quality and ta based handover

Quality and TA based HandoverImplementation

K S Madanpuri

Quality based HandoverImplementation

The system constantly measures the transmission quality for both the up-link and down-link connections. Another term for for transmission quality is BER. A high BER may result from a signal strength that is too low or from interfering signals.Seperate thresholds (QLIMULAFR & QLIMDLAFR) are set for connections using AMR speech codecs due to more robust nature of AMR connections. The separate threshold for AMR makes it possible to avoid unnecessary bad quality handovers for terminals using AMR speech codecs.

The relationship between BER and deci-transformed quality units(dtqu)

is as below BER before

channel decoderValue dtqu

<=0.2% 0 00

0.2 – 0.4 % 1 10

0.4 – 0.8 % 2 20

0.8 – 1.6 % 3 30

1.6 – 3.2 % 4 40

3.2 – 6.4 % 5 50

6.4 – 12.8 % 6 60

>= 12.8% 7 70

Quality based HandoverImplementation

If the quality value, as calculated in the averaging process, exceeds either the Quality LIMit Down-Link (QLIMDL) or the Quality LIMit Up-Link (QLIMUL), the system indicates an urgency codition.QLIMUL and QLIMDL parameters determine thresholds triggering an urgency handover.

Quality based HandoverImplementation

The parameters are QLIMUL: Quality limit up-link for handover.

Numeral 0-100 QLIMDL:Quality limit downlink for handover.

Numeral 0-100 QLIMULAFR: Quality limit up-link for handover

for Terminals using Fullrate speech codecs. Numeral 0-100

QLIMDLAFR: Quality limit down-link for handover for terminal using Fullrate AMR speech codecs. Numeral 0-100

If the quality value, as calculated in the averaging process, exceeds either the Quality LIMit Down-Link (QLIMDL) or the Quality LIMit Up-Link (QLIMUL), the system indicates an urgency codition.QLIMUL and QLIMDL parameters determine thresholds triggering an urgency handover.

TA

What is TA ?

TA

Time

Frequency

0 1 2 3 4 5 6 7

Channel

Time Slot

Air Interface Layers

RadioTransmission

LogicalChannels

Messages

RadioTransmission

LogicalChannels

Messages

Terminal BaseStation

Layer 1Bits

Layer 2Packets

Layer 3Messages

Burst• The information contained in one time slot

on the TDMA frame is call a burst.

• Five types of burst– Normal Burst (NB) – Frequency Correction Burst (FB)– Synchronization Burst (SB)– Access Burst (AB)– Dummy Burst

TA

TA

TA

26 multiframes periodicity = 120 mS 1 multiframe periodicity = 120/26 mS = 4.615 mS1 frame periodicity = 8 Time slotSo, 1 TS periodicity = 4.615/8 mS = 577 micro S1 TS periodicity = 156.25 bitSo, 1 bit periodicity = 577/156.25 = 3.69 micro SThe distance covered in 1 bit periodicity = 3.69*300 = 1106 meteres

TA

Timing advance is a solution specifically designed to counteract the problem of time alignment. It works by instructing the Mis-aligned MS to transmit its burst earlier or later than it normally would. In GSM, the timing advance information relates to bit times. Thus, an MS may be instructed to commence its transmission a certain number of bit times earlier or later, related to previous position, to reach its timeslot at the BTS in right time.

TA

BTS may transmit and receive continuously, whereas the MS transmission and reception times are periodic,it is up to the MS to compensate for propagation delay by anticipating its transmission time.The MS has to send a burst on the uplink way 3TS-2t after receiving a burst on the downlink way; 2t is Timing Advance value.

TA

TA based handover

TALIM : Timing advance limit for handover. Numeral 0-63 bit periods (normal cell)

0-219 bit periods (extended cell range)

TA based HandoverImplementation

If an MS is close to the cell border, defined by the TA, a handover will be triggered. The BSC compares the current average value of TA to the defined TALIM. If TA exceeds TALIM, the BSC tries to hand the MS over the a suitable neighboring cell.

TA based HandoverImplementation

If TA exceeds TALIM, the BSC tries to hand the MS over the a suitable neighboring cell. If no suitable neighbor is available, no handover will be executed. Since GSM defines the maximum TA to be 63 bits periods, the TALIM value must be smaller than 63.

MAXTA : Timing advance limit when an MS is considered lost. Numeral 0-63 bits period (Normal range cell).Numeral 0-219 bits periods (extended range cell).

TA based HandoverImplementation

THANKS

K S MADANPURI