A METHOD OF CERTIFICATION FORLTE SMALL CELLS IN THE HFC NETWORK

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One of the problems associated with installations of LTE Small cells co-located on

the HFC network is interference of LTE Tx signal with QAM signals carried on the

network (LTE Ingress). This paper quantifies the level of leaks that will likely need to

be mitigated to avoid interference, it describes the sensitivity required of a detector

used in this application, and it proposes possible certification procedures that could be

performed at locations where small cells will be installed.

One method for estimation of interference from the LTE Tx signal is an expansion

upon data acquired from drive outs performed at or very close to the LTE frequency.

The test methodology would include the standard steps of measuring leakage by

patrolling the HFC plant, and calculation of leak location. It would then need to be

expanded to include measuring or calculation of the field strength of the LTE Tx signal

at each leak location point, and then a calculation of CNR in the cable depending upon

the level of QAM signal. This method has some disadvantages.

1. Leak source radiation patterns are assumed to be omni-directional, while

in reality they fluctuate significantly and each leak can have nulls or

maximums in the direction of the LTE Small cell.

2. Imprecision of the leak location will cause further inaccuracy of the

re-calculation of leak level with respect to 10 feet and further inaccuracy

of estimating CNR in the cable.

3. Re-calculation of LTE Tx signal level to the point of leak location in most

cases is not very accurate because actual obstructed path losses of signal

is random due to multipath effects, etc.

4. Invisibility to very small leaks due to the sensitivity of existing equipment.

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While in general the drive out data is very useful with regard to general plant

maintenance and monitoring for LTE egress and ingress, these imprecision points

noted above for the LTE small cell scenario show that it is not reliable enough to

be used exclusively in this application. A better method will be described below

that discusses a certification procedure at the LTE Small cell location, with ongoing

monitoring post-certification provided from a combination of drive-out data

highlighting the cumulative level of all leaks at the Small cell location and from SNR at

the LTE band of modems downstream of the Small cell.

The alternative method of estimation of interference is based on measuring QAM

leak levels from the point of the radiation interfering signal, i.e. from the location of

the LTE Small cell. In this case both the radiation pattern of the leak source and the

actual obstructed path losses of the leak signal (including multipath effects) will be

taken into consideration. The allowable leakage level at the 750 MHz LTE band can be

defined from formula (10) in the Arcom Digital Quantifying leakage thresholds for QAM

LTE interference technical paper as follows:

ELeak = 2 Uqam - PTx– GTx- CNR + 20 log D + OPL + 27.67, (1)

where

Eleak - is allowable leak level at distance 3 meters in dBμV/m;

D – is the distance from LTE transmitter to leak source in meters; PTx– is power of LTE transmitter in dBm;

GTx– LTE transmitter antenna gain in dBi;

OPL – is obstructed path loss including leak source radiation pattern loss in dB;

CNR - is threshold of allowable CNR for QAM signal at cable in dB

(33 dB for QAM-256).

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The same leak signal at the location of the LTE Small cell will have level:

EleakLTE = ELeak - 20 log D - OPL + 20 log (3m) = 2 Uqam - PTx– GTx- CNR + 37.21 (2)

Note, that the allowable leak level at the location of the LTE Small cell does not depend

upon the distance to the leak (assumption being that the distance is more than 3

meters). In other words, in the case of measuring the leak level from the Small cell

location, the allowable leak threshold for a leak located, for example, at a distance of

3 meters and for leaks located at distance 30 or 300 meters is the same and defined

by formula (2). Also the allowable leak level measured at the Small cell location does

not depends upon OPL (leak radiation pattern and other obstructed path loses). This

means that all of the above random parameters are already taken into consideration

and that there is no need to measure each leak locally from a distance of 3 meters, or

to calculate LTE level and QAM signal levels at each location – a significantly more

practical approach than the initially described scenario.

That is the good news, the problem becomes that the main challenge of measuring

allowable leak level at the Small cell location is the required very high sensitivity of

the leakage detector. The table below shows the allowable leak level for Small cells

(Ptx = 43 dBm, Gtx = 4 dbi) depending from QAM signal level in cable with a threshold

CNR=33 dB.

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The QAM signal level used should not be the QAM Signal at the point of the Small cell,

but at the lowest level of plant in proximity to the Small cell where there is a potential

leak source. As such, in this expected worst case where the signal level at cable is

10 dBmV, the sensitivity of QAM leakage detector should be better than 0.1 dBμV/m.

Current leakage detectors used in the cable industry can’t provide this sensitivity at the

LTE band even with using directional active yagi antenna. For example, the sensitivity

of Rohde& Schwartz leakage detector EFL110/220 with active yagi antenna at LTE band

is only 11 μV/m, 40 dB worse than the required sensitivity of 0.1 dBμV/m.

To provide the required sensitivity we propose to use a cross-correlation QAM leakage

detector (the same as the QAM Snare product), but with substantially longer signal

accumulation time and with the reference QAM samples captured directly from the

cable network where the Small cell will be installed. The current QAM Snare leakage

detector implemented for drive-out was engineered for the desired sensitivity of

the application and has sensitivity of approximately 3 μV/m at LTE band with an

accumulation time of 1.2 msec and for a dipole loop antenna with gain of 4 dBi.

Increasing the accumulation time from 1.2 msec to 100 msec will increase sensitivity

by at least 25 dB plus an additional 6…8 dB is expected from using a directional active

yagi antenna. The total expected improvement in sensitivity is more than 30 dB or 30

times in μV/m.

The block diagram of the proposed certification system is shown below.

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The system works as follows. Technician sets up the required QAM channels for

measuring leak (because of the different frequency response of leaks it is expected that

multiple channels will need to be tested, likely four different QAMs at the LTE band),

with antenna polarization (horizontal and vertical). Because all directions will need to

be scanned for multiple channels and to avoid human error by scanning too quickly or

in the wrong direction, an automated setup with an antenna rotator will be utilized.

The system starts to scan different directions at azimuth plane and detects leak at

selected QAM channels automatically. The results of scanning are stored in detector

memory in the format: Polarization – Azimuth - QAM channel - Leak level -Time

delay. Then recorded data can be downloaded into computer for analysis and reporting.

An example of the leak detector screen showing a detected leak is shown below.

An alternative technique which could also be used for certification of Small cells is

generation of a CW pilot signal at frequencies between adjacent QAMs at the Small

cell location and then monitoring forward spectrum captured by cable modems (CM)

within the nearest nodes. This method is shown at the picture below. Of course,

generation of CW signal with a power of 20 W (same power as at Small cell) will likely

require FCC approval because un-modulated CW signal can produce interference to

other LTE cells.

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CW TX signal can be placed, for example, at a frequency of 726 MHz between QAM

channels 112 and 113. If this CW Tx signal enters the cable due to bad isolation, then

it will be detected at FWD spectrum captured by CMs. Lab test shows (see below) that

for RezBW = 30 kHz (which is provided in case of capture spectrum at CM) the CW

pilot with relative level -33 dBc (CNR threshold for QAM-256) can be detected between

adjacent QAMs. But for effective detection it will be required to accumulate a number

of captured spectrums from each of CM. This described process and approach would

present a challenge to go in the field to affect repair, because the direction of the leak

would not be known.

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As described, the preferred process would be to certify a location prior to the Small cell

activation using the coherent cross correlation technique with large accumulation time.

Direction of the leak will be indicated, and the leaks can be mitigated using the other

QAM Snare leakage detector tools intended for line technicians, operating at the same

frequencies used for this certification. For ongoing maintenance, existing tools within

the management software provide very useful data with regard to cumulative leak

levels at each Small cell location, and alarms can be generated when the cumulative

number exceeds some threshold level. While there will be inaccuracies in this number

that have been previously discussed, it does provide some useful metrics. An example

is shown below where the leak level existing at any location can be calculated and

displayed.

And lastly, as an additional monitoring tool, a realistic method for detection of LTE

ingress is to place DOCSIS QAM channels or OFDM channels into the bandwidth of the

LTE down-link spectrum and analyze degradation of SNR measured by CM. Using

broadband spectrum capture to detect LTE ingress in all scenarios is not possible

because the QAM signal level at CM input is too low and the corresponding LTE ingress

of -30 dBc is below the noise floor.

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