dBm/10 kHz for wideband modulation with a bandwidth greater than 250 kHz. 434.040 to 434.790MHz. No duty limit. Audio and video applications are excluded. Analogue or digital voice applications are allowed with a max. bandwidth ≤ 25 kHz and with spectrum access technique such as LBT or equivalent. The transmitter shall include a power output sensor controlling the transmitter to a maximum transmit period of 1 minute for each transmission.

Radio amateurs are active across Europe in this band on a secondary basis. 433.050MHz to 433.800MHz, and 434.600MHz to 435.000MHz should be avoided.

433.92MHz is a popular SRD centre channel for off the shelf modules and should be avoided.




433MHz in-building propagation is more favourable than 868MHz, so we are moving in the right direction there. Antenna efficiencies in small packages for personal applications are reduced, so more RF is required for the same ERP in a given enclosure size.

Because bandwidth isn’t available elsewhere, 433MHz may be the optimum choice for some video and audio SRD applications to return from 868MHz.

The 433MHz option is not particularly encouraging for the 868MHz migrant. This allocation is a viable alternative for some current 868MHz applications, but does not offer comparable quality of service for the user. Co-channel interference from other SRD users and radio amateurs is highly probable in most urban locations for significant durations of time.

169MHz SRD band The 169MHz SRD band is a relative newcomer (2005) to Europe band SRD frequency allocations. Pointers: Not for general SRD applications.

169.400 to 169.475 MHz is for tracking, tracing

and data acquisition with ERP up to 500mW. Meter reading with ≤50kHz channels with <10% duty. Asset tracking and tracing with ≤50kHz channels at <0.1% duty.

169.4875 to 169.5875 MHz is for Aids for the hearing impaired Personal Systems at 10mW and Public Systems at 500mW (Individual site licence may be required). Max 50kHz channels.

169.48125MHz and 169.59375MHz for Social

Alarms exclusive use. 12.5kHz channels. 10mW ERP and <0.1% duty.

In building propagation is much superior to 868MHz for SRD applications. Balanced against this, antenna efficiency for a given product size is much reduced. Consequently TX ERP is significantly lower, but on balance in dwelling performance can be superior and result in reduced receiver numbers in distributed systems.

Incidental host noise, from microcontrollers for example, offers a significant design challenge. Many products using 868MHz without

problems will find interference issues with a sensitive 169MHz receiver and integral antenna. This may significantly complicate a move from 869MHz to 169MHz for manufacturers. General EMC standards used for CE attachment, for example EN 55022 Class B, have limits many tens of dB above the noise floor of a co-located 169MHz receiver.

169MHz status REC 70-03 implementation as of 24/3/11: Widespread acceptance, excepting the following. Austria, not implemented, planned. Bulgaria, not implemented, national security needs Denmark, not implemented, PMR band Georgia, not implemented Greece, not implemented Norway, limited implementation Russian Federation, not implemented Ukraine, not implemented

In conclusion the Europe 169MHz allocation offers social alarm manufacturers an ideal opportunity with exclusive channels and low duty cycle limits imposed on the neighbours.

There are higher power users in directly adjacent channels, but not withstanding the additional design considerations, hardware prime cost should rival 869MHz product. 138MHz SRD Band Hot off the press just now is news of the increasing adoption of 138.200MHz to 138.450MHz with 10mW ERP and importantly a 1% duty limit imposed by product design features. Applications are for ‘Non-Specific SRD’. Implementation is likely to be patchy for some time. The UK introduction is planned for 2012. REC 70-03 not implemented as of 24/3/11: Belgium, not implemented. Croatia, not implemented. France Not implemented. Military use. The use of this band by SRDs is not planned Georgia, not implemented Germany, not implemented. Defence systems Hungary, not implemented. Aeronautical mobile applications operate in the band Italy, not implemented. Military application Latvia, not implemented. Exclusive defence systems Liechtenstein, not implemented Poland, not implemented. Military application Russian Federation, Not implemented Slovak Republic, Not implemented Defence systems Slovenia, not implemented not available




Spain, not implemented. Military application Sweden, not implemented Switzerland, not implemented Exclusive defence systems Netherlands, not implemented. Exclusive defence systems Turkey, not implemented. Defence systems Ukraine, not implemented. United Kingdom, Not implemented.

Compared with 868MHz, the propagation comments previously made in the 169MHz section apply to 138MHz too.

REPORT CONCLUSIONS

Whilst characterising LTE interference to terrestrial and cable TV receivers, it appears the regulators have perhaps underestimated the implications for the multiple million installed base of SRD users in Europe? Manufacturers of safety critical social, fire and security alarms operating in the 868MHz SRD band have quite a challenge on their hands, particularly in protecting their installed base of customers. Less critical SRD applications will also suffer intolerable performance degradation and as things stand the commercial impact for manufacturers and the supply chain is in danger of being experienced as a series of unwelcome surprises over time. The size of the global telecoms industry R&D investment in 800MHz LTE deployment plus the commercial and public returns forecast, put the interference issues to the 868MHz SRD band into context. That is, it seems unlikely the current pace of LTE introduction in Europe will be significantly impacted. Specifying and understanding wireless product range performance that’s repeatable is often a vexed and contentious issue, even in minimal interference environments. Some folks are impressed, others disappointed, but if it meets the ‘specification’, it’s OK. The specification and validation may of course be incomplete? Understanding ‘invisible’ interference mechanisms, from wherever they may originate, makes the task difficult. That’s one of the roles of the wireless engineer. The forthcoming LTE interference environment reminds me of these scenarios and I fear that a smoke and mirrors approach may be the default position to explain away poor SRD performance, the parameters of which are not in the technical grasp of product Marketing or Sales functions. In the worst case, you may end up with your end user understanding an awful

lot more about the performance limitations of your 868MHz SRD product in the real world LTE environment than you do. The commercial implications for this are obviously very undesirable. We would very much appreciate your feedback with observations, corrections or opinions on the contents of this report. Please email from our web site. All communications received will be held private and in the strictest confidence. Thank you and kind regards, Martyn.

Documents

LTE 868MHz SRD Report v2