Report on the use of wireless microphones in
a LTE-800 / 4G environment.
Measurements are taken the 27.12.11 between 9AM og 11AM at Fetveien 1, 2007 Kjeller, Norway
Measurements are taken inside the main hall, just inside the door. A test was performed to
se how far into the building we could be, without loosing the signal from the LTE base
station on ”Kjeller”. This to force the LTE modem to perform at maximum power.
The base station we have communicatied with is placed at the entry to the airport at Kjeller. That gives a distance of about 2 kilometres between Fetveien 1
and the base station.
Tests were performed with a Huawei E392 Multimode USB stick - LTE 800 / 2600MHZ USB modem for PC. We assume that the performance of E392 will be typical for LTE modems. The tests were performed on Telenors testnet for LTE-800 in the Lillestrøm area outside Oslo, but Telenor was not present or part in the testing.
Measurements were performed with a Rohde & Schwartz CMS50 frequency analyzer ( 1-1000MHZ), and a Sennheiser receiver – Evolution G3-500 series in E-bånd ( 823-865MHZ)
All tests are performed with recievers with their original filters for their full bandwith. No special filters are applied.
Se pictures on the next page.
NETCOM’s downlink test transmission ( 801 – 811 MHZ )
TELENOR’s downlink test transmission ( 811 – 822 MHZ )
Measurement taken 9.12.11
Purpose:
The purpose of the test, is to establish whether the frequency spectrum called ”Centergap”, that is the area between the ”downlink” og ”uplink” in LTE/4G protocol will be usable for use with wireless microphones. ( 822-832 MHZ ).
”Downlink”:
Bjørn Johnny Valsgård has through several weeks of practical use on the actual location
experienced that the ”downlink” at the present signal level does not create any problems for the wireless microphones in the area. One can put a wireless channel close into a downlink area without experiencing any problems with noise. This seems somewhat logical, since the signal level of the downlink is relativly low and the signal of the wireless microphone locally is much higher. We do however assume, that had the base station been within 100-300 meters, we would experience the same problems that you will se later in the report with the uplink.
Antenna Sennheiser A-2003 Reciever Sennheiser Evolution G3-500 series Antenna distributor Sennheiser ASA-1
Measurement before the LTE-800 modem is turned on:
”Uplink”:
Since the frequency auction is not yet over, this means that the transmissions we now can measure are of the test systems of Telenor. This test system has its uplink at 852-862, which then starts exactly 20MHZ above what the ”centergap” will be, when all the uplink slots are sold and the actual LTE/4G transmissions has begun. For all the measurements you see below you can mentally subract 20MHZ to all frequency axis. Even though we measure at 852-862, we assume that the systems will perform identical at 832-842. When the auction is over, I believe there will be 3 ”slots” alloted, each of 10MHZ, for upto 3 winners of the auction to start their transmissions on. So on the X-axis of these measurements our ”makebelive” centergap is from 842-852MHZ.
As you can see – the frequency band is free of noise.
Measurement no 1 – LTE modem turned on:
No wireless microphones are yet turned on
We se the 852 – 862MHZ Uplink for for the LTE modem is
active. Alt annet er støy/speil
som har oppstått pga at modemet er
påskrudd. All these spikes are IM frequencies that are created by the LTE modem itself.
To force the LTE modem to start transmitting constantly, I created a 1,5GB ZIP file, that I started to transfer to my DropBox account. This forced the modem to perform at maximum speed and signal strength. ( Remember that we are positioned just at the edge of the coverage - inside a building. )
Notice also these lower IMF’s that cover an area about equal to the width of the LTE transmission.
No wireless microphones are yet turned on.
No wireless microphones are yet turned on
No wireless microphones are yet turned on
We see that powerful IMF’s are created, ”mirrors” at
845.575 due to the 853.150 and 860.725
spikes.
This also coincides with the formula for IMF’s: 2A-B.
A B
As we can see, the caracteristica of the LTE-800 modem changes, second by
second.
Preliminary conclution:
IM frequencies occur as a result of the LTE-800 modems own komponents.
Measurement no 3 – LTE modem turned on:
Measurement no 2 – LTE modem turned on:
Measurement no 5 – LTE modem turned on + 1x MIC turned on at 843.700MHZ
Mic at 843.700 MHZ IM frekvens / speil
som har oppstått på 834.550 pga Mic’en og 852.950
IM frequency / mirrors that have arisen at 834.550 due to the mic transmitter and
852.950
Mic at 843.700 MHZ IM frekvens / speil
som har oppstått på 826.850 pga Mic’en og 860.875 IMF’s / mirrors that
have arisen at 826.850 due to the
microphone transmitter and
860.875
Be aware that all measurements with the microphone turned on are done with both the LTE-800 modem and the mic transmitter lying on a table at a distance of 3-4 meters from the reciever antenna. There is therefore no ”body loss” or other factors that diminish the signal of either transmitter. The only thing that can affect signal strenght is the gain of the A-2003 antenna, and the ASA-1 distribution amplifier. This is done to create the worst possible scenario in regards to what a user of wireless systems is likely to encounter when under a LTE-800 transmitters range.
Preliminary conclution:
IM frequencies occur as a result of the LTE-800 modem and a wireless channel.
Measurement no 6 – LTE modem turned on + 1x MIC turned on at 843.700MHZ
Preliminary conclution:
IM frequencies appear at new locations – it would seem randomly, but dependant of how the LTE-800 modem behaves. We can see the IMF here also in measurement no 5, but considerably lower,
Here we see both the IMF’s are back.
There are a couple of minutes between each of these measurements.
Measurement no 8 and 9 – LTE modem turned on +
2x MIC turned on at 843.700 and 836.000MHZ
We have 2 pcs of wireless mic turned on. We se that the result is similar to what we see in measurement no 5 and 6.
IMF between ”uplink” the 2 wireless
channels.
Uplink
RF level RECORDING –
placed on top of an IMF / Mirror frequency
Here we measure the RF strenght of an IMF / Mirror over a periode of 1 minute. We see that the modem fluctuates in signal strenght with a speed of about 1 HZ. This means that IMF’s will come and go at the same frequency. We can count about 55-60 ”peaks” in the minute the recording lasts.
Measurement of IMF’s with Rohde og Schwartz instrument.
The reason this whole test was not done on this unit, is simply that its to slow, and the ”SPAN” or measurable bandwith of the device is to low ( 10MHZ ). But what we see in this little test, confirms the same as we see on the other measurements.
Thoughts and conclutions:
1) IMFs / Mirrors are generated between components of the LTE-800 signal itself – This is shown in measurement no 1-4.
2) IMF’s are generated between the LTE-800 uplink signal and a wireless microphone channel. The strenght and width of these vary dependant of how the LTE modem behaves at the particular time of measurement. This is shown in measurement no 5-7. 3) This test is done with 1 pc of LTE-800 modem active. We can only assume that using
wireless microphones infront of an audience with X pcs of ”IPHONE 5"s in their pockets, and even the carrier of the wireless microphone has such a device in his pocket – the situation will get WORSE and even more unforseable than what we have seen today. 4) A frequency plan to use wireless mic’s in the centergap will be useless!
One will never be able to predict how the LTE signals frequency components will generate IMF’s that will destroy your frequency plan.
5) During the testing we also tuned a reciever to a IM frequency, and listened to this on the sound system. The squelch of the reciever not only opened, but the AF out of the unit was as high as I have ever seen before. The mixer console peak lights went off like a christmas decoration. The AF level was 20-30 dB higher than with a normal use. We then turned on a wireless microphone transmitter at the same frequency. The signal of the WL ”took over” and we had what seemed like a ”normal” situation, that is untill we covered the antenna of the transmitter with our hand, or went into a shadow area in relation to the reciever antenna. In this situation we got considerable ”wheesing” and noise. The ”link budget” - the difference between the noise floor and the signal maximum – got to small. Use under these circumbstances will NOT be reccomended. The risk of blowing your speakers is considerable when you i.e. turn your transmitter OFF.
6) These test were done with equipment that has a 42MHZ bandwith. This is considerably larger than the centergap at 10MHZ. All the systems I know of have filterbanks widths of between 25 and up too 75MHZ. This means that any system that you tune to a
frequency in the centergap will either also pick up parts of the uplink, the downlink or both.
These tests does now answer if a specially built system that has adapted a filterbank for 822-832 MHZ ( the centergap ) will be able to operate without noise. If these filters are good enough they might be able to shield the reciever from being overloaded by IMF’s.
7) The designers of the LTE / 4G system has clearly been aware of the amount of IMF’s that will be generated. This must have been the reason that a ”guard band” of 10MHZ was introduced in the first place. When considering that the ”uplink” area will be a coherent band of 30MHZ from 832 to 862 ( Probably divided into 3 bands of 10MHZ ) one could wonder if the ”guard band” should not have been wider ? This has
probably to do with the fact that the transmission is digital, and their requirement for ”headroom” is considerably lower than for an analogue transmission.
The tests were performed by:
Bjørn Johnny Valsgård bjornv@scandecsystemer.no +47 952 36 711 Bjørnar Riise bjornar@scandecsystemer.no +47 916 23 925 on the 27.12.11.
Consequences for the ”IEM” band – 863-865MHZ.
If one studies the measurements 1-9, you wil se clearly that also the ”IEM” band is also totally ”jammed out” by IMF’s. When the actual LTE transmissions are on, the area 832-863MHZ will be divided by 3 suppliers of LTE. Each of them probably getting about 10MHZ of spectrum. These 30MHZ will be filled up with uplinks from thousands of LTEsubscribers. When only 1 pc of such a modem can generate such problems as we see here, we can conclude that the area just above the 862 limit, will NOT be usable for wireless mic’s or IEM transmissions ( In Ear Monitoring ).
Report on the use of wireless microphones in
a LTE-800 / 4G environment. – APPENDIX A
Measurements are taken the 7.2.12 between 8AM and 9AM at Fetveien 1, 2007 Kjeller, Norway
Measurements are taken inside the main hall, just inside the door. A test was performed to
se how far into the building we could be, without loosing the signal from the LTE base
station on ”Kjeller”. This to force the LTE modem to perform at maximum power.
The base station we have communicatied with is placed at the entry to the airport at Kjeller. That gives a distance of about 2 kilometres between Fetveien 1
and the base station.
Tests were performed with a Huawei E392 Multimode USB stick - LTE 800 / 2600MHZ USB modem for PC. We assume that the performance of E392 will be typical for LTE modems. The tests were performed on Telenors testnet for LTE-800 in the Lillestrøm area outside Oslo, but Telenor was not present or part in the testing.
Measurements were performed with an AARONIA SPECTRAN HF-6080 V4 X (10MHz - 8GHz) Spectum analyzer, with an OmniLOG 90200 antenna. The disctance from the LTE-800 modem to the antenna of the Spectran was about 1 meter.
Page 2 MCS Real-Time Spectrum Analyzer Software was used on the PC for analisys.
The Uplink of the LTE-800 modem is as before done on 852-862MHZ.
Understand also that the LTE modem is operating in a time slot fashion. This means that since it is supposed to share the frequencies with potensially hundreds of other users in the area, it will turn it self on and off constantly depending on the time slots it is allocated from the base station it
communicates with. This means that when we measure, we get only a few measurements pr 50 sweeps that actually shows the modem working in the full range of the band its supposed to without turning off its transmission during the time the spectrum analyzer uses to sweep the wanted range of frequencies. On page 5 this is shown with a couple of examples.
One important thing to understand, before one reads the rest of the report.
In the original report, the measurements were done with a wireless microphone reciever. We could therefore not see what where IMF’s generated in the recievers demodulation, or wether they were actually ”on the air”. This spectrum analyzer is adjusted with such a narrow filter, that what we see, is what is actually ”ON AIR”, and not generated in the recievers demodulator.The tests were performed by:
Bjørn Johnny Valsgård bjornv@scandecsystemer.no +47 952 36 711 on the 7.2.12.
Measurement no 1 – LTE modem turned on:
No wireless mic’s turned on
862 MHZ 865 MHZ 852 MHZ 862 MHZ 865 MHZ 852 MHZ
As we can see on both of these sweeps the modem has IMFs actually ”on the air” way outside its specified uplink area. This is the case both below and above the 852-862 range the modem is supposed to work in.
Page 4
Measurement no 2 – LTE modem turned on:
2 pcs of wireless mic’s turned on
862 MHZ 865 MHZ 863,1 MHZ 864,1 MHZ
The upper image shows one ”sweep”, the lower includes an red line where the max level at each frequency is showed over the length of the recording. As we can see the modem uses only parts of the range at a certain moment in time, but over time it uses the whole range.
The microphones work fine as long as they are ”on”, when listened to in the speaker system, but the moment we turn them off, the noise from the LTE modem triggers the squelch in the recievers and blasts through with 20-30dB + level in comparison to the audio level from the microphone when in use. Again we point out the dangers both in relation to peoples hearing, and the very real possibility of damaging your speaker system.
862 MHZ 865 MHZ 863,1 MHZ 864,1 MHZ
Other sweeps from measurement no 2 –
LTE modem turned on:
2 pcs of wireless mic’s turned on
These images show other sweeps where the modem seems to be using only part of the frequency band, but I believe this is only a mismatch between the time it takes to do a sweep, and the time slot allocated to the modem from the base station. The images shown on the page 3 and 4 are better ”timed”. All of the images below come from the same ”recording”
Measurement no 2 – LTE modem turned on:
Max / Min / Average of 40 sweeps
2 pcs of wireless mic’s turned on
