Senin, 23 Mei 2011

3GPP LTE and LTE Advanced

LTE PLANNING

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LTE Tutorials

Selasa, 17 Mei 2011

Introduction to Adaptive Antenna System (AAS)

An Adaptive Antenna System (AAS) can focus its transmit energy to the direction of a receiver. While receiving, it can focus to the direction of the transmitting device. The technique used in AAS is known as beamforming or beamsteering or beamshaping. It works by adjusting the width and the angle of the antenna radiation pattern (a.k.a. the beam).

beamforming or beamsteering (AAS) in WiMAX, the main lobe is focused to the receiver

Figure. WiMAX BS with AAS beamforming capability
The main lobe is directed toward a subscriber station (SS).

Combined with multiple antennas in the Base Station (BS), AAS can be used to serve multiple Subscriber Stations (SSs) with higher throughput. A technique known as SDMA (Space Division Multiple Access) is employed here where multiple SSs that are separated (in space) can transmit and receive at the same time over the same sub-channel.

Multiple antennas in the WiMAX Base Station combined with AAS results in higher throughput per Subscriber Station

Figure. WiMAX BS with multiple antennas and AAS
AAS and multiple antennas combined in the BS can increase per user data rate.

AAS also eliminates interference to and from other SSs and other sources by steering the nulls to the direction of interferers. AAS is an optional feature in WiMAX and not included in WiMAX certification. But due to its effectiveness in improving performance and coverage especially in Mobile WiMAX case, many vendors integrate AAS capability into their products.


Handoff mechanism in Mobile WiMAX

For implementing a mobile network, a handoff mechanism must be defined to maintain uninterrupted user communication session during his/her movement from one location to another. Handoff mechanism handles subscriber station (SS) switching from one Base Station (BS) to another. Different handoff techniques have been developed. In general, they can be divided into soft handoff and hard handoff.

soft handoff in a voice-centric network

Figure. Soft Handoff
A SS maintains multiple connections. Delay is very minimal

Soft handoff is used in voice-centric cellular networks such as GSM or CDMA. It uses a make-before-break approach whereas a connection to the next BS is established before a SS leaves an ongoing connection to a BS. This technique is suitable to handle voice and other latency-sensitive services such as Internet multiplayer game and video conference. When used for delivering data traffic (such as web browsing and e-mail), soft handoff will result in lower spectral efficiency because this type of traffic is bursty and does not require continues handover from one BS to another.

hard handoff in Mobile WiMAX

Figure. Hard Handoff
A SS maintains a connection to a single BS at any given time.

Mobile WiMAX has been designed from the outset as a broadband technology capable of delivering triple play services (voice, data, video). However, a typical Mobile WiMAX network is supposedly dominated by delay-tolerant data traffic. Voice in Mobile WiMAX is packetized (what is called VoIP) and treated as other types of IP packets except it is prioritized. Hard handoff (HHO) is therefore used in Mobile WiMAX. In hard handoff, a connection with a BS is ended first before a SS switches to another BS. This is known as a break-before-make approach. Hard handoff is more bandwidth-efficient than soft handoff, but it causes longer delay. A network-optimized hard handoff mechanism was developed for Mobile WiMAX to keep a handoff delay under 50 ms.

Minggu, 15 Mei 2011

How to Plan MAIO and HSN

For telecom engineers, it is very help full to know about MAIO. Basically there is a set of frequencies where the channels are hop to avoid interference that set of frequencies is called MA-LIST. suppose MALIST contained frequencies [A,B,C,D,E,F,G,H,I,J,K,L].

Then MAIO corresponds the starting point of hopping sequence. suppose there are four TRx in one sector. then MAIO will be, for example, 0,2,4.
for first TRx for traffic, the MAIO is 0. means that hopping sequence starts from first frequency in the MALIST, that is frequency A. and for MAIO 2 it starts from frequency C and for MAIO 4 it starts from frequency E.

Also HSN (hopping sequence number), ranges from 0 to 63 . if we use HSN as, for example 4 it means that for MAIO 0, the sequence starts from frequency A and repeats frequencies in following manner.

For MAIO 0 and HSN 4
A,F,K,D,I,B,G,L,E,J,C,H,A,F

For MAIO 2 and HSN 4
C,H,A,F,K,D,I,B,G,L,E,J,C,H

For MAIO 4 and HSN 4
E,J,C,H,A,F,K,D,I,B,G,L,E,J

Sabtu, 14 Mei 2011

Call Drop optimization

1. Radio Link Time-Out

Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the message can be decoded the counter is incremented by 2. However, the value can not exceed the initial value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by RLINKUP for timeout in the BSC. If the mobile moves out of coverage and no measurement reports are received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request message is sent to the MSC. To be sure that the mobile has stopped transmitting, the BSC now waits RLINKT SACCH periods before the timeslot is released and a new call can be established on the channel.
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2. Layer 2 Time-Out
If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.

3. Release Indication
When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the mobile station and a Release Indication to the BSC. The system does only react on Release Indication if it is received during a normal disconnection situation. If such a message is received unexpectedly this will usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of measurement reports. It is also possible that the release will be normal depending on when the Release Indication is received.
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4. MSC Time-Out
Normal Release:
If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC will send a Channel Release (cause: normal release) and then deactivates the SACCH.
Reject (only SDCCH):
If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear Command to the BSC and the call is cleared by Channel Release (cause: normal release).
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5. Assignment to TCH
Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:
a. There must be a TCH channel available, i.e. no congestion
b. The locating algorithm must have received at least one valid measurement report.
If either of the criterion is not fulfilled, Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be sent to the mobile station and a Clear Request to the MSC.
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TCH Drop reason (1)
The classification of TCH Drop Reasons are arranged in the order of priority:
1.Excessive Timing Advance
2.Low Signal Strength
3.Bad Quality
4.Sudden Loss of Connection
5.Other Reasons

Excessive Timing Advance
The TCH Drop counters due to Excessive Timing Advance will pegged when the during the time of disconnection, the last Timing Advance value recorded was higher than the TALIM Parameter. This drop reason is commonly apparent to isolated or island sites with a wide coverage area.
Action:
Check if the cell parameter TALIM is < "63"
Solution:
Set TALIM to a value close to 63.
Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.


TCH Drop Reasons (2)
Low Signal Strength on Down or Uplink or Both Links
The drops counters due to Low Signal Strength will be pegged when the Signal Strength during the last Measurement Report before the call dropped is below the LOWSSDL and/or LOWSSUL Thresholds. LOWSSDL and LOWSSUL are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Signal Strength are below the thresholds, only Drop due to Low SS BL will pegged. Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage will result in dropped calls. Building shadowing could be another reason.
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Action:
Check coverage plots.
Check output power.
Check power balance and link budget.
Check if Omni site.
Check antenna configuration & type.
Check antenna installation.
Perform drive tests & site survey.
Check TRX/TS with high CONERRCNT.

Solution:
Add a repeater to increase coverage in for example a tunnel.
Change to a better antenna (with higher gain) for the base station.
Add a new base station if there are large coverage holes.
Block/Deblock TRX

TCH Drop Reasons (3)
Poor Quality on Down or Uplink or Both Links
The drops counters due to Bad Quality will be pegged when the Signal Strength during the last Measurement Report before the call dropped is above the BADQDL and/or BADQUL Thresholds. BADQDL and BADQUL (expressed in DTQU) are BSC Exchange Property parameters which is used only for statistics purposes and does not affect the behavior of calls. If both UL and DL Quality are above the thresholds, only Drop due to BAD Quality BL will pegged.
Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or TCH. Faulty MAIO assignment can cause frequency collisions on co-sited cells especially on 1x1 Reuse. External interference is also one possible cause of problem on quality.
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Action:
Check C/I and C/A plots.
Check Frequency Plan (Co-BCCH or Co-BSIC Problem).
Check MAIO, HOP, HSN parameters.
Check FHOP if correctly configured (BB or SY).
Check for External Interference.
Perform drive tests.

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Solution:
Change BCCH frequency.
Change BSIC.
Change MAIO, HOP, HSN.
Change FHOP.
Record RIR or on-site Frequency Scanning to identify source of interference.
Use available radio features.

TCH Drop Reasons (4)
Sudden Loss of Connection
Drops due to Sudden Loss are drops that have not been registered as low signal strength, excessive timing advance, bad quality or hardware (other) reasons, and the locating procedure indicates missing measurement results from the MS.
There are some common scenarios that could lead to Sudden Loss of connections such as very sudden and severe drops in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc., very sudden and severe occurrence of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults, Synchronization or A-bis link fault (transmission faults), and
MS Faults.

Action:
Check BTS Error Logs, Alarms and Fault Codes.
Check CONERRCNT per TRX and TS.
Check Transmission Link (A-bis).
Check for DIP Slips.
Check LAPD Congestion.
Correlate Handover Lost to Drops due to Sudden Loss

Solution:
Fix Hardware Faults and Alarms.
Reset TRX with high CONERRCNT.
Ensure that Synchronization and A-bis Link are stable.
Change RBLT with high DIP Slips.
Change CONFACT or increase Transmission Capacity
Investigate HO Lost Problem

TCH Drop Reasons (5)
TCH Drops due to Other Reasons
TCH drops due to Other Reasons are computed by subtracting the sum of drops due to Excessive TA, Low SS, Bad Quality and Sudden Loss from the Total TCH Drop Counts. Drops due to Other Reasons are generally associated with hardware problems, transmission link problems on A-bis, Ater or Ainterfaces, and sometimes Handover Lost.

Action:
Check BTS Error Logs.
Check Alarms and Fault Codes.
Check CONERRCNT per TRX and TS.
Check Transmission Link (A-bis).
Check for DIP Slips.
Correlate Handover Lost to Drops due to Other Reasons

Solution:
Fix Hardware Faults and Alarms.
Reset TRX with high CONERRCNT.
Ensure that Synchronization and A-bis Link are stable.
Change RBLT with high DIP Slips.
Investigate HO Lost Problem


Problem reason of drop in SDCCH

Low Signal Strength on Down or Uplink
The reason for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure.
Action: Check coverage plots.Check output power. Perform drive tests. Check BTS error log
Solution: Add new sites. Increase output power. Repair faulty equipment.

Poor Quality on Down or Uplink
Action: Check C/I and C/A plots. Check frequency plan. Perform drive tests.
Solution: Change frequency. Use available radio features.

Too High Timing Advance
Action: Check if the cell parameter TALIM is < style="font-weight: bold;">Solution: Set TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for cochannel cells.

Mobile Error
Some old mobiles may cause dropped calls if certain radio network features are used. Another reason is that the MS is damaged and not working properly.
Action: Check MS fleet.
Solution: Inform operator.

Subscriber Behavior
Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing illadvised locations to attempt calls, etc.
Action: Check customer complaints and their MS.

Battery Flaw
When a subscriber runs out of battery during a conversation, the call will be registered as dropped call due to low signal strength or others.
Action: Check if MS power regulation is used. Check if DTX uplink is used.

Congestion on TCH
The SDCCH is dropped when congestion on TCH.
Action: Check TCH congestion
Solution: Increase capacity on TCH or using features like Assignment to another cell, Cell Load Sharing, HCS, Dynamic Half-Rate Allocation and FR-HR Mode Adaptation etc

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