8/14/2019 Footprint Part 4 of 6 Noise and Infrastructure

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FootprintChanging the RF cost performance paradigm

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Introduction

Noise is a fact of life in radio communication systems, withsome background noise always present.

Noise can also emanate from many sources including:- Cable, filters and other passive elements which

exhibit a loss. Intermodulation Distortion (IMD), and The environment - broad band RF interference from

electricity static, ionic clouds or corona discharge,electric appliances and tools, automobiles, lightningand spurious RF etc.

The radio receiver will see any interference as unwanted

noise, and reducing noise levels or noise rise at theBTS receiver is key to network health.

Any internal or external interference is likely to result insome degree of deafening in a TDMA receiver or robbingof Interference Margin in CDMA systems. The result isreduced coverage, capacity or both, causing:

Dropped Calls. Subscriber Churn. Lost Revenue, and High OPEX

Figure 1

Noise and Network Topology

In todays wireless networks, mobile connectivity ismaintained by handing a call from one cell to the nextalong the subscribers route. The point of handover is

determined by the subscribers detected signal quality, orBit Error Rate (BER), with the actual acceptable signal andnoise level ratios varying between TDMA and CDMAtechnologies.

RF theory suggests the noise floor level is the sum of allthe noise sources and unwanted signals within a system.

It limits the accuracy of the smallest measurements, sincedetectable signal amplitude must be no less than the noisefloor for TDMA systems, or within a tolerable level under itfor spread spectrum technology.

Receiver noise levels degrade further as evolvingtechnologies utilize wider channel bandwidths.

ChannelBandwidth

kTB Thermal 50ohm Noise Floor

GSM 250 kHz -119 dBm

CDMA 1.25 MHz -112 dBm

WCDMA 5 MHz -107 dBm

Table 1

The specified sensitivity of a BTS receiver inconjunction with expected noise levels determinesthe geographic layout of the network and thehardware needed to support it.

Higher than anticipated noise levels or noise risechallenge planned cost performance modelsthrough reduced practical receiver sensitivity,coverage and capacity.

We have previously sighted Holma and Toskala (WCDMA& UTMS Nokia Finland 2004) who suggest 1dB loss inCDMA and UTMS receiver sensitivity can mean as much as11% loss of coverage.

Interference Generated Noise

Increased receiver interference adds to the noise floorcausing the rejection of the subscriber call that wouldotherwise be of acceptable quality.

Figure 2

Figure 2 is the display from a spectrum analyser onMaximum Hold, capturing wide band noise resulting

from the presence of PIM in the RF interconnection overtime. In this case GSM900 and WCDMA800 were co-located with resulting PIM products degrading the receiver

Noise and the RF Infrastructure (Part 4 of 6)

Coverage and capacity are critical cost performance metrics in wireless systems, with both generally limited by degradedreceiver performance from the introduction of unwanted noise. This interference can come from multiple possible sources,

but is commonly introduced into the system by deteriorating components or less than perfect RF infrastructure.

8/14/2019 Footprint Part 4 of 6 Noise and Infrastructure

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FootprintChanging the RF cost performance paradigm

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noise floor to as high as -95dBm/30kHz. This loss inreceiver sensitivity is loss in coverage and capacity.

(Case studies supporting these findings are provided inpart 5 of this series of papers.)

De-Sensitization, Saturation and Blocking,

Signal to Noise Ratio (SNR) is also degraded when largeinterfering signals saturate a receiver, compromisingsignal gain and creating distortion. The degradationworsens as the interference increases in power. This effectis often called blocking, saturation or de-sensitization.

Every component in the receiver has a non-linearcharacteristic, and when subjected to saturation, both in-band and out-of-band harmonics and intermodulationproducts can be generated inside the receiver eventhought they were not present in the input signal.

Uplink Noise Rise in CDMA Systems

In the spread spectrum wireless environment uplink noiserise is defined as the relationship between the total noiseinterference a receiver must contend with in proportion tonormal background thermal noise floor.

CDMA system noise rise is the total of all prospectiveinterference, remembering that every subscriber session isevery other sessions interference.

As the session numbers increase, subscriber noise risenaturally increases toward a level setting the maximumcapacity of the receiver.

Typically RF power control at both ends of the wirelesslink is used to minimise the session interference andmaximise coverage and capacity. A limit is imposed whenthis rise over thermal exceeds a preset magnitude. eg,

6dB.

There are many scenarios that will determine how thenatural noise rise will affect the receive system, but onceit reaches the preset limit, only lowering the average bitrate and hence interference power density can increasecapacity.

Subscriber uplink noise rise is a normal function of ahealthy spread spectrum receiver, however any unwantednoise in the system will rob capacity and coverage.

Working with Noise

As discussed above, there are multiple possible noisesources. Removing one does not necessarily eliminate theothers but it does narrow the options.

Changes in the cellular RF environment to meet demandfor more services and lower cost performance, combinedwith the pressure to mix new technologies with thosecurrently used, introduces a potential IMD nightmare.

IMD is often the root cause of noise rise, and as the RFspectral density at the BTS increases, the possibility ofnoise being present in the form of Passive Intermodulation(PIM) increases. This is a direct result of non-linearity inthe RF system caused by poor quality components or

physical construction.

It is therefore essential the antenna and the supportinginfrastructure is built using the same Quality Control (QC)standards as the radio equipment and associatedcomponents.

(The previous paper in this series titled Working with PIM- part 3 of 6, introduces PIM, discussing how this can beavoided at the BTS.)

Traditionally TDMA technology created some challengeswith PIM, but narrow bandwidth channels made workingaround prospective interference problems easier whenthere was plenty of available spectrum. PIM productseither fell out of the receiver band or only affected somechannels. Where they fell out of band, the cause was

either ignored, or frequency planning provided a solution.Second order products were never considered to be aproblem.

Spectrum licensing cost limitations and wider bandwidthtechnology has forced a review of this practice triggeringthe need to use every bit of available spectrum.

CDMA technology with its 1.25 MHz bandwidthrequirement was more challenging from the start. Withthe pre-selection filters found in narrow band systemsunable to be used, IMD is a very likely cause of noise riseas broadband PIM products potentially enter the receiverdirectly, or as a component producing receiver IM bymixing with local oscillator components. Second orderproducts are also more likely to be a problem.

The best and most cost effective way to avoid receivenoise rise due to any IMD derivative is to eliminate orsignificantly reduce its presence. This means correctingthe source.

Testing for PIM at the BTS is proving to be the mosteffective way to confirm this has been achieved.

Summary

Utilizing a PIM test as an RF infrastructure performancemetric is now being used or reviewed by wireless networkoperators around the world.

Working together, Summitek Instruments and Triasx areaddressing the growing global interest in InterconnectionQuality Analysis (iQA), and have created anInterconnection Technology Centre (iTC) to provideindustry support.