# Presentation: 102926

## Measurement and Processing to Determine Land-mobile Radio (LMR) Channel Occupancy

- Randy Hoffman
- ISART 2005

## Topics of Discussion

- Who are we?
- Purpose of measurements
- Challenges
- Techniques for measurement
- Post processing methods

## Radio Spectrum Measurement Program - Mission 

- To provide the Executive Branch, critically needed:
- Radio spectrum data
- Data analysis
- Reports and summaries

**Notes:**
Briefly discuss ITS’s mission. Emphasize our reporting to the Executive Branch as apposed to the FCC which reports to the Congress.
We collect data, perform technical analysis, and provide reports summarizing the results.

## Slide 4

**Notes:**
Briefly show where NTIA sits in the Department of Commerce.  NTIA headed by Assistant Secretary Mike Gallagher who reports directly to Commerce Secretary Carlos M. Gutierrez .

## National Telecommunications and Information Administration

**Notes:**
Briefly identify ITS and OSM.  Emphasize that we are sister organizations what work closely together. OSM more policy oriented. ITS more purely technical.  

## Responsibilities

- Under Departmental Organization Order 25-7, issued 5 October 1992, and amended 3 December 1993, the NTIA Office of Spectrum Management (OSM) is responsible for identifying and making arrangements for measurements necessary to provide NTIA and the various departments and agencies with information to ensure effective and efficient use of the spectrum.
- The Radio Spectrum Measurement System (RSMS) resides at ITS and is tasked to perform spectrum measurements as required to fulfill this mission.

**Notes:**
OSM identifies the need and makes arrangements for measurements. The Radio Spectrum Measurement System (RSMS) resides at ITS for the purpose of making the measurements necessary to answer questions.

## Why LMR Measurements?

- Update usage trend information comparing data collected in mid 70’s – for future LMR spectrum management.
- To determine current usage:
- to help with decisions regarding how to make federal LMR radio operations more efficient.
- Provide information for realistic design of possible shared trunked system.

**Notes:**
 Making comparison with mid 70’s to show how usage levels are changing and provide one basis for predicting future levels. Usage trends, up to this point are completely unknown.
2) Want to get a better idea of current usage in order to determine how the LMR bands can be used more efficiently and to provide information for realistic design of possible shared trunked systems.

## Challenges

- Wide dynamic range
- Need for high sensitivity (low noise figure)
- Strong local signals
- 100 dB Dynamic Range

**Notes:**
1) Need to be able to see weak signal without overloading the system due to strong local signals.

## Challenges

**Notes:**
This plot shows the maximum, mean, and median values for 400 LMR channels in the range 144-149 MHz 
 One can readily see that that range of power levels for the signals is several 10’s of dB.
  Therefore, to be able to low signals and, at the same time accommodate local strong signals, the system must have a wide dynamic range capability.

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Need to resolve individual channels with a minimum of 12.5 kHz spacing
- Requires:
- narrow resolution bandwidth
- sharp roll-off
- large stopband attenuation.

**Notes:**
1) How do we achieve rigorous requirements for resolution bandwidth and, at the same time, acquire data rapidly?

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Differentiation from system noise
- Need to know what is system noise and what is signal

**Notes:**
 System noise, itself has a range of values that at any one moment can appear to be an LMR signal.
 How do we differentiate system noise from LMR signal?

## Challenges

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Differentiation from system noise
- Impulsive noise
- Need to reduce the probability that impulsive noise is mistaken for a signal

**Notes:**
 Due to electrical machinery and automotive ignitions
 Wideband, affecting the entire band at the same time.
 Can give a false indication of LMR channel usage 

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Differentiation from system noise
- Impulsive noise
- Strong out-of-band signals
- System overload
- Complicate in-band thresholds

**Notes:**
 Can overload the system if not properly reduced through filtering.
 Channel 7 produces some unwanted signal power in the 171- to 174-MHz region 
 This complicates how we determine what is LMR signal and what is due to the TV station

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Differentiation from system noise
- Impulsive noise
- Strong out-of-band signals
- Strong in-band signals
- System overload

**Notes:**
 Strong signals can complicate the measurements process for several reasons
 They can overload the system and force amplifiers into non-linear regions of operation.

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Differentiation from system noise
- Impulsive noise
- Strong out-of-band signals
- Strong in-band signals
- Intermodulation products

- IM = +/- mF1 +/-nF2

**Notes:**
 Strong signals can produce intermodulation products. 
 These are due to non-linearities of the system, when there are two or more strong signals
 They occur at frequencies described by the the equation, where the products of greatest concern are when m+n is and odd number. 
 While they don’t occur frequently, they can give a false indication of an LMR signal 

## Challenges

- Wide dynamic range
- Narrow channel spacing
- Differentiation from system noise
- Impulsive noise
- Strong out-of-band signals
- Strong in-band signals
- Sideband noise

**Notes:**
 Strong in-band signals can result in sideband noise and give the false indication of LMR channel usage. 
 Generated either by the sideband skirts of the flattop window, local oscillator noise, or ADC jitter.

## Measurement System

**Notes:**
 Three separate systems with omni-directional, vertically polarized discone antennas, 35 ft high
  138-174 MHz, 225-400 MHz,  406-420 MHz
 Preselector used to attenuate strong out of band signals
 Notch filtering for strong local signals that are present much of the time.
 Attenuation, if necessary, for strong signals.
 PreAmp to reduce the noise figure
 Use Agilent E4440A in base mode
 Digitized at 30 Msamples/s in 5.5 MHz bandwidth chunks

## Measurement Procedure

- Careful site selection
- Signals detected by measuring RF energy at the channel frequency – inferring LMR transmission if signal greater than system noise.
- Digitized at 30 Msamples/s in 5.5 MHz bandwidth chunks
- Do fft with flattop window, decimated to 12.5 kHz  or 15 kHz spacing with samples centered on the channels, and downloaded via internet connection.
- Acquisition every 200 ms (up to 480 channels)
- Capability for message length statistics.
- Measure for 4 minutes / 5.5 MHz span and switch to next span.  Sufficient data to characterize the channel every hour.  Measure 24 hours/day for a week.
- Throw away data during overload conditions

## Median Processing

**Notes:**
 We take the median of each 5 acquisitions 
Reduces the variance of the system noise and can set a lower threshold for signal detection 
 Reduces fluctuations in amplitude due to modulation
 Reduces the impact of impulsive noise
 3 out of the 5 acquisitions must be elevated to affect the median

## Resolution Bandwidth

**Notes:**
 Flattop window
 80 dB adjacent channel rejection
 3 dB BW = 5.5 kHz; 80 dB BW = 15 kHz 
 Narrow enough to prevent adjacent channel leakage, even with 6 kHz maximum freq deviation
 Some amplitude fluctuation with 6 kHz maximum freq deviation, but median processing reduces this phenomenon. 
 Show Measurement here

## Post Processing – Intermodulation

**Notes:**
 Because all channels are acquired simultaneously, it is possible to predict intermodulation products
 Characterize the intermodulataion products for our system

## Post Processing – Intermodulation

**Notes:**
1) This show the relationship of 

## Post Processing – Intermodulation

## Post Processing – Intermodulation

## Post Processing – Sideband Noise

## Post Processing – Min Threshold

**Notes:**
 Signal considered to be present if the power in a channels exceed  a threshold
 Threshold needs to be as low as possible to give maximum detection of signals
 Threshold needs to be high enough to prevent false signals due to system noise

## Post Processing – Min Threshold

## Post Processing – Impulsive Noise

**Notes:**
1)  Can do this because all of the channels are acquired simultaneously

## Summary

- Challenges and Solutions
- Wide Dynamic Range
- Narrow Channel Spacing
- Differentiation from System Noise
- Impulsive Noise
- Strong Out-of-band Signals
- Strong In-band Signals
- Overload
- Intermodulation Products
- Sideband Noise
- Report – to be published this year