Troubleshooting Radio Links Guide

WALKairTM Troubleshooting Radio Links

Alvarion Ltd. All rights reserved

The material contained herein is proprietary, privileged, and confidential. No disclosure thereof shall be made to

third parties without the express written permission of Alvarion Ltd. Alvarion Ltd. reserves the right to alter the

equipment specifications and descriptions in this publication without prior notice. No part of this publication shall be

deemed to be part of any contract or warranty unless specifically incorporated by reference into such contract or

warranty.

WALKair Troubleshooting)

ii

About This Manual This manual summarizes the different scenarios that a field WALKair installer may stumble upon and the corrective actions he may take in order to have the WALKair link up and running.

The content of this manual starts with step by step trouble shooting and gradually proceeds to network problems and analysis methods.

The structure of the document is as following:

Step-By-Step Air Link Setup Trouble Shooting – this section is aimed at basic air link problems; Easy to use as the user is guided by questions and actions towards the solution to the problem.

Understanding Air Link Setup Process – In this section the user will find a more detailed description of how the air link is established following by a relevant LCI message in each phase.

Understanding E1 Alarm Forwarding in WALKair – Many service problems begin with physical layer problems. This section describes the E1 alarm mechanism and how it is implemented in WALKair as a point to multi point system.

Possible Network Problems and Causes – This section introduces possible problems that occur due to a network problem and not necessarily a specific link. All are problems that really happened in the field and are a result of installation, planning or commissioning problems.

Analysis Methods – This section provides tools and methods for analyzing the occurrence of network radio problems. Some of the tools are inherent in WALKair and WALKnet products and some requires the use of external measurement equipment.

NOTES: This manual requires the reader to be familiar with the latest WALKair systems different manuals content and release notes.

Part 1: Air link is not rising

iii

Table Of Contents Chapter 1..............................................................1

Step by Step Air link Setup Trouble Shooting....................................... 1 Part 1: Air link is not rising? ................................................................ 1

Is the Terminal scanning frequencies? ....................................................2 Is the Terminal scanning frequencies continually? ..................................3 Is the Terminal scanning intermittently?.................................................4

Part 2: Air link is up, but non-optimized? ............................................ 8 Does the link budget comply with the theoretic value given by the radio planner (up to ±3 dB)? ............................................................................9

Chapter 2............................................................14

Understanding the Air link Setup Process........................................... 14 Base Station Activities ....................................................................... 14 Terminal Station Activities ................................................................ 15

Chapter 3............................................................20

Understanding E1 Alarm Mechanism in WALKair................................ 20 E1 Interface Modes............................................................................. 20

Unframed.............................................................................................20 Transparent – Framed no Signaling ......................................................21 Common Channel Signaling – Framed with V5 Signaling.......................21

Framing Format ................................................................................. 21 Double Frame Format ..........................................................................22 CRC4 Multi-Frame Format ...................................................................23

Fractional E1 Alarms .......................................................................... 24 AIS.......................................................................................................24 RAI ......................................................................................................24


iv

Time Slot 0 and Fractional E1 Service ................................................24 TS BU Rx direction is disconnected ...................................................... 25 TS BU Tx direction is disconnected ..................................................... 26 Radio Link Loss ................................................................................... 27

Chapter 4........................................................... 28

Possible Network Problems and Causes ...............................................28 Base Station Antenna Coverage ..........................................................28

Frequency Reuse in the same base station ........................................... 30 Random Interference ..........................................................................31

Chapter 5........................................................... 32

Analysis Methods................................................................................32 Diagnosis of an interference problem..................................................32 Measuring Interference .......................................................................33

Measuring Uplink ................................................................................ 34 Measuring a WALKair Signal ...............................................................35 Detecting Cable Gain Configuration Problems.....................................35

Base Cable Gain Configuration ............................................................ 36 Terminal Cable Gain Configuration ...................................................... 40


1

Chapter 1

Step by Step Air link Setup Troubleshooting

Part 1: Air link is not rising In this part of the manual step-by-step questions will guide you through a rising of a link.

Before making any failure analysis at the Terminal side (Why doesn't the link rise?), make sure that the Base is indeed operative. This may be done either by:

� Having an operative Terminal in the same sector and making sure that the Base Antenna is aligned correctly (The Terminal must be within the effective Base Antenna lobe. See Annex B for further clarifications).

� Measuring 48 V at the IF-Cable connector at the BS-RFU side.

Once it has been determined that the Base is operative, the problem is presumed to be at the Terminal site.

The next phase is to make sure that the Terminal is indeed scanning frequencies. The next questions shall guide through the first phase.

In order to see which frequency the Terminal is currently scanning, connect to the LCI at the TS-BU and press - and <ENTER>, for a lower level message status (DEBUG).


2

Is the Terminal scanning frequencies? When connected with LCI to the TS-BU the following message should appear in DEBUG message status (after the TS-BU completes its boot process):

Print Display level: INFO

TS#10> Enter Option No: Scanning Frequencies

Scanning Frequencies




Print Display level: DEBUG

TS#10> Enter Option No: Fr#60290: DSP EVENT: long no power

Fr#60290: Fixed Attenuator Gain: Tx 0.00000 Rx 0.00000

Attempt sync on RF Band Index 18, Freq [Up1030750, Dn680750]

Fr#60294: Cable Gain: Tx -24.00000 Rx -47.00000

Fr#60311: DSP EVENT: long no power


Attempt Sync on RF Band Index 19, Freq [Up1032500, Dn682500]




Attempt Sync on RF Band Index 20, Freq [Up1034250, Dn684250]



In this case:

� Enable the Terminals' administrative status.


3

Is the Terminal scanning frequencies continually?

In this case the terminal scans frequencies and does not detect any power on each of the frequencies and doesn't "stop" on a specific frequency. A continuous printout on the screen of the frequency appears.

If this is the case:

� Make sure that all the radio configurations are correct (especially RFU-Head-Type).

� Measure the IF cable for short/cut out.

� Measure 48 V (minimum, nominal should be around 56 V) at the TS-BU IF port.

� Measure 48 V (minimum, nominal should be around 56 V) at IF cable (TS-RFU side). This test shall make sure that 48 V is supplied to the TS-RFU.

If all the measurements and configurations are OK, do the following:

If there is another operative link in this sector:

� Align Terminal Antenna.

� Confirm Base Antenna alignment.

� Replace TS-RFU.

� Replace TS-BU.

If there are no other operative links in this sector:

� Confirm Base antenna alignment.

� Confirm 48V at the IF cable (RFU-BS side).

� Replace BS-BU.

� Replace IF-MUX (before replacing, check fuse in back of IF-MUX).

� Replace BS-RFU.


4

� Replace BS-Antenna.

Is the Terminal scanning intermittently?

This is the case when the terminal scans and once in a while stops and tries to establish a link after synchronizing on a specific frequency. Scanning Frequencies

Sync on RF Band Index 0, Freq [Up999250,Dn649250] - wait for SEARCH





Changing the display mode to DEBUG will supply more information about the cause of the failure.

Each message provided by the TS-BU indicates a different failure. These messages are divided into two different categories (for each category a different question is asked and answered):

� Misconfiguration problem.

� Transmission/Receiving power problem.

Q: Misconfiguration Problem?

There are two different symptoms that indicate two different kinds of misconfiguration:

1 Having no Terminals at all configured at the base will result in the next message:



Fr#11466: DSP EVENT: pwr detect


5

Fr#11759: DSP EVENT: rx ready

Fr#11764: Missing TS number 0


2 Having a Terminal not defined and enabled at the Base (but other Terminal-Customer-ID are), will result in the following message:




Fr#405567: DSP EVENT: rx ready

Sync on RF Band Index 0, Freq [Up999250,Dn649250] - wait for SEARCH

Fr#415566: Time Out: WAIT_SEARCH


Resolution:

In this case, make sure that the terminal is registered at the BS-BU. Make sure that this is the correct BS-BU at the sector (by frequency).

Q: Transmission/Receiving Power Problem?

Each of the following messages may appear as a result of:

� Weak signal received at the Terminal.

� Weak signal received at the Base (as a result of a weak signal transmitted by the Terminal).




Fr#1904: DSP EVENT: sync fail


6


Print Display Level: INFO

TS#10> Enter Option No: Scanning Frequencies


Sync on RF Band Index 0, Freq [Up999250, Dn649250] - wait for SEARCH


=>AirLink removed (By RLC, Fr#16).

=>AirLink removed (By Rx-CRC, Fr#13548014).

In order to pinpoint the problem source (Base or Terminal) the next question needs to be answered:

Q: Are there other TSs operative in the same sector?

A: Yes, there are other TSs operative at the same sector. (if not go to the following answer).

In this case:

Verify that the Terminal reaches the next phase upon other frequencies (BS-BU) at the same sector:

Sync on RF Band Index 0, Freq [Up999250, Dn649250] - wait for SEARCH

Fr#415566: Time Out: WAIT_SEARCH

If the Terminal does reach such a state:

� Verify configuration at relevant BS-BU.

� Replace BS-BU.

� Replace TS-BU.

If the Terminal does not reach the phase mentioned upon the operative frequencies:

� Align the TS-RFU.


7

� Replace TS-RFU.

� Replace TS-BU.

� Verify alignment of BS Antenna.

� Replace BS-BU.

No, this is the only TS in the sector.

In this case:

At the Terminal location:

� Align the TS-RFU.

� Replace TS-BU.

� Replace TS-RFU.

If all the above fails at the Terminal location, at the Base location:

� Replace BS-BU.

� Replace BS-RFU.


8

Part 2: Air link is up, but non-optimized?

A WALKair link is approved to be an operative (and optimized) one if all of the following questions are answered affirmatively:

� Does the link budget comply with the theoretical calculation given by radio planner (up to ±3 dB)?

� Are Up and Down SNR figures above 22 dB? If not, see the WALKair advance troubleshooting manual for more information.

� Were errors recorded on the link in the link approval process? If not, see the WALKair advance troubleshooting manual for more information.

� Is the link stable (does not fail once in a few minutes/hours)? If not, see the WALKair advance troubleshooting manual for more information.

NOTE: If the link was established on an incorrect frequency, check RFU-Head-Type configuration.

A step-by-step process will guide you through a process to correct the particular problem of the link.


9

Does the link budget comply with the theoretic value given by the radio planner (up to ±3 dB)?

The theoretic calculation of fade margin is the expected attenuation between the Base and Terminal Antenna. By pre-knowing the fade margin, one may predict the power received/transmitted at the Terminal.

The compliance between the expected power and the power reported by the system may vary by ±3 dB. Please see Appendix A for an example of fade margin calculation in cases of free space, clear line of sight situations and its use in the system.

In a case when the power calculation does not comply with the power reported by the system the following should be applied.

First make sure that the RFU-Head-Type is configured correctly - mind the next note:

NOTE: In v4.2 and below please make sure that for RFU type labeled as A and B the RFU revision type should be A, and for RFU type labeled as C revision B should be configured.

In case of another link in the same sector

This is the case when there is another operational link in the same sector. There are deviations between the expected and reported power at the Terminal. The deviation may appear in three types (A,B and C):

Type A

Deviation Type:

Transmit power reported is higher than expected and Received power is reported lower than expected


10

Resolution

Terminal site:

� Make sure that IF-cable gain is configured properly.

� Align the Terminal RFU Antenna.

� Make sure that there are no near objects in front of the TS-RFU. Such an object may be a metal bar or buildings that block the line of site to the Base.

Base site:

� Make sure that there are no objects in front of the Base Antenna.

Figure 1: Antenna With Clear View

� Make sure that the Terminal is within the Base Antenna effective lobe (see Annex B for effective Base Antenna Lobe). If not, align the Base Antenna (vertically and horizontally).

Type B

Deviation Type:

� Transmit power reported is higher than expected and Received power reported is as expected.

� Transmit power reported is as expected and Received power reported is higher than expected.

Resolution


11

� Check that the cable gain at the Terminal is not higher than real.

Type C

Deviation Type:

� Transmit power reported is as expected and Received power reported is lower than expected.

� Transmit power reported is lower than expected and Received power reported is as expected.

Resolution

� Check that the cable gain at the Terminal is not lower than real.

� If all other fails:

� At the Terminal site:

Replace TS-RFU. •

•

•

•

Replace TS-BU.

� At the Base (only if the current BS-BU is not connected to an operational Terminal):

Connect the BS-BU to a different port in the IF-MUX.

Replace the BS-BU.

There are no other links in this sector.

This is the case when there is only a single Terminal in a sector. The deviations between the reported and the calculated power Transmitted/Received by the Terminal are of three types.

Here are the three different types and the resolution of each deviation type.

Type A

Deviation Type:

Transmit power reported is higher than expected and Received power reported is lower than expected


12

Resolution:

Base site:

� Make sure that IF-cable gain is configured correctly (see Preface: Cable Gain for more details).

� Make sure that RFU head type is configured correctly as appears on the silver label on the RFU. (Except 3.5GHz RFU revision B that has to be configured as revision A)

� Make sure that there are no close objects in front of the Base Antenna (Figure 1).

� Make sure that the Terminal is within the Base Antenna effective lobe (see Annex B for effective Base Antenna lobe). If not, align the Base Antenna (vertically and horizontally).

Terminal Site

� Make sure that IF-cable gain is configured properly.

� Align the Terminal RFU-Antenna.

� Make sure that there are no near objects in front of the TS-RFU. Such object may be a metal bar or buildings that block the line of site to the Base.

NOTE: Only if all other measure taken have failed:

Replace BS-BU. Replace IF-MUX. Replace BS-RFU. Replace TS-BU. Replace TS-RFU.

•

•

•

•

•

Type B

Deviation Type

� Transmit power reported is higher than expected and Received power reported is as expected

� Transmit power reported is as expected and Received power reported is higher than expected


13

Resolution

� Check that the cable gain at the Terminal and at the Base is not higher than real.

Type C

Deviation Type

� Transmit power reported is as expected and Received power reported is lower than expected

� Transmit power reported is lower than expected and Received power reported is as expected

Resolution

� Check that the cable gain at the Terminal is not lower than real.

� If all else fails:

� At the Terminal site:

Replace TS-RFU. •

•

•

•

•

•

•

Replace TS-BU.

� At the Base:

Connect BS-BU to a different port in the IF-MUX.

Replace BS-BU.

If all the links in the sector have the same phenomenon, replace IF-MUX.

If all the links in the sector have the same phenomenon, replace BS-RFU. (If possible switch to redundant RFU and antenna).

Replace Base Antenna. (If possible switch to redundant RFU and antenna).


14

Chapter 2

Understanding the Air link Setup Process

This chapter provides a better understanding of the process in which the WALKair system sets up an air link between a BS BU and the TS BU. Various link initialization scenarios are described, including full descriptions of TS LCI messages during the link establishment leading to easier trouble shooting.

First the Base Station activities in the process will be described and next the TS activities.

Base Station Activities � Installing base station and frequency selection

As the BS BU enabled and frequency is selected, a PRBS is transmitted on the desired channel, resulting a continues 1.75 MHz of bandwidth radio signal, as if traffic is transmitted. (All information available on Installation Manual and Commissioning Manual)

� Configuration:

� List of associated terminal stations

� Services (Not mandatory for link establishing)

Terminal Station Activities

15

� Search messages are sent to the associated terminal stations, allocated over the EOC channel. Embedded Operation Channel (EOC) is used for signaling, Management & Maintenance EOC/Signaling. The Base Station will provide the Terminal Station all the necessary service configuration parameters over the EOC channel (in the end of the process). The Terminal Station sets EOC signaling bit until it receives an access from the Base Station. The search message will continued to be sent to each terminal the haven’t completed initilization with the base yet.

Terminal Station Activities � Installing the terminal station

(All information available on Installation Manual and Commissioning Manual)

� Configuration: � Radio parameters

� ID Number

� Scanning Frequencies In this stage the TS is scanning frequencies and transmits nothing into the air. The scanning will start from the first frequency index in the configured frequency sub band to the last one cyclically and it will continue to do so until power will be detected and the MODEM is able to synchronize on the received signal.

On the LCI capture below the “Scanning Frequencies” message can be viewed, below it, more detailed information is displayed. By pressing the “-“ symbol, the display level shifts to Debug, then the following information can be observed: � Attempt sync on RF Band Index 18 – The RF frequency

the TS BU is currently tuned on and waiting for power detect.


16

� The configured cable configured in the system for this frequency index

� DSP EVENT: a series of events that are presented by the DSP MODEM according to the relevant scenario: •

•

•

•

long no power – No power has been detected. pwr detect – Power has been detected. The reason for the power (modulation, center frequency, etc.). It could be interference as well.

sync fail – After power has been detected, the MODEM attempts to synchronize on the received signal, but fails to do so.

rx ready – The MODEM succeeded to demodulate the tuned signal.

Scanning Frequencies Scanning Frequencies Scanning Frequencies Scanning Frequencies TS#20> Enter Option No.:- Print Display Level: DEBUG Attempt sync on RF Band Index 18, Freq [Up3433625,Dn3533625] Fr#588375: Cable Gain: Tx -23.00000 Rx -23.00000 Fr#588392: DSP EVENT: long no power Fr#588392: Fixed Attenuator Gain: Tx 0.00000 Rx 0.00000 Attempt sync on RF Band Index 19, Freq [Up3435375,Dn3535375] Fr#588396: Cable Gain: Tx -23.00000 Rx -23.00000 Fr#588413: DSP EVENT: long no power Fr#588413: Fixed Attenuator Gain: Tx 0.00000 Rx 0.00000 Attempt sync on RF Band Index 20, Freq [Up3437125,Dn3537125] Fr#588417: Cable Gain: Tx -23.00000 Rx -23.00000 Fr#588434: DSP EVENT: long no power Fr#588434: Fixed Attenuator Gain: Tx 0.00000 Rx 0.00000 Attempt sync on RF Band Index 21, Freq [Up3438875,Dn3538875] Fr#588438: Cable Gain: Tx -23.00000 Rx -23.00000 Fr#588454: DSP EVENT: long no power

� Synchronization Sync on RF Band Index 17, Freq [Up3431000,Dn3531000] – Synchronization on WALKair air protocol frame succeeded. This message appears right after the message rx ready.


17

� Authentication wait for SEARCH – In this stage, the TS BU waits for the search message coming from the base station. The search message transmitted on the base EOC channel contains an ID number that is being compared to the ID number of the TS, as they match the process of link initialization continues. If a search message was received but ID’s do not match the following message is displayed: Missing TS number 0

If it does match the following will be displayed: DLC search


18

The following is a flow chart describing the three phases, Scanning, Synchronization and authentication.

ScanningFrequency

Power Detect ?"Long no pwr"

Synchronization &DemodulationSucceeded ?

"wait forsearch"

Continue to PowerEqualization

"Pwr Detect"

"rx ready"

"DLC search"

"sync fail"

"missing TS # 0"or

"Time Out: WAIT_SEARCH'

Next frequencyindex

Figure 2: Link Initialization Phases


19

� Power Equalization In this stage the first transmission of the TS BU begins by transmitting an initial low power signal. Followed by commands from the base station the TS BU increases or decreases the transmitted power in order to meet a desired received power level at the base station. In other words this is the RTPC process – Remote Transmit Power Control. This process continues as long as the link is up.

� Distance measurement The BS BU is measuring the distance to the TS BU in order to compensate over the various delays to each terminal. Since the terminal is not mobile, this process happens only during the initialization phase.

In the end of the measurement the following is displayed: Estimated dist: 0 Adjustment: 0 symbols. (Debug Init Dist: 0)”; “DLC distance “; “DISTANCE –8.

� Equalizer Training The BS BU MODEM learns the path characteristics from a preamble series transmitted from the terminal station. DSP equalizer in the BS BU compensates over it. The process is automatic and continues as long as the link is up. No message is displayed on the LCI.

� TS Sends Parameters Before the base station will provide the Terminal Station all the necessary configuration parameters over the EOC channel, the terminal station will send its hardware information to the base.

� Operational Service The service begins as the hardware parameters of the TS BU meet the requirement of base station service configuration. After completing and passing this stage, the air link is formally established.


20

Chapter 3

Understanding E1 Alarm Mechanism in WALKair

The purpose of this chapter is to describe adherently the signaling over the E1 and fractional E1 services in general and in WALKair. The WALKair system is complied with the G703 standard, still the signaling issue in Point to Multi Point fractional E1 service remains confusing. The main aspects of the signaling discussed in this chapter are framing, alarming and CRC (on channel 0), examples will follow the explanations.

E1 Interface Modes

Unframed In the unframed mode the E1 Framer is not looking on time slot 0. It does not reconstruct the 125µsec frame. In this mode there is not any meaning to the 32 time slot structure. The whole traffic is used a one 2Mbit/stream.

In the conversion to the BI internal highway the E1 transceiver generate a 125µsec frame. The frame start is set once bit synchronization is achieved. Every time a different start location will be selected. Base on this random frame start the content of the internal highway timeslots is defined.

Framing Format

21

In unframed mode there is no meaning to FE1 service.

Transparent – Framed no Signaling In this mode the E1 framer is set to work in framed mode. Hence, in addition to reconstructing the 2Mbit/Sec stream, the framer also construct a 125µsec frame based on time slot 0.

The BS/TS BU refers to time slot 16 as a regular traffic time slot.

Common Channel Signaling – Framed with V5 Signaling

In this mode the E1 framer is set to work in framed mode.

Time slot 16 is assumed to have V5.x HDLC common channel signaling protocol protocol. The BS/TS BU will terminate this time slot and will transfer the relevant information over the EOC channel (Embedded Operation and Control Channel - system over head channel).

Starting WALKair version 5.4 HDLC signaling is supported over channels 15 and/or 31 as well, depended on the user configuration. The same process will be with channels 15 and 31, hence, they will be terminated by the BS BU and the relevant signaling for each TS will be sent over the overhead channel respectively.

Framing Format The framing structure is defined by the contents of time-slot 0


22

Double Frame Format Table 1: E1 Time Slot zero of double frame format

1 2 3 4 5 6 7 8

Frame Containing the Frame Alignment Signal

Si 0 0 1 1 0 1 1

Note 1

Note 1 Frame Alignment Signal

Frame not Containing the Frame Alignment Signal

Si 1 A Sa4 Sa5 Sa6 Sa7 Sa8

Note:

1. Si bits: reserved for international use. If not used, these bits should be fixed to ‘1’. 2. Fixed to ‘1’. Used for synchronization. 3. Remote alarm indication: In undisturbed operation ‘0’; in alarm condition ‘1’. Sa bits: Reserved for national use. If not used, they should be fixed at ‘1’.

Framing Format

23

CRC4 Multi-Frame Format Table 2: E1 time slot zero of multi - frame format

Sub-

Multi-frame

Frame

Num

1 2 3 4 5 6 7 8

0 C1 0 0 1 1 0 1 1

1 0 1 A Sa4 Sa5 Sa6 Sa7 Sa8

2 C2 0 0 1 1 0 1 1


4 C3 0 0 1 1 0 1 1


6 C4 0 0 1 1 0 1 1

1


0 C1 0 0 1 1 0 1 1


2 C2 0 0 1 1 0 1 1


4 C3 0 0 1 1 0 1 1

5 E* 1 A Sa4 Sa5 Sa6 Sa7 Sa8

6 C4 0 0 1 1 0 1 1

2

7 E* 1 A Sa4 Sa5 Sa6 Sa7 Sa8

E: Spare bits for international use. Sa : Spare bits for national use. C1 …C4 : Cyclic redundancy check bits. A: Remote alarm indication.


24

Fractional E1 Alarms

AIS In general, Alarm Indication Signal is transmitted whenever an RAI is received by an E1 interface.

AIS is different in point to multi point fractional E1 service than full E1 frame point-to-point service. AIS in point to point systems is known as transmission of “All Ones” in all time slots except time slot 0 whereas in fractional E1 service all ones will appear only on the time slots that service is assigned on them.

RAI Remote Alarm Indication is transmitted if the incoming E1 interface losses synchronization of the incoming data, the threshold for that is FAS error ratio greater than 10e-3. RAI sets bit A of the NFAS to “1”.

Time Slot 0 and Fractional E1 Service

In case of framed E1 structure WALKair doesn’t transfer time slot 0 from the receive direction of the E1 frame (to the BU TS/BS) towards the air. In the transmit direction (the outgoing E1 frame from the BU interface), time slot 0 is recreated. The information of time slot 0 is created as follows:

� CRC is calculated according to the time slots coming assigned to the port. Hence, the CRC code is different in BS BU port than from the port in the TS BU containing fractional E1 service.


25

� RAI – Depended on the receive direction of the interface.

� AIS – Depended on the AIS alarm coming from the corresponding E1 interface on the other side of air communication. The alarms are forwarded from BS to TS BU and vice versa over the EOC channels.

� Alarm forwarding scenarios

The following are examples of signaling scenarios in WALKair. The sign red X symbol represents the point of failure. In all the examples E1 service is applied as a physical layer, second layer may be a Leased Line / V5.X / Frame Relay service.

TS BU Rx direction is disconnected In this case the incoming E1 frame to the TS BU telecom interface is disconnected.

The TS BU (ID 20 in the diagram) will transmit RAI to the corresponding telecom equipment.

The BS BU will transmit AIS, all ones only on the time slots that are assigned to the TS BU ID 20 with the faulty connection; those time slots are ones that service is applied to them, V5, Leased Line service or Frame relay. The other time slots that are assigned to TS ID 10, the one working free of fault, will continue to transfer data/voice.This is called fractional E1 AIS.

On the other hand in a vice versa scenario, for instance, if the equipment in the BS BU side was disconnected in the same way as the previous scenario, all the TS BU’s will transmit AIS.


26

Radio Link

AIS

*

RAI

AIS

E1 ServiceE1 ServiceRadio Link

ID 10 ID 20

Figure 3: TS BU Rx Connection is Disconnected

Radio

Link

RAIAIS

E1 S

ervi

ce

TS BU Tx direction is disconnected

The transmitted E1 frame from the TS BU is disconnected, the information doesn’t get to the telecom equipment in the CPE side.

Figure 4: TS Tx transmission towards its telecom equipment is disconnected


27

The telecom equipment towards the BU will generate RAI and the BU will transmit AIS alarm in all time slots in response.

Radio Link Loss

E1 S

ervi

ce

Radio

Link

AIS

AIS

In radio loss scenario, AIS will be transmitted in all 32-time slots of the E1towards the telecom equipment in both sides, BS and TS BU. Even if the telecom equipment in the TS side will be disconnected, AIS will still be transmitted.

Figure 5: Radio Link Loss


28

Chapter 4

Possible Network Problems and Causes

Some link problems do not concern only a specific air link. Since the air is shared with other links and probably other systems, problems can be common to more than one link. Below are examples of cases with common problems to a few terminals.

Base Station Antenna Coverage The effective Base antenna lobe is defined as the field in which the antenna gain is not less than 3 dB from its maximum gain.

The effective lobe is a function of the antenna opening angles (azimuth and elevation). These angles are dependant on the type of antenna chosen for a specific radio planning. An installation mistake can be that the sector antenna is not pointed in the preplanned heading direction. As a result, the terminals that were supposed to be covered will have a degraded performance, lower received power and higher transmit power than expected (measured value compared to calculated value by radio planning or free space). See the diagram below:

Base Station Antenna Coverage

29

Original SectorCoverage

Mistaken ShiftedSector Coverage

Heading of BaseStation Antenna

Terminals with lowreceive power and hightransmit power thanexpected

Figure 6: Base Station Antenna Disalignment

Furthermore, the effect of interfering signals on the uplink performance will be greater as well as the downlink interference to other sector links. An example of this is when the same frequency as this sector is used in the opposite sector that is supposed to point 180° backwards.


30

Frequency Reuse in the same base station

In the following scenario there is an attempt to reuse the same frequency between 2 opposite sectors on the same base station. Mistakenly, the 2 sectors aren’t transmitting the same power (could be deliberately or a user mistake), so that one sector is causing greater interference to the other.

The red sector in the illustration below is transmitting at a greater power than the blue one. The result is a lower C/I ratio to the terminals associated to the blue sector, hence, low SNR, bit errors, while the red sectors performance is not degraded.

Naturally it is obvious that frequency reuse will degrade the uplink and downlink SNR, however the degradation should be symmetrical to both sectors.

Base Station

C/I

FundementalCarrier

InterferingCarrier

C/I

FundementalCarrier

InterferingCarrier

Figure 7: Frequency Reuse in Same Base Station

Random Interference

31

See page 31 for details on how to measure C/I value.

Random Interference The WALKair system works in regulated frequency bands. Hiring radio spectrum should assure freedom from external interference spectrum. A deployed network normally has interferences but deterministic ones that are due to reuse of frequencies. External interferences are those that do are not come from signals transmitted by the deployment of the WALKair system, but probably by other operators, radars or other kinds of radio transmitter. In some cases the required spectrum or some of it may not be clear of interference and errors will occur. Interference can come in variety of ways; short bursts ones, over wide band of spectrum, constant ones.

The symptoms of these kinds of interference can be either the appearance of bit errors or even the loss of the air link for a while. Interference can be detected easily prior to the deployment of the system by performing a radio survey as recommended in the WALKair installation manual. As the spectrum is occupied by the signals of the WALKair system it becomes very difficult to detect interference because it requires disabling links/carriers that are suspected of suffering from interference. The chapter “Analysis Methods” offers a way to evaluate interference based on the measurements presented by WALKnet performance monitoring.


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Chapter 5

Analysis Methods

Diagnosis of an interference problem

When the symptoms of a problem are detected it is possible to observe the WALKnet system’s air performance behavior and in most cases determine whether the cause is radio interference or something else.

Activate the air performance monitoring for the required terminal and display in 15 minutes intervals the last 24 hours and follow the instructions below.

Start by differentiating the performance of the uplink channel from the downlink channel. In this way the analysis can be more focused. An interfered with link is characterized by one or more of the following symptoms:

� Bad FER (Frame Error Rate), less than 10e-6

� Minimum SNR lower than 22dB in one or more intervals of 15 minutes

� More than 1% of Error Seconds

� Appearance of unavailable seconds

Measuring Interference

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It is not enough to conclude only by these symptoms that there is interference. The same symptoms may appear if the Received signal of a terminal or base station is varied over a large range of power and also drops to lower values than expected. In case of low received power the SNR is degraded naturally to the lower signal.

In the case of random interference usually low SNR along with bit errors will appear when the received power remains constant, in other words, the signal level is constant and the interference level is increasing.

Constant interference is harder to analyze, especially in deployed networks, because there is no reference of times where the link had good performance relative to a time where random interference appears. However certain power levels should match certain values of SNR and when the SNR value is constantly low and the Received power is ok, then this might be an indication of interference.

Deployment of a network takes in account during the radio-planning phase the predicted level of interference (one that is caused due to reuse of frequencies) for every customer. It is important to know these values and compare them with the system performance.

After it is determined that the errors are caused either by a stronger level of interference than predicted in radio planning or by random interference we should verify it by measurement. In order to measure interference we should disable the transmission of the interfered carrier and use a spectrum analyzer as described in the next section.

Measuring Interference In order to rule out an interference problem, one may measure the interference in the WALKair IF.


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Measuring Uplink An Uplink IF measurement is executed by connecting the Spectrum Analyzer to the Rx port (any free port) of the IF-MUX. An illustration of the connection appears in the following figure:

Figure 8: Uplink IF Measurement

Measuring a WALKair Signal

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Measuring a WALKair Signal The following table summarizes the WALKair signal characteristics versus the setting of a spectrum analyzer that is used to measure it.

WALKair Signal Characteristics Spectrum Analyzer Settings for Proper Power Measurement

1.75 MHz wide 64QAM modulated signal.

RBW: Optimal is 1.75 MHz. 1 MHz will give close results.

Pseudo Random Signal. Detection type: normal. Storage type: Average (more than 100 counts).

TDMA transmission: Downlink is a constant transmission of the BS, ergo the signal is constantly operative.

Uplink is operative only when bandwidth is allocated for the TS transmitting.

In case of Downlink the measurement is exact.

In case of Uplink measurement, the power measured is a function of how many timeslots are allocated for the transmitting TS.

NOTE Using a calibrated spectrum analyzer with the above settings shall result in a measurement error of up to ±2 dB.

Detecting Cable Gain Configuration Problems

The cable gain in WALKair is the configuration as set by the WALKair commissioner that informs the system of the IF cable attenuation (connecting the indoor unit (BU) with the outdoor unit (RFU)).


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The system uses the configured figure in its calculation of the overall gain of the system.

NOTE: Cable gain is always set as a negative figure.

Incorrect cable gain configuration may cause a number of different scenarios described below:

Base Cable Gain Configuration At the Base, variation of the configured figures from the real cable gain shall directly affect the power transmitted by the Base and the power transmitted by the Terminal.

Transmit Cable Gain

In the Tx path (see Figure 9), a higher configured cable gain (in absolute value) than the Real cable attenuation (in absolute value) shall result in higher transmitted power. Lower configured cable gain (in absolute value) than the real cable attenuation (in absolute value) shall result in lower transmitted power. Example:

� If the real cable attenuation is 15 dB, and the cable gain is configured at 10 dB (+10 dB in absolute value), (variation of 5 dB lower), then the power transmitted shall be 5 dB lower. Thus the power transmitted shall be:

Expected transmitted power - deviation = Real transmitted power.

+18 dBm (expected by the system) - 5 dB (deviation) = 13 dBm

� If the real cable attenuation is 15 dB, and the cable gain is configured at -20 dB (+20 dB in absolute value), (variation of 5 dB higher), then the power transmitted shall be 5 dB lower. Thus the power transmitted shall be:

Expected transmitted power + deviation = Real transmitted power.


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18 dBm (expected by the system + 5 dB (deviation) = 23 dBm.

NOTE: One always must have in mind the 1dB compression point when transmitting higher than optimal transmitted power.

Figure 9: Transmit Cable Gain

Receive Cable Gain In the Rx path the Base regulates the Terminals' Transmit power (using the RTPC mechanism). The Terminals' power is set to a point where the Receive power at the base is the MODEM working point (default is -80 dBm).

As the gain of each unit in the system is pre-known (RFU, IF-MUX, cable gain), a power of -80 dBm is estimated by the system. The system measures the power received by the BU and then subtracts the pre-known overall gain of the system.

Thus, an incorrectly configured cable gain will cause a wrong estimation of the power received by the Base.


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In the Rx path (see Figure 10), a higher configured (larger absolutely) cable gain than the real cable attenuation shall result in lower Terminal Transmit power and thus lower "real" received base power (poorer SNR). A lower configured (smaller absolutely) cable gain than the real cable attenuation shall result in higher transmitted Terminal power and thus higher "real" received base power (better SNR).

Figure 10: Receive Cable Gain

Example:

� If the real cable attenuation is 15 dB, and is configured at 10 dB, (variation of 5 dB lower), then the power transmitted by the Terminal shall be 5 dB higher, and the actual received power level at the Base antenna shall be 5 dB higher. Thus the power received shall be:

power receive expected + deviation = power received

-80 dBm + 5 dB = -75 dBm


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� If the real cable attenuation is 15 dB, and is configured at 20 dB, (variation of 5 dB higher), then the power transmitted by the Terminal shall be 5 dB lower, and the actual received power level at the Base antenna shall be 5 dB lower. Thus the power received shall be:

power receive expected - deviation = power received

-80 dBm + 5 dB = -85 dBm

NOTE: The real power received by the Base may be evaluated by the receive SNR. When an SNR of 25 dB is obtained, the real power received at the Base antenna is -80 dBm.


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Terminal Cable Gain Configuration At the Terminal the configured cable gain does not directly affect the power transmitted by the Terminal but only limits it in extreme cases (see note at the end of the paragraph).

Transmit Cable Gain

In the Tx path (see Figure 11), a higher configured cable gain (in absolute value) than the real cable attenuation (in absolute value) shall result in higher reported Transmit power, and does not affect the power transmitted (set by Base). A lower configured cable gain (in absolute value) than the real cable attenuation (in absolute value) shall result in lower reported Transmit power, although the real Transmit power does not change.

Figure 11: Terminal Transmit Cable Gain


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Example:

� If the real cable attenuation is 10 dB, the power transmitted is 0 dBm (set by Base), and the cable gain is configured at -15 dB (+15 dB in absolute value), (variation of 5 dB higher), then the Transmit power reported by the TS-BU shall be 5 dB higher. The real power transmitted is still 0 dBm. Thus the Transmit power reported shall be: real power transmitted + deviation = reported power

0 dBm + 5 dB = 5 dBm

� If the real cable attenuation is 10 dB, and the cable gain is configured at -5 dB (+5 dB in absolute value), (variation of 5 dB lower), then the power reported by the TS-BU shall be 5 dB lower. The real power transmitted is 0 dBm. Thus the transmit power reported shall be:

real power transmitted - deviation = reported power

0 dBm - 5 dB = -5 dBm


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Receive Cable Gain

In the Rx path (see Figure 12), a higher configured cable gain (in absolute value) than the real cable attenuation (in absolute value) shall result in higher reported Receive power, and does not affect the power transmitted (set by Base). A lower configured cable gain (in absolute value) than the real cable attenuation (in absolute value) shall result in lower reported received power, although the real received power does not change.

Figure 12: Terminal Receive Cable Gain

Example:

� If the real cable attenuation is 10 dB, the power received is 0 dBm (received from Base), and the cable gain is configured at -15 dB (+15 dB in absolute value), (variation of 5 dB higher), then the Transmit power reported by the TS-BU shall be 5 dB higher. The real power transmitted is still 0 dBm. Thus the Transmit power reported shall be:

real power received + deviation = reported power

-65 dBm + 5dB = -60 dBm

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� If the real cable attenuation is 10 dB, the power received is -65 dBm (received from Base) and the cable gain is configured at -5 dB (+5 dB in absolute value), (variation of 5 dB lower), then the power reported by the TS-BU shall be 5 dB lower. The real power transmitted is still -65 dBm, and the transmit power reported shall be:

real power transmitted - deviation = reported power

-65 dBm - 5 dB = -70 dBm

NOTE: Although the Terminal Transmit power is automatically set by the Base using the RTPC process, there is maximal Transmit power limitation at the Terminal Radio parameters menu - Modem working point option. This option will limit the Transmit power of the Terminal if the power reported by the TS-BU shall exceed the limitation. If an incorrect cable gain is inserted (at the Tx path), a link may not rise as a result of insufficient power transmitted.


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Appendix A - Calculating received and transmit power levels of a terminal station based on a clear line of sight link and using free space model.

All calculations of fade margin in this Annex apply to good weather conditions (that is without rain, snow, fog or any other harsh conditions).

In order to calculate fade margins in Free Space the following formula may be used:

)log(205.92 kmGHzdB DFL ×+

When:

L - The Free Space loss, in dB.

F - The frequency Band (3.5 Ghz; 10.5 Ghz; 26 Ghz).

D - Distance from Base to Terminal, in km.

In order to find the loss between the two antennas (Terminal and Base), the Gain of each antenna needs to be subtracted from the Total Fade Margin Loss. All the antenna gains appear in the following table:

Table 3: Antenna Gains and Opening Angles

Band Gain [dBi]

Elevation Opening Angle [deg]

Antenna Type and Azimuth Angle [deg]

26Ghz 36 ±2.6° Terminal

26Ghz 42 ±1.3° Terminal

26Ghz 18 ±9° Base 45° Vertical

26Ghz 18 ±9° Base 45° Horizontal

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Band Gain [dBi]

Elevation Opening Angle [deg]

Antenna Type and Azimuth Angle [deg]

26Ghz 15 ±9° Base 90° Vertical

26Ghz 15 ±9° Base 90° Horizontal

3.5Ghz 18 ±20° Terminal

3.5Ghz 16 ±9° Base 60° Vertical

3.5Ghz 16 ±8° Base 60° Horizontal



10.5Ghz 25 ±8° Terminal



10.5Ghz 16 ±4.5° Base 90° Vertical

10.5Ghz 16 ±4.5° Base 90° Horizontal

Thus the loss between Terminal and Base shall be:

)()()()( dBAntennaalminTerdBiAntennaBasedBinargMFadedBTotal GGLL

Now that the Total Loss is calculated, the power expected to be transmitted/received by the Terminal may be calculated as well:

)()()( dBTotaldBmBaseTxdBmRx LPP

)()()( dBTotaldBmBaseRxdBmTx LPP

Example:


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In a WALKair 3.5 GHz system, the base antenna lobe is 60° and the distance between Base and Terminal is 1 km. The Base is working in default MODEM working point.

What is the expected Terminal Transmit/Receive power?

Figure 13: Example for Calculating Expected Terminal Transmit/Receive Power

Resolution:

First, calculate the Free Space fade margin:

dBinargMFadeL 103)15.3log(205.92 ×

Now subtract the two antenna gains. At 3.5 GHz, the gain of a 60° is 16 dBi. The gain of Terminal antenna at 3.5 GHz is 18 dBi.

Thus the total loss is:

dBdBdBdBiTotalL 691618103 −−

Default Base MODEM working point values at 3.5 GHz is -80 dBm Receive power and +18 dBm Transmit power.

Thus the power received and transmitted at the Terminal should be:

dBmdBdBmRxP 516918

dBmdBdBmTxP 116980

NOTE: When reported by the system these values may vary by ±3 dB.

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Documents

Troubleshooting Radio Links Guide