MW Frequency Planning

Frequency Planning

Table of Contents

• Introduction

• ITU-R Frequency Plans Arrangement

• Channel Planning Rules

• Frequency Re use Concept

• Cross Polar Interference Cancellers ( XPIC )

• Frequency Planning Examples

• Cross polar antenna

Introduction

Frequency planning of microwave links involves assignment of radio frequencies in such a way that mutual interference between the various paths does not contribute harmfully to the overall performance and availability.

All frequencies used in a radio-relay network are normally selected from an established frequency plan, established either by an international or national organization.

Introduction (Cont’d)

Within the ITU, Radiocommunication Bureau ( ITU-R ) is responsible for providing RF-channel arrangements, i.e. frequency plans. These plans are published as recommendations or reports

Frequency planning requires use of certain basic rules while setting up the frequency plans.

ITU-R Frequency Plan Arrangements

ASSIGNED FREQUENCY BANDWIDTH

LOWER SUB-BAND UPPER SUB-BAND

GO/LO CHANNELS HI/LO CHANNELS

T-R SPACING

CENTER GAPT-T SPACING

F0

CENTER FREQUENCY

F1 F2 Fn F1’ F2’ Fn’

Frequency Plan Arrangements

• The frequency band is divided into two half bands, one half band for the go or transmitter ( Tx ) frequencies, the other one for the return or receiver ( Rx ) frequencies.

• The half bands have space for paired radio channels.

• To each of the channels in the lower half band corresponds one with the same channel number in the upper half band.

• The duplex separation is the same for all channels.

Frequency Plans Arrangements (Cont’d)

Basically three ways of utilizing these frequency plans, depending on the type of equipment, interference considerations and need for spectrum efficiency.

– Alternated Channel Arrangement

– Co-channel Arrangement

– Interleaved Arrangement

Frequency Plan Arrangements (Cont’d)

For each arrangement TU-R Recommendations

specify:

• the T/R spacing,

• the adjacent channel spacing ( co-polar and cross-polar ),

• the number of channels.

• the center frequency of the band,

Alternated Channel Arrangement

• Most widely used utilization

• Every second channel is utilized using the same polarization, whereas adjacent channels operate on opposite polarizations.

Co-channel arrangement

• Every radio-channel is utilized twice for independent traffic on opposite polarizations for the same path.

• Where frequency congestion is a problem, co-channel operation may be a solution.

Interleaved arrangement

• A a compromise of the alternated mode and the co-channel mode.

• Typically applicable to low capacity systems where the bandwidth of the signal may be less than the channel separation.

• In this arrangement the center frequency of the radio channel on the opposite polarization is half-way between the adjacent channels.

Interleaved Frequency Plan

Example:ITU Spectrum Allocations

Frequency Band Selection

Band

2 GHz

6-8 GHz

10 GHz

11 GHz

13-18 GHz

23-38 GHz

Pro

Best propagation - no power fading

(decoupling, ducting)

Effective space diversity

Lowest outage in non-ducting areasBest high capacity, long-haul

performance Very effective space diversity Good discrimination to interference and long-delayed reflections

Good performance on longer paths

Effective space diversity

Low rain outage in thunderstorm areas

Wide RF spectrum ( 1000 MHz ), so many high-capacity channels are available

Narrow and wideband channels available

Uncrowded bands (2000 MHz @ 18 GHz)

Few bandwidth constrictions

Con

Wideband links are vulnerable to dispersive fading. Reduced fade margins due to lower antenna gains (more outage). Higher interference levels with wide beamwidth antennas. Subordinated to PCS. High clearance paths are vulnerable to reflections.

Longer paths are vulnerable to power fades due to ducting and decoupling in an adverse climate, requiring higher path clearances in some areas. Bands are crowded in some areas.

Bandwidth limited to 5 MHz maximum which accommodate 4-16 T1/E1 channels.

Rain outage is a major factor in some areas.

Path lengths are limited in thunderstorm areas.

Very rain sensitive - e.g. needs 12-16 dB more fade margin ( or 50% shorter paths ) at 23 GHz than 18 GHz for equal outage in rain areas.

Frequency Plans and Principles

Recommendations specifying the individual radio channel are made by the Radio communications sector of the ITU. These ITU-R Recommendations specify:

– the center frequency of the band,

– the T/R spacing,

– the adjacent channel spacing ( co-polar and cross-polar ), and

– the number of channels.

• Frequency are always assigned in pairs.

• HI/LO Channel Planning Rule states that “all radios at a site shall either transmit HI ( Upper band frequencies ) or Lo ( Lower band frequencies ).”

• Designate the Lower Frequency Band or Go Channel as Fn

or “transmit LO”.

• Designate the Upper Frequency Band or Return Channel as Fn’ or as “transmit HI”

• Radio Rings shall be planned with even number of links to avoid HI/LO Rule violation. See Next slide.

Frequency Planning Rule

Radio Rings Channel Planning

Example: Channel Assignment

LOWER SUB-BAND UPPER SUB-BAND

GO/LO CHANNELS HI/LO CHANNELS

Class Work: Frequency Planning

Channel Low Frequency (MHz) High Frequency (MHz)

1 7750 8050

2 7780 8080

3 7810 8110

Special Cases: Passive Repeaters

HI/Lo rule do not apply for sites equipped with passive and RF repeaters.

Frequency Re use Concept

Frequency re-use refers to a situation where the same frequency pair is re-used in a route.

Why? : To conserve frequencies, minimize spares, andmaximize route capacity.

The most efficient re-use plan is one in which a single pair of frequencies is reused within a multi-hop system.

Frequency Re-use Concept (Cont’d)

Frequency Re-use causes co channel intersystem interference from isolated systems and also may cause interference in the following cases:

• Two frequency plans used at the repeater or nodal sites.

• Co-path use of a single frequency on opposite polarizations to double the route capacity.

• Overshoot in two- and four-frequency plans.

Two frequency plan places the same transmit and receive frequencies on both sides of a repeater site.

Creates two possible co-channel interference cases:

Receive backside reception and Transmit backside radiation.

Overshoot ( at sites further down the route )

Two-Frequency ( One-Pair ) Plan

Two-Frequency ( One-Pair ) Plan

FF1

SITE BSITE A SITE CFF1

F0F1 F1’

Receiver Backside Reception

FF1

F1

F1

SITE B

SITE ASITE C

• Receiver backside reception is produced by the victim signal’s antenna receiving co-channel interference signal from its backside.

• Worst case, victim side on the “front” side fades and the interferer remains at full strength.


• Solution: The interfering signal must be attenuated below the victim receiver’s threshold value to that value which results in an acceptable amount of threshold C/N degradation.

• The only protection is the front-to-back ratio of the victim signal’s receive antenna(s).


• The required F/B ratio of the victim receiver’s dish is determined by its fade margin plus the C/N ( BER = 10-3 ) ratio that produces the allowable threshold degradation for the particular modulation used.

Example:

Say if a 1 dB threshold degradation is permitted and Fade Margin to meet the outage objective is 35 dB and C/N is 32 dB at BER 10-3, the required front to back antenna ratio is calculated as 67 dB.

The above value can only be met at 4 GHz and above by using a “ High Performance” or even better antenna with a shielding drum or shroud around the perimeter.,

SITE B

FF2

FF2

F2

SITE A

SITE C

Transmitter Backside Radiation

• Transmit backside interference will be received by the victim at the level of equal to the wanted signal less the antenna F/B ratio of the interfering transmitter’s antenna.

• If at the same height both signals experience equal fades no significant interference case exist.

Co-Path Cross-Polarization

• The use of the same RF channel on the opposite polarization is used to increase the channel capacity on digital routes.

• Very effective on short non fading paths, but ( with degradations or loss of normal 35 dB X-pol discrimination due to specular multipath ) requires sophisticated X-pol interference cancellers ( XPIC ) on longer fading paths.

V

H

F1V

F2H

F3V

F4H

F1’V

F2’H

F3’V

F4’H

1

1

1

1

1

1

1

1

V

Co-Path Cross-Polarization (Cont’d)

H

Conventional RF Channel Arrangement4xSTM-1

Cross Polar Interference Cancellers (XPIC)

• Cross Polar Interference Cancellers ( XPIC ) in the receiver, remove the unwanted signal that has leaked from the opposite polarization into the wanted one.

• With an XPIC the XPD can be as low as 15-20 dB before the performance is degraded.

XPIC Application Cases:

• In some complex 1:N networks, perhaps where N>7

• On longer specular fading hops not equipped with effective Space Diversity and IF Combining protection

• In long high-frequency links above 10 GHz with very high rain intensities which precipitate slant rainfall.

When XPIC is unnecessary?

• In most 1:N links configured with effective Space Diversity and IF Combining protection

• In short and high inclination links with little or no fade activity not requiring Space Diversity

• In short high frequency ( above 15 GHz ) links

• If the cross-polar discrimination of antennas is insufficient to solve the overshoot problem using a single frequency pair, a second set of frequencies is required.

• Overshoot cases between alternate hops in a full growth route may still exist. In this case, the frequency and polarization should be alternated on every second hop.

Four Frequency Plan

Four Frequency Plan (Cont’d)

• The “backside” interference cases diminish or vanish in a four-frequency plan.

F1 F2 Fn F1’ F2’ Fn’

FF2’FF1’

FF2FF1FF1FF1’

Overshoot in two- and four-frequency plans

In the above case, a victim site receives a co channel unwanted signal following a different path. This case is similar to Receive Backside reception described earlier.

F1

F1

F1

Overshoot

Path

Overshoot: Worst Case Scenario

• The victim signal fades to threshold while the interfering signal does not.

• The protection for the victim is from azimuth ( and horizon ) discriminations plus additional free space loss due to extra distance traversed by the interfering signal relative to the desired signal.

• This case is similar to Receive Backside Reception as discussed earlier.

Overreach Calculation: Example

Assume the minimum threshold C/N ratio is 34 dB, the ratio of two path lengths is 3. The composite fade margin required to meet outage objectives is 35 dB.

The extra distance will attenuate the interfering signal by 20 log 3 = 10 dB and the balance is [ C/N – attenuation due to distance – CFM ] as shown below:

= 34dB – 10dB + 35dB = 59 dB

It can be seen that the rest of the attenuation must be provided by, antenna azimuth and path polarization discriminations.

• If the interference signal is problematic the only practical way to improve the situation, assuming antennas with good side lobe suppression are already in use' is to change the polarization of one of the two paths.

• This way the interfering signal experiences the antenna’s cross-polar discrimination, which is significant.

• For this reason, overreach problems are solved by alternating the polarization every two hops.


• For this reason, overreach problems are solved by alternating the polarization every two hops.


1H

1H

1V

1V

H = Horizontal Polarization

V = Vertical Polarization Victim Site

Class Exercise: 3G Network Frequency Planning

Co Polarized Antennas

For Co polarized operation, both antenna must be either Vertically or Horizontally Polarized

DTxCpDRxCp

Vertical (Horizontal)Vertical (Horizontal)

Azimuth degrees from the main lobe

Antenna Radiation Pattern

Effective Antenna Gain at 5 degrees off-boresight is:

43.0 dB – 30 dB = 13 dB

Cross Polar Operation (Cont’d)

• In a cross polar operation, the disturbing station transmits the signal with opposite polarization to the disturbed station.

• However, the suppression of the cross-polar component in the transmitting antenna is not infinite. In the back-lobe direction most antennas have approximately the same radiation level for both polarizations; giving no cross-polar discrimination in that particular direction.

• The disturbing transmitter always transmits both a vertical and a ( smaller ) horizontal component

• The victim receiving antenna will receive both polarizations.

• Generally, the two components will experience a different discrimination in the receiving antenna.

• So, the received interference level at the disturbed station is the sum of a vertical and a horizontal component.

Cross Polar Operation

Cross-polarization

• In most cases one of the two components will dominate, depending on the relationship between DTxXp DRxXp and for the particular angles for a given path.

• A good cross-polarization enables full utilization of the frequency band by using both the vertical and the horizontal polarization planes. Typical values are 30 dB for standard antennas and 40 dB for antennas especially designed for cross-polar operation. The discrimination always has the largest value in the main lobe direction.

Section 8B: Interference

Introduction

Causes of Interference

Interference Classes

Overreach Interference

Nodal Interference

Intersystem Interference

Radio Frequency Interference Mechanism

Radio Threshold Degradation

Allowable Level of Interference

Interference Calculation Example

Interference Calculation Steps

Introduction

Any unwanted signal

Copy of wanted signal delayed due to multipath propagation

An adjacent channel’s signal traveling over the same link

A signal from another radio link or RF source

Interference Classes

There are two interference classes:

Between the same system ( Intra System Case ): Controllable

Between different systems ( Inter System Case ): Uncontrollable

Intrasystem

This type of interference is caused by an undesired signal generated within the system. Two possible cases:

Intrasystem Interference

• Overreach interference

• Spur or junction interference.

Co channel Overreach Interference

Site D

Site A

Site B

Site C

F1

F1

F2

F1

• Plan for a longer overreach path to add additional free-space loss to the undesired signal.

• Maximize antenna discrimination against the overreach path.

• Use a suitable route such that the overreach path can be blocked by the terrain.

Solutions: Overreach Interference

Co channel Nodal Interference

F1F2 F1F1

F1F1

F1F2

F1F2F1F1

Spur Interference

F1Nodal Interference

Use antenna with a very high front-to-back ratio.

Worst case: Switch to 4 Frequency plan

Solutions Co channel Nodal Interference

Caused by reception of an unwanted transmission from of a different system.

Can be coordinated by adjusting the antenna discrimination and receiver selectivity, and choosing proper frequency plans.

Best Option: Coordination, control, and sometimes compromising on the radio channel through direct negotiation with the other operator on the same Frequency bands.

Note: With proper Government run Spectrum management coordination shall not happen.

Intersystem Interference

Example: Intersystem Interference

MSC 1

MSC 2

BSC 2

Hub

External EMI Example: Desensitization

Receiver Overload: • The excessive unwanted signal drives the receiver into

overload condition even though its signal is well out-of-band.

• Overload happens when the affected receiver’s pre-selection filter is not adequate.

• Overload destroys the normally-required linear amplification process and causes intermodulation distortion and serious data errors.

Causes of Interference

Tx

Rx

Tx

Rx

Path

• PA Non Linearities

• LO Phase Noise (Jitter) on recovered carrier and Clock

• OSC drifts

• Filter Selectivities

• Thermal Noise

• Reflection Echo Feeder system

• T/R Spacing

• F/B Ratio Antennas at nodal junctions

EXTERNAL CAUSES

• Other Networks

• Radar

• Satellite

• Own Network

• Unknown ( difficult to trace)

Difficult to Control

INTERNAL CAUSES

Internal Interference

• Internal Interference mechanisms cause long-term performance degradation.

• Internal Interference and are responsible for low level continuous background error ratio which is even present with high RSL.

• Degradations due to internal interference are quantified as Carrier to Noise ( C/N ) ratio penalty or “implementation margin.”

• It is these degradations that cause the practical C/N versus BER to deviate from the theoretical performance

• At high C/N ratio the digital radio performance becomes asymptotic to the low level background ( dribble ) BER or Residual bit error ratio ( RBER )

• The poorer the performance, the greater the required C/N for a given BER.

• Question: What causes these Residual bit errors ?

Internal Interference (Cont’d)

Residual Bit Error Ratio ( RBER )

RBER The constant bit error ratio is determined solely by the bit error contribution of the radio itself called Residual bit error ratio

Radio Frequency Interference ( RFI ) Mechanisms

Analog Systems

The categories of RFI noise mechanisms in analog systems are

• Carrier beat

• Sideband-to-sideband beat

• Carrier-to-sideband beat

• Threshold Degradation.

Digital systems outage is based on BER and fade margin degradation due to the degradation of Radio Threshold

Digital Radio Receiver Threshold Cmin

• The minimum wideband carrier power ( Cmin ) at the input to a receiver that will produce a usable baseband out put is called the receiver threshold or receiver sensitivity.

• Cmin, the receiver threshold is dependent on:

– the wideband noise power present at the input of a receiver,

– the noise produced within the receiver, and

– the noise sensitivity of the basaeband detector.

Digital Radio Receiver Threshold Cmin (Cont’d)

Specified by Radio vendors: Two Digital Radio Thresholds:

• The 10-6 BER ( or other BER<10-6 ) Static Threshold is for factory and in-service field verification of receiver noise and interference levels, measured manually with attenuators.

• The 10-3 BER Dynamic Threshold is for outagecalculations and “hands-off” field measurements in a normal fading environment with BER ( and, perhaps, chart recorder ) network management instrumentation.

Threshold degradation Causes and Effects

Excessive Interference or High Noise Figure

Increased Radio Noise

Degraded C/N ( dynamic) and T/I( static ) radio thresholds

Reduced fade margin

Increased Outages

RF, Noise, Interference Levels

Dynamic Performance

(Fade Outages to 10-3

BER)UNFADED RECEIVE SIGNAL

LEVEL

SYSTEM GAIN (dB)

NET PATH LOSS (dB)

(– dBm)

Static Performance

(Manual Measurements to 10-6

BER)

CTHERMAL(OR FLAT)

FADE MARGIN

TFMdB

FLAT +INTER-

FERENCEFADE

MARGINIFMdB

DISPERSIVEFADE

MARGINDFM

dB

COMPOSITEFADE

MARGINCFM

dB+ =

C/N RATIOdB

(UNFADED)

C/ RATIOdB

T/I RATIOdB

C/I RATIOdB

N

T

10-6 STATIC THRESHOLD (-dBm)

INTERFERENCE LEVEL = –N +6 dBm

(CO-CHANNEL)

PO

WE

R S

CA

LE

, d

Bm

THERMAL NOISE = – dBm N6 dB (1 dB TOTAL NOISE INCREASE)

10-3 OUTAGE (– dBm)C/N RATIO

dB1 dB DEGRADATION

DUE TO INTERFERENCE

T

C

II

N = -KTB (W)

Where,

N = Digital receiver Thermal Noise power,

K = Boatman's constant ( 1.38 x 10-23 )

T = Receiver Temperature ( in Kelvin's = 290 k )

B = Receiver Noise Bandwidth ( Hz )

Radio Thermal Noise Level

In radio receivers the random motion of electrons due to thermal energy results in an average noise power as shown in the formula below:

N ( dBm ) = 10 log KTB x 10-3

= 10 log KT x 10-3 + 10 log B

For 1 Hz bandwidth at room temperature,

Radio Thermal Noise Level Expressed in dBm:

N = 10 log ( 1.38 x 10 –23 x 290 ) x 103 + 10 log 1 MHz

N = 10 log ( 1.38 x 10 –23 x 290 ) x 103 + 10 log 106

N = 10 log ( 1.38 x 10 –23 x 290 ) x 103 + 60 log 10

N = - 174 dBm + 60 = 114 dBm

Radio Thermal Noise Level

N (dBm) = -114 dBm + 10 log BW + NF

Where,

NF = Radio Noise Figure ( dB )BW = Transmission speed ( mbps )/Modulation Eff. ( Hz )

For radio link planning use N as shown below:

Example: Radio Thermal Noise Level

N = Digital receiver Thermal Noise Floor, dBm

= -114 + 10 log BW + NF

where

BW = Receiver 3 dB noise bandwidth, MHz

= Symbol RF transmission or Baud rate, Mbaud

= Data rate/mod. efficiency ( 6 bps/Hz for 64 QAM )

= 47 Mbps ( DS3 )/6 bps/Hz = 7.8 MHz/Mbaud

NF = 5 dB

N = -114 + 10 log 7.8 + 5 = -100 dBm

Threshold Degradation Calculation

Neff = 10 log { 10N/10 + 10 I/10 } *dBm+………………….(1)

where,

N = Receiver Thermal Noise [dBm]

I = Co channel Interference level [dBm]

Effective Noise offset, Δnef = N - Neff *dB+………………(2)

Degraded Threshold, Tdegraded = T - Δnef *dBm+………..(3)

Where: T is the receiver Threshold., [dBm]

Threshold Degradation Calculation (Cont’d)

Neff = 10 log { 10N/10 + 10 (I-/T/I)/10 } *dBm+………………….(1)

where,

N = Receiver Thermal Noise [dBm]

I = Off channel Interference level [dBm]

T/I = Threshold to Interference Ratio [dBm]

Effective Noise offset, Δnef = N - Neff *dB+………………(2)

Degraded Threshold, Tdegraded = T - Δnef *dBm+………..(3)

Where: T is the receiver Threshold., [dBm]

Threshold Degradation Calculation (Cont’d)

For a radio the calculated Thermal Noise ( N ) is -100 dbm. The Interference is twice the calculated radio N number. The dynamic ( 10-3

outage ) threshold of the victim receiver is – 75 dBm. Allowed co-channel interference level is –106 dBm. Calculate the radio threshold degradation

Neff = 10 log { 10N/10 + 10 I/10 } [dBm]

= 10 log { 10-100/10 + 10 106/10 } [dBm]

= 1 dB

Threshold Degradation

= - 75 + 1 = - 74 dBm

Example: Typical 45 mbps (64 QAM) Digital Radio RF, Noise, Interference Levels

Dynamic Performance

(Fade Outages to 10-3

BER)UNFADED RECEIVE SIGNAL

LEVEL

SYSTEM GAIN

(105 dB)

NET PATH LOSS

(65 dB)

(–35 dBm)

Static Performance

(Manual Measurements to 10-6

BER)

CTHERMAL(OR FLAT)

FADE MARGIN

TFM42 dB

FLAT +INTER-

FERENCEFADE

MARGINIFM

41 dB

DISPERSIVEFADE

MARGINDFM61 dB

COMPOSITEFADE

MARGINCFM

40.8 dB+ =

C/N RATIO65 dB

(UNFADED)

C/N RATIO25 dB

T/I RATIO31 dB

C/I RATIO71 dB

N

T

10-6 STATIC THRESHOLD (-75 dBm)

INTERFERENCE LEVEL = –106 dBm(CO-CHANNEL)

PO

WE

R S

CA

LE

, d

Bm

THERMAL NOISE = –100 dBm N6 dB (1 dB TOTAL NOISE INCREASE)

10-3 OUTAGE (–77 dBm)C/N RATIO

23 dB1 dB DEGRADATION

DUE TO INTERFERENCE

T

C

II

Problem:

For a radio the calculated Thermal Noise ( N ) is -100 dbm. The Interference is –100 dBm. The dynamic ( 10-3 outage ) threshold of the victim receiver is – 75 dBm. The Thermal fade margin is 40 db to meet the link objectives.

Calculate the effect of doubling the interference level on the fade margin. Also calculate the degraded radio threshold level ?

Threshold Degradation Calculation: Due to Co-Channel Interference: Class Work

Digital radio Interference Objectives

• The allowable level of interference into a digital microwave radio is based solely upon its impact on the dynamic ( 10-3 outage ) threshold of the victim receiver a it effects the link’s fade margin, and therefore, its outage time.

• It is calculated that a 3 dB loss of a fade margin doubles the outage time for non diversity link and quadruples the outage.

Allowable Level of Threshold Degradation due to Interference

• As a general guideline from US Telecommunications Industry Association ( TIA ) Bulletin TSM-10-F, Interference Criteria for Microwave Systems. Bulletin 10-F states:

• An interfering signal should not degrade the victim’s receiver static and dynamic thresholds ( and fade margins ) by more than 1 dB.

• The co channel Interference shall be 6 dB below the kTb noise floor of the receiver. ( I=N=6dB ).

Calculation Example:

N = Digital receiver Thermal Noise Floor, dBm

= -114 + 10 log BW + NF

where

BW = Receiver 3 dB noise bandwidth, MHz

= Symbol RF transmission or Baud rate, Mbaud

= Data rate/mod. efficiency ( 6 bps/Hz for 64 QAM )

= 47 Mbps ( DS3 )/6 bps/Hz = 7.8 MHz/Mbaud

NF = 5 dB

N = -114 + 10 log 7.8 + 5 = -100 dBm

Interference Allowed: = - 100 dBm + ( -6 dB ) = 106 dBm

Calculation of Allowable level of C/I

Allowable C/I = 10 log [ 10 – (CFM+ C/N)/10 – 10 – (TFM + C/N)/10 ]

Where

C/I = Unfaded RF carrier-to-interference ratio to meet the CFM objectives, dB = TFM ( to 10-6 BER ) + T/I dB

CFM = Composite Fade margin to meet the link’s outage objective, to 10-3 BER

TFM = Thermal fade margin with out interference, to 10-3 BER, dB

C/N = Receiver’s carrier-to-noise ratio for 10-3 ( outage point ), dB

Allowable co channel interfering signal ( I ) = - RSL dBm + (-C/I,)UNFADED

Example:

• A digital radio is configured with 64 QAM radios and from link Budget calculations the Thermal Fade Margin is 35 dB. Also from the the outage objective is met with a 34 db composite fade margin ( CFM ) meeting the guideline of 1 db standard CFM = TFM – 1 dB degradation due to interference. For this radio C/N 9 10-3 BER) = 22 dB. Calculated RSL = - 42 dBm.

• Solution:

C/I = 10 log [ 10 – (34+ 22)/10 – 10 – (35 + C/N)/10 ]

= 63 dB

From the link budget the RSL is -42 dBm, therefore the allowable co channel I signal level is

I allowable= -42 dBm + 9 –63 dB0 = - 105 dBm.

Class Tutorial Example: Allowable Interference (I) level

Example:

Assume the previous hop is shorter and the outage objective is met with a CFM relaxed 9 dB to 25 dB, all other link parameters being unchanged. ( 35 db thermal fade margin ( TFM ), C/N 22 dB ).

Use I dB degradation guideline and calculate the allowable co channel interference. Comment on the results of your calculations. Answer: -89 dBm

Class Tutorials: Two Frequency Plan: Nodal Interference

An

Std Ant:1.8 m

Gain: 40 dBi

F/B ratio: 48 dB

Std Ant:1.8 m

Gain: 40 dBi

F/B ratio: 48 dBStd Ant:3 m

Gain: 45 dBi

F/B ratio: 52

Std Ant:3 m

Gain: 45 dBi

F/B ratio: 52

F

FF’

Radio Data: C/I = 15 dB ( Interference

Objectives )

From Link Budget:

Nominal Receive Level: 40 dBm

Fade Margin: 40 dB ( Performance )

F’

FF’

F’

SITE B

SITE A SITE C

Step 1: Calculate C faded carrier at the victim site ( site B ).

C faded carrier = NRSL – FM

Step 2: Calculate the interference (I) signal at the victim site ( site B )

I = C unfaded from the interferer ( site C )- the F/B ratio ( site B )

Step 3: Calculate faded C/I from (1) and (2) above as:

C/I faded = C faded carrier / I

Class Tutorials: Calculation Steps: Nodal Interference

To improve the C/I faded ratio first try to balance the receive levels at the victim site.

Is C/I faded > C/I minimum

No

YesDone

From (3)

Class Tutorials: Nodal Interference

• Upgrade the antenna at the wanted transmitter site to the next higher size and see if the new C/I faded > C/I minimum ratio is now acceptable.

• Repeat the previous interference analysis with the above antenna to ensure that the new interference level will not degrade the link performance or C/I faded > C/I minimum.

• If after balancing the C/I ratios improve but is still not good enough. Go back to the original configuration and change the antenna at the nodal or victim site in the direction of the wanted transmitter with a high-performance antenna with F/B of 70 dB with the same gain ( 45 dBi ).

Class Tutorials: Nodal Interference

4m

Feeder Loss: 5

dB

Class Tutorial: Four Frequency Plan:Overshoot Interference Case

Assume:

XMT Power: 30 dBm

RCVR Threshold: - 80 dBm

C/I = 15 dB:

RPE for 3m and 2.4 m antennas ( see antenna patterns )

Site 4

F1

F1’

F1 (H) F1’

F1(H)

F2’ (H) F 2

F1’

2.4 m

3m

4m

3m

2.4 m

50 km 40 km 70 km

140 km

Feeder Loss: 4 dB Feeder Loss: 3 dB

Overshoot Path

Site 1

Site 2 Site 320 deg 5 deg

Class Tutorial: Four Frequency Plan:Overshoot Interference Case

Objectives:

(1) To determine the C/Iunfaded at Site 4. Unwanted signal. Co-channel Case.

(2) See the effect of 30 dB fade on the performance of link 3–4.

(3) To determine C/I ratio at site 4, if it is needed to double the capacity of link 3-4 by only replacing the existing antennas with dual polarized antennas. ( Ignore any overshoot from Site 1. )

Network Configuration for Calculation

Site 4

F1(H)

F1’

4m

70 km

Feeder Loss: 3

dB

Site 35 deg

3m

Feeder Loss: 4

dB

Overshoot Path = 140 km

20 deg

Site 1

F1 (H) Interference

F1 (H)

4m

Calculation Steps

Step 1:Calculate Unfaded receive carrier level at Site 4 from Site 3. Call it CRX4Step 2:Calculate the interference level at Site 4 say IRX4. From antenna patterns read antenna gain at Site 1, 20 degrees off-boresight, copolar and for site 4, 5 degrees off-boresight, copolar.Step 3:Calculate Unfaded receive carrier level at Site 4 from Site 3. Call it CRX4

Calculations Steps (Cont’d):

Step 4:

Calculate the interference level at Site 4 say IRX4.

• From antenna patterns read antenna gain at Site 1, 20 degrees off-boresight, copolar

• Similarly read antenna gain for site 4, 5 degrees off-boresight, co-polar

Note:

For both sites read antenna directivity from Y axis for discrimination angles at X axis then subtract these values form Antenna Gain and

determine the effective antenna gain.

Step 5:

Compare the impact of unwanted signal at site 4 and if do not meet the C/I objectives change the channel polarization plan and re calculate to see if it will meet the C/I objectives. See the next slides as hints to solve this problem.

Calculations Steps (Cont’d):

Cross Polar Operation Interference Calculations

Tx

LTXATX

GTX

DTxCp

Tx

LTXATX

GTX

DTxXp

Rx

IVARX

GRX

DRxXp

Rx

IH

GTX

DRxCp

ARX

Vertical Component

Horizontal Component

FSL

FSL

VICTIM SITE ( H )INTERFERING SITE ( V )

• The disturbing transmitter will always transmit both a vertical and a ( smaller ) horizontal component and the receiving antenna at the disturbed station will receive both polarizations, but the two components will generally experience a different discrimination in the receiving antenna.

• So, the received interference level at the disturbed station is the sum of a vertical and a horizontal component. In most cases one of the two components will dominate, depending on the relationship between DTxXp DRxXp and for the particular angles for a given path.

Cross Polar Operation Interference Calculations

General Formula Cross Polar Operation

The general formula for the calculation of interfering signal level for Xpolar operation is ( the wanted and interfering signals have opposite polarization ):

I = 10log(10((LTx - ATx + GTx - DTxXp - FSL + GRx - ARx - DRxCp)/10)

+ (10((LTx - ATx + GTx - DTxCp – FSL + GRx - ARx - DRxXp)/10))

Where,I(V)(H) Level of single interference signal in dBm.LTx Output level of the disturbing transmitter in dBm.ATx Feeder/branching Losses in dB in the transmitting station.

GTx Antenna gain for the transmitting antenna in dB. DTxCp Co-polar antenna discrimination in dB for the transmitting station.DTxXp X-polar antenna discrimination in dB for the transmitting station.FSL Free space attenuation in dB.

Where ( contd.)

GRx Antenna gain for the receiving antenna in dB ( victim site ).ARx Feeder/branching loses in dB in the receiving station.DRxCp Co-polar antenna discrimination in dB for the receiving station.DRxXp X-polar antenna discrimination in dB for the receiving station.

Note:If more than one interfering signal is present, the total interference level, LTOTAL, must be obtained.


Again, splitting up the formula gives the following sub-equations:The net output power of the co-polar signal component ( referred to the receiver ) from the disturbing station equals:

LTx - ATx + GTx – DTxXp

The attenuation of this co-polar signal in the receiver is given by:

GRx - ARx - DRxCp


The net output power of the cross-polar signal component ( referred to the receiver ) from the disturbing station equals:LTx - ATx + GTx - DTxCpThe attenuation of this cross-polar signal in the receiver is given by:GRx - ARx - DRxXpAdding the input level of the co-polar and the cross-polar interference signal on a power basis gives the total interference level at the disturbed receiver.


Cross polar Antenna Radiation Pattern Example

Effective Antenna Gain at 5 degrees off-boresight is:

43.0 dB – 30 dB = 13 dB

Class Work:

• Generally during preliminary network planning, standard performance antennas are used, and no level adjustments are made to reduce interference problems.

• It is important to do the interference calculations to determine if the hop objectives are met and if not what action must be taken to solve the interference problem.

Calculation Example (Co-polar operation)

Tx

LTXATX

GTX

DTxCp

Rx

IVARX

GRX

DRxCp

Vertical Component

FSL

INTERFERING SITE (V) VICTIM SITE (V)

LTx 29.0 dBmGTx 40.3 dBATx 2.5 dBDTxCp 0.0 dB

GRx 43.6 dBARx 3.1 dBDRxCp 46.0 dB

The distance between the sites is 12 km and the link frequency is 7 GHz: FGSL calculates as 130.dB. Calculate Co channel Interference at the victim site. ( Answers: -69 dBm)

I = LTx - ATx + GTx – DTxCp + GRx - ARx – DRxCp- FSL

Andrew 3.0m HP antenna

Calculation Example (Cross-polar operation)

TxLTX

ATX

GTX

DTxCp

TxLTX

ATX

GTX

DTxXp

RxIV

ARX

GRX

DRxXp

RxIH

GTX

DRxCp

ARX

Vertical Component


FSL

FSL

VICTIM SITE (H)INTERFERING SITE (V)

LTx 29.0 dBmGTx 40.3 dBATx 2.5 dBDTxCp 0.0 dBDTxXp 3 0.0 dB

GRx 43.6 dBARx 3.1 dBDRxCp 46.0 dBDRxXp 53.0 dB

Andrew 2.0m HP antenna

Vertical Component

Since the interfering site is transmitting on horizontal polarization, therefore, transmitted signal towards the victim site is DTxXp .

Since the victim is using vertical polarization and will receive the co-polar component and the antenna discrimination equals DRxCp .

I = LTx - ATx + GTx – DTxXp + GRx - ARx – DRxCp- FSL= −99 3 dBm




The horizontal component is the co-polar component at the transmitting station Downtown. Consequently the antenna discrimination at the transmitter equals DTxCp .

Horizontal polarization is the cross-polar component at the receiver, so the antenna discrimination equals DRxXp. The horizontal component of the interfering signal is found by:

I = LTx - ATx + GTx – DTxCp + GRx - ARx – DRxXp- FSL= −76.4 dBm

Conclusions:

• Calculations shows that the combined interference level is much higher than approximately -105 dBm to avoid any threshold degradation ( < 1 dB degradation ).

• Solutions: To get the required T/I ratio only try to use antenna with better discriminations such as HP antennas, typically 10 dB improvement in antenna discrimination. At higher cost to fully solve the problem, use of SHXP antennas.

• If SHXP antennas do not solve the problem, change the frequencies.

Interference Workshop

Step by Step Manual procedure to Calculate Interference

Step 1:

Obtain the following information to determine where interference analysis is needed in the network:

Frequency Bands

If the network frequency plan uses different frequencies then there is no need to analyze these links for interference. For example use of 7 and 13 GHz bands together.

Available Frequency Pairs

For example a two frequency plan as opposed to a four frequency plan. Number of pairs determine how often the frequencies are repeated.

System Layout ( to scale )

This will indicate possible cases of interferences in the network.

Distance between paths

Distances over 150 Km an be ignored, however short distances or co located sites must be analyzed for interference.

Terrain

Mountains between sites will block an interfering signal, however sites located on mountain tops must be investigated for possible interference.

Exaggerated Azimuths

90 – 180 degrees may not require interference analysis.

Step 1 (Cont’d)

The purpose of Step 2 is to fill an “ Interference Case Worksheet” with necessary data and determine the maximum allowable interference level. Interference Case Worksheet is included in Appendix A.

Antenna Patterns

These should be based on polarizations schemes such as HH, VV, HV, VH used in the network and shall be available for both, the victim receiver and the interfering transmitter.

Path Calculations

These should include: Victim Path, Interfering Path and interfering path to victim receiver.

Step 2

Step 2 (Cont’d)

System Layout Drawing

Step 3: Antenna discriminations

For the following network in the following slide, find the angle between the desired path and the potential interfering path. For example:

• Calculate azimuth from the interferer to victim and then calculate the difference between this azimuth and the working azimuth.

• Calculate azimuth from the victim back to the interferer and then calculate the difference between this and the working azimuth.

• Locate the antenna pattern for the pertinent XMT/RCV antennas. There are two patterns: 1) at the XMTR antenna causing the interference and 2) at the victim RCVR

• Find the antenna discrimination designated on the pattern at the appropriate discrimination angle.

Step 3: Antenna Discrimination Angles Calculations (Cont’d)

Threshold to Interference (T/I) Curve:

Step 4

• Calculate the C/I objective as C/I = T/I + FM, where FM is the required Fade Margin to meet the performance objective. Compare with the calculated interference level.

• T/I can be found from the radio T/I curve. Fade Margin can be read from the Path Calculation Worksheet.

Step 5

Consider any of the following options to meet the required interference objectives:

Antenna Changes:

- Use larger diameter antenna to increase gain

- Use High performance antennas to add achieve higher levels of discrimination

- X pole discrimination

Adjust Power Output

Add Pads

Use other frequency assignments

Re route the link

RFI Reduction Techniques

• Antenna Characteristics

• Angle of Interference

• Transmitter Power Selection

• Geographical Separation ( Distance, Path Blockage )

• Frequency Separation

• Cross polarization

How to Determine if Interference is Corrupting your System?

• An interference problem can often be categorized by its frequency characteristics.

• Unwanted signal almost certainly enters the receiver through the antenna.

• A spectrum analyzer can be connected to the operating system antenna as a substitute to measure, display and identify unwanted signals.

• Obtain a broadband, spectrum analyzer and as first step in recognizing if interference has corrupted a receiver, learn the characteristics of the wanted signal.

• Monitor the spectrum analyzer. If interference is overlapping the intended receiver signal, it will be relatively obvious on the spectrum analyzer display.

• Carry out the process for few days by sweep the entire network.

• Link Engineering Software will only indicate if Intra system RFI is a problem.



• Second Step: Analyze the frequency domain using a spectrum analyzer to measure wanted signal frequency, power, harmonic content, modulation quality. This is the wanted signal.

• If interference is overlapping the intended receiver signal, it will be relatively obvious on the spectrum analyzer display.

MW Frequency Planning

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