Interference Reduction

Interference reduction Siemens

MN1789EU10MN_0002 © 2004 Siemens AG

1

Contents

1 Power control 3

1.1 Objectives 4

1.2 Measurement preprocessing for power control 6

1.3 Power control decision 8

1.4 Power control execution 12

1.5 Power control for AMR calls 22

1.6 Service dependent power control 24

2 Frequency hopping 27

2.1 Introduction 28

2.2 Frequency hopping system 32

3 Discontinuous Transmission DTX 37

4 Channel allocation due to interference level 41

5 Exercises 47

6 Solutions 53

Interference reduction

Siemens Interference reduction

MN1789EU10MN_0002

© 2004 Siemens AG

2



3

1 Power control


MN1789EU10MN_0002

© 2004 Siemens AG

4

1.1 Objectives

The objective of power control is to adapt the transmit power of the MS as well as of the BTS to the reception conditions. For example a mobile station MS 1 located near the BTS can use a lower transmit power than a mobile station MS 2 at the edge of a cell to achieve the required uplink quality.

There are two advantages of power control:

� reduction of the average power consumption (especially in the MS),

� reduction of the interference experienced by co-channel (for adjacent) channel users.

Power control is applied separately for the uplink and the downlink and separately for each logical channel). It can be enabled/disabled using the following flags (administered within the PWRC object):

Specification Name DB Name Meaning

EN_MS_PC EMSPWRC Flag to disable / enable classic / enable adaptive uplink power control.

EN_BS_PC EBSPWRC Flag to disable / enable classic / enable adaptive downlink power control.

It should be noted that downlink power control is not applied for downlink bursts using the BCCH frequency.



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T

X

P

W

R

MS 1

T

X

P

W

R

MS 2

BTS

Fig. 1 Required MS transmit power depending on its distance to the BTS


MN1789EU10MN_0002

© 2004 Siemens AG

6

1.2 Measurement preprocessing for power control

Any control loop is based on measurements of the controlled system parameters. For PC purposes, for each call in progress and for both links (downlink/uplink), measurements of:

� received signal level

� received signal quality

are carried out over each SACCH multiframe, which is 104 TDMA frames (480 ms) for a TCH and 102 TDMA frames (471 ms) for a SDCCH.

Every SACCH multiframe the MS sends in the next SACCH message block the downlink measurements on the dedicated channel (averaged over one SACCH multiframe) via the Measurement Report message to the serving TRX of the BTS. This means that any SACCH message will report the averaged data for the previous reporting period only. The TRX performs the uplink measurements on the dedicated channel. The measurement reports (uplink and downlink) referring to the same SACCH multiframe are used as input parameters for the Measurement Report averaging procedures, implemented within the BTS.

This means that the adjustment of the transmit power of the MS and the BTS is based on following measurement values (refer to Chapter 4.2.1 Measurement Preprocessing for Handover):

� RXLEV_DL_FULL / SUB � - RXLEV_UL_FULL / SUB

� RXQUAL_DL_FULL / SUB � - RXQUAL_UL_FULL / SUB

The measurement values are preprocessed within the BTS in the same way as for the handover process, i.e. a gliding average window and a weighting of FULL and SUB values is used. The parameters for measurement preprocessing for power control are administered in the object PWRC and are listed in the table below.

Power control correction

This mechanism is necessary to ensure full handover functionality if BS power control and frequency hopping is enabled.

Normally, if BS PWRC is enabled the MS is informed about this by a flag in the SYS_INFO. This flag makes the MS suppress measurement reports derived from the BCCH carrier in order to avoid the measurements to be falsified by the ‘full power’ part of the BCCH.

If frequency hopping is disabled - which could be the case after frequency redefinition due to failure of a TRX- the MS may hop on the BCCH carrier only. In this case all measurement reports are suppressed (or declared ‘not valid’) by the MS - which means that no handover is possible.



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Enabling the power control correction mechanism has the following results:

a) The BS PWRC flag is set to ‘0’ in the SYS_INFO even if the parameter EBSPWRC=CLASSIC or ADAPTIVE.

b) The MS thus provides valid measurement reports even for the BCCH carrier.

c) The BTS takes care that the ‘full power’ part from the BCCH carrier is correctly substracted from the measurement reports.

Parameters for measurement preprocessing power control

Specification Name DB Name Range Meaning

A_QUAL_PC PAVRQUALAQUALPC

1-31 Averaging window size for RXQUAL values, used for power control decisions

W_QUAL_PC PAVRQUALWQUALPC

1-3 Weighting for RXQUAL_FULL values

A_LEV_PC PAVRLEV ALEVPC

1-31 Averaging window size for RXLEV values, used for power control decisions

W_LEV_PC PAVRLEV WLEVPC

1-3 Weighting factor for RXLEV_FULL values

EN_BS_PWRC_CORR EBSPWCR TRUE/ FALSE

Power control correction in case of BS-power control and frequency hopping


MN1789EU10MN_0002

© 2004 Siemens AG

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1.3 Power control decision

For power control decision the average values of RXLEV_UL/DL and RXQUAL_UL/DL are compared with some preset thresholds (O&M parameters). A flow chart of the power control decision process is shown in the figure below.

The power control decision is primarily based upon the received signal quality, rather than on the received signal level. The reason behind this, is that the transmitter power directly affects the quality of the radio link regardless of the overall received signal level, which may be dominated by co-channel interference. If the controlled variable (its average value) lies in the tolerance defined by the thresholds, then no control action is taken, i.e. a “deadband” type of control response is produced. This introduces stability into the control process and guarantees an adequate speech quality. Note, that the controlled variables involved in the algorithm are used in accordance with their coding, e.g. RXQUAL_XX = 0 corresponds to the least BER (best signal quality: BER<0.2%).



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PC Decision

Process

RXQUAL_XX<U_RXQUAL_XX_P

RXLEV_XX<L_RXLEV_XX_P

+ 2 x POW_RED_

STEP SIZE

Power decrease

no

yes

x

RXQUAL_XX>

L_RXQUAL_XX_P

Power increase

yes

x

RXLEV_XX>

U_RXLEV_XX_P

Power decrease

yes

no

RXLEV_XX<

L_RXLEV_XX_P

Power increase

yes

no

no

x

x

yes

no

Fig. 2 Flowchart of the power control decision process


MN1789EU10MN_0002

© 2004 Siemens AG

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The test

RXLEV_XX > L_RXLEV_XX_P + 2 x POW_RED_STEP_SIZE

should prevent the control loop from oscillating, i.e. a power decrease decision for quality reasons should not be followed by power increase decision for signal level reasons. Note, that the O&M parameter POW_RED_STEP_SIZE is defined in terms of a difference between two transmit power levels. An unit power level step corresponds to a nominal 2 dB step in the variation of the transmit power. Field measurements at SIEMENS have shown that even at very low received power levels a good quality for a radio link can be obtained with a relative high probability. Because sudden fades may deteriorate the quality very rapidly, if the above condition is satisfied, the received power level is then compared with the corresponding lower threshold to ensure a required minimum power level on the radio link.

Parameters for Power Control Decision

The parameters for power control decision are administered in the object PWRC and are listed in the table below.

Specification Name

DB Name Range Meaning

L_RXLEV_DL_P L_RXLEV_UL_P

LOWTLEVDLOWTLEVU

0...63 RXLEV threshold on downlink/uplink for power increase

U_RXLEV_DL_P U_RXLEV_UL_P

UPTLEVD UPTLEVU

0...63 RXLEV threshold on downlink/uplink for power decrease

L_RXQUAL_DL_PL_RXQUAL_UL_P

LOWTQUADLOWTQUAU

0...7 RXQUAL threshold on downlink/uplink for power increase

U_RXQUAL_DL_PU_RXQUAL_UL_P

UPTQUAD UPTQUAU

0...7 RXQUAL threshold on downlink/uplink for power decrease



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RX QUAL

Power Increase

(bad level)

Power Increase

(bad quality)

Power Decrease

(high level)

Power Decrease

(good quality)

7

63

RXLEV

0

U_RXQUAL_XX_P

L_RXQUAL_XX_P

L_RXLEV_XX_P

2 x POW_RED_STEP_SIZE

U_RXLEV_XX_P

Fig. 3 Illustration of the thresholds used in the power control decision process

The following integrity conditions on the power control thresholds have to be observed:

U_RXQUAL_XX_P < L_RXQUAL_XX_P XX = UL, DL;

L_RXLEV_XX_P < U_RXLEV_XX_P XX = UL, DL.

Furthermore the power control thresholds have to harmonize with the ones for handover, i.e.:

U_RXQUAL_XX_P < L_RXQUAL_XX_P < L_RXQUAL_XX_H XX = UL, DL;

L_RXLEV_XX_H < L_RXLEV_XX_P < U_RXLEV_XX_P XX = UL, DL.


MN1789EU10MN_0002

© 2004 Siemens AG

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1.4 Power control execution

1.4.1 Conventional power control

Having detected within the decision process that a change of transmit power should be carried out, the value for this change has to be fixed. If power control would only be based on RXLEV, the difference between RXLEV and the level threshold could be used as the step size for power control decision. However, since the main objective is to maintain an acceptable speech quality and since there is no direct simple relation between a change of the transmit power level and the resulting change of RXQUAL, the strategy is to change the transmit power in small steps and then analyze the resulting RXQUAL. This means that fixed step sizes for power increase and power decrease are used given by the parameters:

� POW_INCR_STEP_SIZE: 2, 4 or 6 dB

� POW_RED_STEP_SIZE: 2 or 4 dB,

To be able to react fast enough on sudden drops of link quality one should have

POW_INCR_STEP_SIZE > POW_RED_STEP_SIZE.

The maximum range for uplink power control is given by:

� [13 dBm, Min (MS_TXPWR_MAX, P)] for a GSM-MS Phase 1

� [5 dBm, Min (MS_TXPWR_MAX, P)] for a GSM-MS Phase 2 and

for a GSM 850 MS

� [0 dBm, Min (MS_TXPWR_MAX, P)] for a DCS1800-MS and

for a PCS 1900-MS

where P is the maximum RF output power of the MS (power class) and MS_TXPWR_MAX the maximum transmit power allowed in the respective cell. The minimum step size for transmit power adjustment is 2 dB.



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For downlink power control the range is determined by the maximum output power PBTS of the BTS and the static reduction BS_TXPWR_RED of the BTS output power:

BS_TXPWR_MAX = PBTS - 2 * BS_TXPWR_RED

BS_TXPWR_RED = 0, 1, ... 6, Unit: 2 dB.

The range for downlink power control is then given by:

BS_TXPWR_MAX - 30 dB ... BS_TXPWR_MAX with a step size of 2 dB.

In the case of a power control decision a MS/BS Power Control message is created wherein the MS/BS is requested to adjust its transmit power level to:

REQ_TXPWR = CONF_TXPWR + POW_INCR_STEP_SIZE (Power Increase)

REQ_TXPWR = CONF_TXPWR - POW_RED_STEP_SIZE (Power Decrease)

where CONF_TXPWR is the confirmed power level used by the MS or BTS on the concerned channel. If these values of REQ_TXPWR are not within the range for power control, the nearest value within the range is used instead.


MN1789EU10MN_0002

© 2004 Siemens AG

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Having requested a transmit power REQ_TXPWR, the power control decision process is suspended and it is waited for a confirmation that the transmit power of the MS/BTS is adjusted to requested value, i.e.

CONF_TXPWR = REQ_TXPWR.

If such a confirmation is not received within an interval of P_CONFIRM SACCH multiframes, the power control decision process is immediately resumed using the most recently reported confirmed value.

If a confirmation is received, the power control decision process is suspended for a certain number of SACCH multiframes given by the parameter P_CON_INTERVAL. The reason for this is to allow an observation of the effect of one power control decision before initializing the next one; by this means the power control process is stabilized. Thus it is recommended to set

P_CON_INTERVAL > A_QUAL_PC

in terms of a number of multiframes.

The processes and time relations are illustrated in the figure below:

� case 1: requested transmit power confirmed

� case 2: requested transmit power not confirmed



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request

TXPWR

(2)requested transmit power

not confirmed

(1) requested transmit power confirmed

CONF_TXPWR

= REQ_TXPWR

suspension of PC decision

P_CON_INTERVAL

P_CONFIRM

resume

Power Control

resume

Power Control

(2)(1)

time

Fig. 4 Timer usage in the conventional power control process


MN1789EU10MN_0002

© 2004 Siemens AG

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1.4.2 Adaptive power control

The adaptive power control for circuit switched services applies increment steps, which are automatically adapted to the system's need according to the signal quality (RXQUAL) or to the signal level (RXLEV). The adaptation steps are dynamically calculated by the system according to current radio conditions.

Calculation of the stepsize:

Fast power increase is applied, if the signal quality average is below L_RXQUAL_XX_P and the signal level average is below L_RXLEV_XX_P.

stepsize A [dB]:=abs(RXLEV – 0.5*(U_RXLEV_XX_P+ L_RXLEV_XX_P))

Fast power increase is applied, if the signal quality average is above L_RXQUAL_XX_P and the signal level average is below L_RXLEV_XX_P.

stepsize B [dB]:=abs(L_RXLEV_XX_P – RXLEV))

Standard power increase is applied, if the signal quality average is below L_RXQUAL_XX_P and the signal level average is above L_RXLEV_XX_P.

stepsize [dB]:= POW_INCR_STEP_SIZE * 2dB;

Standard power reduction is applied, if the signal quality average is above U_RXQUAL_XX_P and the signal level average is above U_RXLEV_XX_P.

stepsize [dB]:= POW_RED_STEP_SIZE * 2dB;

In case of a power reduction for calls using baseband hopping over the BCCH carrier the resulting power level reduction is checked against a operator-definable absolute maximum reduction (number of 2dB steps) .



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RX QUAL

Power Increase

(bad level)

Adaptive stepsize B

Power Increase

(bad quality)

Static stepsize

Power Decrease

(good quality)

Static stepsize

7

63

RXLEV

0

U_RXQUAL_XX_P

L_RXQUAL_XX_P

L_RXLEV_XX_P

2 x POW_RED_STEP_SIZE

U_RXLEV_XX_P

Power Increase

(bad quality)

Adaptive stepsize A

Fig. 5 Definition of the areas for the different step sizes used in the adaptive power control process


MN1789EU10MN_0002

© 2004 Siemens AG

18

Differences in measurement preprocessing for adaptive power control

For the averaging process of the measurements two arrays of values are used for uplink and downlink and for quality and level.

Sample values for non-DTX channels are not entered n-times into the averaging window any more, but every value is entered once. Additionally, the current weight factor is stored in parallel, as shown in the figure below. Thus the time needed to fill the averaging window is always constant. The averaging value is calculated by adding up all sample values within the averaging window multiplied by their weight. The total is divided by the ‘total weight’.

The delay timer is only started if a power change decision was made due to quality reasons to allow a new set of RXQUAL values to be received and evaluated. Since the time needed to fill the averaging window depends only on the RXQUAL window length set by the operator, the delay timer is calculated from that value and is

Delay time = Averaging window length * Duration of a SACCH period.

Furthermore, all sample values in the RXLEV averaging windows are corrected by the respective power level change as if they were already received with the changed power until the power change is confirmed by the MS. No suspension time is used.



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0 4321 98765 3130

10 14131211 01081015 00sample

Averaging window size, e.g. 8

1 2111 01112 00weight

Fig. 6 Averaging process in case of adaptive power control


MN1789EU10MN_0002

© 2004 Siemens AG

20

Parameters for Power Control Execution

Specification Name DB Name /Object

Range Meaning

MS_TXPWR_MAX MSTXPMAX/ BTS

2...15 0...15

Maximum TXPWR an MS may use in the serving cell 2 = 39 dBm, 15 = 13 dBm (GSM900)0 = 30 dBm, 15 = 0 dBm (DCS1800, PCS1900) Range for uplink power control: 13 dBm ... MAX (GSM, phase 1) 5 dBm ... MAX (GSM, phase 2) 0 dBm ... MAX (DCS, PCS) MAX = Min (MS_TXPWR_MAX, P) P: power class of the MS

BS_TXPWR_RED PWRRED / TRX

0...6 Static reduction of the TRX output power: BS_TXPWR_MAX = PBTS - 2 * PWRRED Range for downlink power control: BS_TXPWR_MAX - 30 dB ... BS_TXPWR_MAX

POW_INCR_ STEP_SIZE

PWRINCSS / PWRC

DB2 DB4 DB6

Step size for power increase in dB

POW_RED_ STEP_SIZE

PWREDSS / PWRC

DB2 DB4

Step size for power reduction in dB

P_CONFIRM PWRCONF /PWRC

1...31 Maximum interval for waiting for a confirmation of the new transmit power level. unit: 2 TSACCH

P_CON_INTERVAL PCONINT / PWRC

0...31 Minimum interval between changes of the RF transmit power level (time for suspension of a power control decision after a power control execution) unit: 2 TSACCH

PC_MAX_BS_TX_POWER_RED_LEV

PCMBSTXPR / PWRC

0…15 Maximum BS power reduction for calls using baseband hopping over the BCCH carrier.



21

Relations to be observed:

To avoid an oscillating power control due to level the following unequations shall be fulfilled:

POW_RED_STEP_SIZE < POW_INCR_STEP_SIZE

and

POW_INCR_STEP_SIZE < U_RXLEV_XX_P - L_RXLEV_XX_P


MN1789EU10MN_0002

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22

1.5 Power control for AMR calls

For AMR calls the power control is implemented basing on the same principles as for 'normal' calls. The difference, however, is the scale for the quality of a connection. Instead of the RXQUAL values a more precise C/I value is used for the comparison of actual values with given thresholds. The C/I values are derived from the RXQUAL (bit error rate) as shown in the table below:

RXQUAL C/I RXQUAL C/I

6.88 ... 7 1 3.13 ... 3.37 14

6.63 ... 6.87 2 2.88 ... 3.12 14

6.38 ... 6.62 4 2.63 ... 2.87 15

6.13 ... 6.37 5 2.38 ... 2.62 16

5.88 ... 6.12 6 2.13 ... 2.37 16

5.63 ... 5.87 7 1.88 ... 2.12 17

5.38 ... 5.62 8 1.63 ... 1.87 17

5.13 ... 5.37 8 1.38 ... 1.62 18

4.88 ... 5.12 9 1.13 ... 1.37 18

4.63 ... 4.87 10 0.88 ... 1.12 19

4.38 ... 4.62 11 0.63 ... 0.87 19

4.13 ... 4.37 11 0.38 ... 0.62 19

3.88 ... 4.12 12 0.13 ... 0.37 20

3.63 ... 3.87 13 0 ... 0.12 20

3.38 ... 3.62 13

For AMR calls, the RXQUAL threshold LOWTQUAX is replaced by the C/I [dB] threshold LOWTQUAMRXX, and the RXQUAL threshold UPTQUAX is replaced by the C/I [dB] threshold UPTQUAMRXX.



23

Specification Name DB Name /Object Range Meaning

L_RXQUAL_AMR_DL_P L_RXQUAL_AMR_UL_P

LOWTQUAMRDL LOWTQUAMRUL / PWRC

0…30 AMR quality lower threshold on downlink/uplink for power increase

U_RXQUAL_AMR_DL_P U_RXQUAL_AMR_UL_P

UPTQUAMRDL UPTQUAMRUL / PWRC

0…30 AMR quality upper threshold on downlink/uplink for power decrease


MN1789EU10MN_0002

© 2004 Siemens AG

24

1.6 Service dependent power control

The feature Service dependent Power control distinguishes fourteen service groups. This concerns Circuit-Switched services (CS) on Half Rate (HR), Full Rate (FR), Enhanced Full Rate (EFR), Adaptive Multi-Rate (AMR), Advanced Speech Call Items (ASCI), Voice Broadcast Services (VBS), Voice Group Call Services (VGCS), and High Speed Circuit-Switched Data services (HSCSD).

For each service group relevant threshold parameters for power control can be defined individually.

The different service groups are summarized in the table below:

Service Group Description

SG-1 Signaling on hopping channel

SG-2 Signaling on non-hopping channel

SG-3 CS speech (FR, EFR, ASCI VBS, ASCI VGCS) on hopping channel

SG-4 CS speech (FR, EFR, ASCI VBS, ASCI VGCS) on non-hopping channel

SG-5 CS speech (HR) on hopping channel

SG-6 CS speech (HR) on non-hopping channel

SG-7 CS data up to 9,6kbit/s or HSCSD 9,6kbit/s on hopping channel

SG-8 CS data up to 9,6kbit/s or HSCSD 9,6kbit/s on non-hopping channel

SG-9 CS data up to 14,4kbit/s or HSCSD 14,4kbit/s on hopping channel

SG-10 CS data up to 14,4kbit/s or HSCSD 14,4kbit/s on non-hopping channel

SG-11 CS speech (AMR-FR) on hopping channel

SG-12 CS speech (AMR-FR) on non-hopping channel

SG-13 CS speech (AMR-HR) on hopping channel

SG-14 CS speech (AMR-HR) on non-hopping channel

If parameters are set for a specific service group, the system will use those values for the power control algorithm, otherwise the global parameter settings are used.



25

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PWRC

PWRC parameters:

• NAME

• EBPWRC

• EMSPWRC• …

• SG1PCPAR

• SG1PCPAR

• …

• SG14PCPAR• …

• PCMBXTSPRL

Settings for BSC

area for all SGs

that are not

activated

PWRC parameters for SG1:

• enableBSPowerControl

• enableMSPowerControl• …

•

• PCUpperThresholdQualUL

PWRC parameters for SG14:

• enableBSPowerControl

• enableMSPowerControl• …

•

• PCUpperThresholdQualUL

Relevant for

Service groups

activated in the

BSC area

Fig. 7 Principle of service dependent power control


MN1789EU10MN_0002

© 2004 Siemens AG

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27

2 Frequency hopping


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© 2004 Siemens AG

28

2.1 Introduction

The principle of Frequency Hopping used within GSM is that successive TDMA bursts of a connection are transmitted via different frequencies - the frequencies belonging to the respective cell according to network planning. This method is called Slow Frequency Hopping (SFH) since the transmission frequency remains constant during one burst (in contrast to fast frequency hopping where the transmission frequency changes within one burst).

The effect of Frequency Hopping is that link quality may change from burst to burst, i.e. a burst of high bit error rate may be followed by a burst of low bit error rate since

� short term fading is different on different frequencies,

� the interference level is different on different frequencies.

Since the information for one speech frame is interleaved over 8 successive burst and the successful decoding of a speech frame depends on the average bit error rate over these 8 bursts, a speech frame may be decoded even if there were some bursts with bad quality. Without hopping in general all 8 bursts are either good or bad. Hence the benefit of Frequency Hopping is a kind of equalization of the link quality of all connections by

� frequency diversity - averaging of short term fading,

� interference diversity - averaging of interference.

To achieve interference diversity, connection in different co-channel cells may not hop in a synchronous, but in an uncoordinated way. The corresponding hopping mode is called (Pseudo) Random Hopping: uncorrelated hopping sequences are used in different co-channel cells. Hence the probability of a collision (interference) of two specific connections in co-channel cells is inverse proportional to the number of frequencies used in the hopping sequence. A hopping sequence is characterized by the so-called Hopping Sequence Number HSN.

In GSM there are 63 uncorrelated pseudo random hopping sequences (HSN = 1, ..., 63). Furthermore, there is a Cyclic Hopping Mode, characterized by HSN = 0.

These two Hopping Modes are illustrated in Fig. 5.



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+:interference averaging

- :no optimum frequency diversity

+:optimum frequency diversity

(averaging of Rayleigh fading)

- :no interference averaging

connection 2

connection 1

cell 2 (same frequencies as cell 1)cell 1

TDMA frame

Hopping Sequence Number HSN = 1

Pseudo Random Hopping:

f4

f3

f2

f1

f4

f3

f2

f1

TDMA frame

e.g. HSN = 27 for reuse cell

collision probability = 1 / no. frequencies

f4

f3

f2

f1

TDMA frame

Hopping Sequence Number HSN = 0

Cyclic Hopping:

Fig. 8 Illustration of cyclic and pseudo random hopping


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Frequency Hopping Gain:

1. Averaging of Short Term Fading

Signal to noise ratio required to obtain 0.2 % residual BER for class 1b bits:

frequency hopping diversity TU3 TU50 HT100

none none 11.5 7.5 6.8

2 frequencies none 10.0 6.5 6.7



none yes 6.8 - -

2 frequencies yes 5.5 - -



Typical Frequency Hopping and Antenna Diversity Gain S/N for GSM

For DCS1800, roughly the same values can be assumed at half the speed (e.g. TU25 instead of TU50).

TU50 Typical Urban 50 km/h

HT100 Hilly terrain 100 km/h

For low speed of the MS, each doubling of the hop step number results in an 1.5 dB improvement. For higher speed and for hilly terrain, frequency hopping does not give additional gain, since short term fading is already averaged by motion. The gain of FH increases with increase number of frequencies.



31

2. Interference Levels:

In the following a simple example for the effect of interference averaging is given:

Let us assume that if the sum of all interferences is so high, that C/I is less than 5.0 (7 dB), a call is disturbed. It is recommended to obtain a C/I > 10 dB in most areas of a cell. We give an example for C/I ratios in a cell with 4 frequencies with and without frequency hopping.

CELL A: f1, f2, f3, f4

Interferer CELL B: f1, f2, f5, f6

Interferer CELL C: f3, f4, f5, f6

f1 f2 f3 f4

MS to BTS interference level

without FH

0.10 C/I = 10 dB

0.14 C/I = 8.5 dB

0.25 C/I = 6 dB

0.28 C/I = 5.5 dB

MS to BTS average interference level with

FH

0.19 C/I = 7.2 dB

BTS to MS interference level

without FH

0.10 C/I = 10 dB

0.10 C/I = 10 dB

0.28 C/I = 5.5 dB

0.28 C/I = 5.5 dB

BTS to MS average interference level with

FH

0.19 C/I = 7.2 dB

Interference levels for cell A

This means that without FH two calls have low link quality (C/I < 7 dB), whereas with FH the quality of all calls is just above the threshold of C/I = 7 dB.

Frequency Hopping Techniques in the SBS

The SBS supports both, baseband and synthesizer hopping.

In baseband hopping, the bursts belonging to one connection are switched to different high frequency boards in the BTSE. Therefore, the maximum number of frequencies used for the hopping is restricted to the number of carrier units configured for the cell.

In synthesizer hopping the carrier unit itself changes the frequency so that the number of frequencies used for the hopping can extend the number of configured carrier units.


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32

2.2 Frequency hopping system

In this chapter we discuss the implementation of a frequency hopping system in SBS and the different algorithms for the evaluation of hopping sequences. A hopping sequence is the time dependent sequence of used output frequencies. One parameter used in the creation of a frequency hopping system is the so called Mobile Allocation MOBALLOC which is the list of all frequencies used in the sequence. There are two possibilities to administrate these sequences:

Cyclic hopping:

The cyclic hopping is mainly used to combat the Raleigh fading. The frequencies are used one after another. If the MOBALLOC = f1, f2, f3, f4 is created a cell, the parameter for allocating cyclic hopping is Hopping Sequence Number HSN = 0. The result is a hopping f1, f2, f3, f4, f1, f2, f3, f4,..... The start frequency for the hopping is administrated with the parameter Mobile Allocation Index Offset MAIO. Its value depends on the number of given frequencies.

Pseudo Random hopping:

If another value for HSN is taken, the output sequence is pseudo random. The Random hopping is used mainly to improve the C/I level for example in cells with the same frequencies if a different HSN is given.



33

Objects and parameters in SBS for administration of frequency hopping:

With the SBS system frequency hopping is configured by creating frequency hopping system objects and by assignment of radio channels to these hopping systems.

Specification Name

DB Name / Object

Range Meaning

HOPP_MODE HOPMODE / BTS

BBHOP SYNHOP

Flag indicates whether baseband or synthesizer hopping is used

BTS_ISHOPPING HOPP / BTS

TRUE FALSE Flag to enable/disable FH within the BTS

FH_SYS_NUMBER FHSY / FHSY

1...10 Number to identify a frequency hopping system

HSN HSN / FHSY

0...63 Hopping sequence number 0: cyclic hopping 1...63: random hopping

MA MOBALLOC/ FHSY

BCCHFREQ, CALLF01, : CALLF63

Mobile allocation: list of frequencies within the FH system (maximum number of frequencies = 64).

FH_SYSTEM_ID FHSYID / CHAN

0...10 Specifies the frequency hopping system (given by FHSYN) to which a channel shall belong. (0: no hopping)

MAIO MAIO / CHAN

0...63 Mobile allocation index offset: defining the starting frequency (number in the MA frequency list) for a hopping sequence at a certain frame number FN, i.e. different channels using the same FH system shall have different MAIOs.


MN1789EU10MN_0002

© 2004 Siemens AG

34

Base Band Frequency Hopping Configuration

TRX 0 f0

TRX 1 f1

TRX2 f2

TRX 3 f3

RTSL0 BCCH

FHSYID = 0 MAIO = 0

SDCCH FHSYID = 2 MAIO = 0

TCH FHSYID = 2 MAIO = 1


RTSL1 SDCCH

FHSYID = 1 MAIO = 0




RTSL2 TCH

FHSYID = 1 MAIO = 0




RTSL3 TCH

FHSYID = 1 MAIO = 0




RTSL4 TCH

FHSYID = 1 MAIO = 0




RTSL5 TCH

FHSYID = 1 MAIO = 0




RTSL6 TCH

FHSYID = 1 MAIO = 0




RTSL7 TCH

FHSYID = 1 MAIO = 0




FH_SYSTEM_ID: FHSYID=0 -> No Hopping

FH_SYSTEM_ID: FHSYID=1 -> MOBALLOC=BCCHFREQ&CALLF01&CALLF02&CALLF03

FH_SYSTEM_ID: FHSYID=2 -> MOBALLOC=CALLF01&CALLF02&CALLF03

Example for channel configuration for Base Band Frequency Hopping



35

Synthesizer Frequency Hopping Configuration

TRX 0 f0

TRX 1 f1

TRX2 f2

TRX 3 f3

RTSL0 BCCH FHSYID = 0 MAIO = 0

SDCCH FHSYID = 1 MAIO = 0



RTSL1 SDCCH FHSYID = 0 MAIO = 0




RTSL2 TCH FHSYID = 0 MAIO = 0
























FH_SYSTEM_ID: FHSYID=0 -> No Hopping

FH_SYSTEM_ID: FHSYID=1 -> MOBALLOC=CALLF01&CALLF02&…&CALLF07

Example for channel configuration for Base Band Frequency Hopping


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36

MAIO management for Synthesizer Frequency Hopping

Reuse pattern 1x3

For the reuse pattern 1x3 all the frequencies for hopping are divided into three groups and each site of the network will be assigned with the same groups.

Co-channel interference between sectors is avoided by the frequency plan. Adjacent channel interference between sectors can be avoided up to 50% frequency usage per cell on the assigned frequencies by synchronizing sectors, assigning identical HSNs to sectors and assigning MAIOs to TRXs deliberately.

RF ...

Sector 0 f1 f4 f7 f10 f13 f16 f19 f22 f25 ...



MAIO TRX1 TRX2 TRX3 TRX4 TRX5 TRX6 TRX7 TRX8 ...

Sector 0 0 2 4 6 8 10 12 14 ...

Sector 1 1 3 5 7 9 11 11 15 ...

Sector 2 0 2 4 6 8 10 12 14 ...

Reuse pattern 1x1

All the hopping frequencies are assigned to each cell on each site. Co- and adjacent channel interference between sectors can be avoided up to 16.7% frequency usage per cell on the assigned frequencies by synchronizing sectors, assigning identical HSNs to sectors and assigning MAIOs to TRXs deliberately:

MAIO TRX1 TRX2 TRX3 TRX4 TRX5 TRX6 TRX7 TRX8 ...

Sector 0 0 6 12 18 24 30 36 42 ...

Sector 1 2 8 14 20 26 32 38 44 ...

Sector 2 4 10 16 22 28 34 40 46 ...



37

3 Discontinuous Transmission DTX


MN1789EU10MN_0002

© 2004 Siemens AG

38

DTX has been developed for satellite systems some years before. In GSM it is used the first time in a mobile communication system. The goal is to reduce MS power consumption and to reduce the interference in a cell. During a normal conversation, the participants speak only 50% of time. Each direction of transmission is occupied about 50% of time. DTX is a mode of operation where the transmitters are switched on only for those frames containing useful information. The difficulty is to find techniques to distinguish noisy speech from real noise even in a noisy environment. These algorithms are implemented in the VAD (Voice Activity Detection) function. The background acoustic noise has to be evaluated in order to transmit characteristic parameters to the receive side. The receive sides generates a similar noise called comfort noise during periods where the radio transmission is cut.

SBS parameter for DTX / VAD administration:

Specification Name

Object DB Name

(values)

Meaning

DTX indicator uplink

BTS DTXUL

(MAYFSHNH,

SHLFSHNH,

SHNFSHNH,

MAYFMAYH,

SHLFSHLH,

SHNFSHLH)

Discontinuous transmission uplink

MAYFSHNH (MS may use DTX for FR TCHs, shall not for HR TCHs).

SHLFSHNH (MS shall use DTX for FR TCHs, shall not for HR TCHs).

SHNFSHNH (MS shall not use DTX for FR TCHs, shall not for HR TCHs).

MAYFMAYH (MS may use DTX for FR TCHs, may use for HR TCHs).

SHLFSHLH (MS shall use DTX for FR TCHs, shall for HR TCHs).

SHNFSHLH (MS shall not use DTX for Full Rate TCHs, shall for Half Rate TCHs)

DTX indicator downlink

BTS DTXDLFR

(true, false)

Discontinuous transmission downlink for FR calls

BTS DTXDLHR

(true, false)

Discontinuous transmission downlink for HR calls



39

The goal of DTX is to reduce the speech data rate from 13 kbit/s (user speaking) to 500 bit/s. This low rate is enough to encode the background noise. This means instead of one frame of 260 bits per 20 ms only one frame per 480 ms is sent. These so called SID frames (Silence Description Frames) are sent at the start of every inactivity period, then all 480 ms, as long as the inactivity lasts between BTS and MS. Between TRAU and BTS these comfort noise frames are sent all 20 ms. The time behavior looks like following:

TRAU <-> BTS

SSSSCCCCCCCCCCCCCCCSSCCCCCCCS

BTS <-> MS

SSSSC C C C SSC C S

where S = speech frame

C = comfort noise frame

The so called TRAU frame contains not only these 260 bit:

Number of bits, UL Number of bits, DL

frame synchronization 35 35

discrimination: speech, data, full rate, half rate

5 5

time alignment 6 6

bad frame indication 1

DTX mode 1 1

other info 3 1

speech 260 260

spare 5 9

Contents of a speech block for transmission at 16 kbit/s, details in GSM 08.60


MN1789EU10MN_0002

© 2004 Siemens AG

40



41

4 Channel allocation due to interference level


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© 2004 Siemens AG

42

The BTS performs measurements of the (uplink) received level on all its idle traffic channels (TCHs) in the same way as for the busy channels, i.e. the level values in [dBm] in each TDMA frame are averaged over one SACCH period. The averaged value is mapped on a RXLEV value (0, ..., 63).

Since these measurements are related to idle channel the measured level is interpreted as interference level caused by a mobile in another cell using this channel or an adjacent channel (or by an external source of interference).

The information on the interference level can be taken into account at channel selection to assign the channel with the lowest interference level, which is expected to guarantee the best link quality.

An improvement of link quality by this mechanism is only possible if there is a choice, i.e.

� in cells with low traffic load where some idle channels are available for assignment

� if the interference level on idle channels differs significantly (this is not the case if random frequency hopping is applied).

The following section describes the classification and selection of traffic channels due to interference level in more detail.

Having measured the interference level for some SACCH multiframes, an arithmetic mean RXLEV_IDLE of a certain number of successive interference level samples, given by a parameter INTAVE, is calculated if the channel has been idle for the whole measurement period INTAVE * TSACCH.

Using this averaged interference level, an idle traffic channel is classified as a channel of

Interference Band 1 if 0 < RXLEV_IDLE < X1

Interference Band 2 if X1 < RXLEV_IDLE < X2



Interference Band 5 if X4 < RXLEV_IDLE < X5 = 63

where X1, X2, ... X5 denote the boundaries (O&M parameter) of the five interference bands.

0 < X1 < X2 < X3 < X4 < X5 = 63



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Interference Band 5

X5

X4

X3

X2

X1

0

low quality

high quality

RXLEV_IDLE

Interference Band 4

Interference Band 3

Interference Band 2

Interference Band 1

Fig. 9 Thresholds for the interference bands


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© 2004 Siemens AG

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If a traffic channels has not been idle during the whole measurement period, it is not taken into account in the classification procedure.

Having classified all the traffic channels, which have been idle during the whole measurement period, the TRX immediately sends the result of this classification to the BSC (RF Resource Indication message) where it is used in the channel allocation process.

The period for sending the RF Resource Indication message can be set by a parameter RF_RES_IND_PERIOD.

The time relations between measurements in the BTS and information sending to the BSC are illustrated in the figure below.



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RF RES IND

(TRX -> BSC)

time

SACCH_

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averaging

period

INTAVE = 5

RF resource indication period

RF_RES_IND_PERIOD = 15

averaging

period

INTAVE = 5

RF RES IND

(TRX -> BSC)

Fig. 10 Timers used for the determination of interference levels


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© 2004 Siemens AG

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In the case of a call setup as well as in the case of a handover a traffic channel out of the best non-empty interference class is allocated.

A condensed form of the information on the “quality” of the idle traffic channels at a certain BTS is sent from the BSC to the MSC using the RESOURCE INDICATION message. This message contains, for a specified cell (BTS), the number of idle TCHs, which are available separately in each of the five interference bands.

Parameters for Channel Allocation due to Interference Level

The parameters are administered in the object BTS and are listed in the table below:

Specification Name DB Name Range Meaning

EN_INT_CLASS INTCLASS TRUE/ FALSE

Flag to enable the classification procedure in the BTS

INTAVE INTAVEPR averaging Period

1...31

Period for averaging the measured interference level. A traffic channel, which has not been idle during the whole measurement and averaging period, it is not take into account in the classification procedure. unit: TSACCH

X1, X2, X3; X4 INTAVEPR interference Thresholds Boundaries

0...63

Boundaries of the interference classes/ bands.

RF_RES_IND_PERIOD RFRSINDP 0...255 Period for sending the classification information to the BSC. unit: TSACCH

Relations:

0 <= X1 < X2 < X3 < X4 <= 63

INTAVE < RF_RES_IND_PERIOD



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5 Exercises



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Exercise 1

Title: Power Control

Query

Consider a mobile station (of GSM phase 1) of maximum output power PMS = 39 dBm. Within the cell serving that MS the following parameters are set:

MS_TXPWR_MAX = 33 dBm BS_TXPWR_MAX = 43 dBm POW_INCR_STEP_SIZE = 6 dB POW_RED_STEP_SIZE = 4 dB

A classic power control is enabled. Furthermore, the following thresholds for the uplink power control decision are set:

L_RXQUAL_P = 4 L_RXLEV_P = 20

U_RXQUAL_P = 3 U_RXLEV_P = 30

In the table below some examples for the measured and average (uplink) RXLEV and RXQUAL as well as for the last confirmed TXPWR are given. What are the corresponding values for the new commended TXPWR?

Example Last confirmed

TXPWR (dBm)

RXQUAL RXLEV New commanded

TXPWR

1 25 2 23 ?

2 21 4 32 ?

3 15 2 30 ?

4 31 5 25 ?

5 17 5 34 ?



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Exercise 2

Title: Channel Allocation due to Interference Level

Query

Consider a cell with the following parameters or interference classification:

INTAVE = 2 RF_RES_IND_PERIO = 4

X1 = 10 X2 = 20 X3 = 30 X4 = 40 X5 = 63

The following levels have been measured by the BTS on its channels each SACCH frame

1 2 3 4 5 6

B: busy channel

SACCH frame

Chan 1 B 19 21 23 15 15

Chan 2 B B B B B B

Chan 3 12 14 14 16 14 14

Chan 4 20 22 18 18 17 18

Fig. 11 Exercise 2

To which interference bands do channel 1-4 belong at SACCH frame 7?

Which channel is allocated this time in the case of an assignment request?



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6 Solutions



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Solution 1

Title: Power Control

Query

Example 1: 2 = RXQUAL < U_RXQUAL_P → Power decrease by 4 dB

but: RXLEV - 4 dB = 19 < L_RXLEV_P → no power control

Example 2: no power control due to quality, but due to level

32 = RXLEV > L_RXLEV_P → power decrease by 4 dB TXPWR (new) = 21 - 4 dBm = 17 dBm

Example 3: 2 = RXQUAL < U_RXQUAL_P → power decrease 4 dB TXPWR (old) - 4 dB = 11 dBm < 13 dB lower transmit power for

phase 1 MS) → TXPWR (new) = 13 dBm

Example 4: 5 = RXQUAL > L_RXQUAL_P → power increase by 6 dB TXPWR (old) + 6 dB = 37 dBm lower than PMS, but greater than MS_TXPWR_MAX = 33 dBm = TXPWR (new)

Example 5: Though there is a good RXLEV > U_RXLEV_P we have

5 = RXQUAL > L_RXQUAL_P → power increase by 6 dB TXPWR (new) = (17 + 6) dBm = 23 dBm



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Solution 2

Title: Channel Allocation due to Interference Level

Query

Since RF_RES_IND_PERIOD = 4, the relevant resource indication message has been sent directly after SACCH frame 4 using the levels in frames 3 and 4 for averaging and classification.

� AV_RXLEV (chan1) = ½ (21+23) = 22 → interference band 3

� channel 2 busy within measurement period → interference band 5

� AV_RXLEV (chan3) = ½ (14+16) = 15 → interference band 2

� AV_RXLEV (chan4) = 18 → interference band 2

→ channel 3 or 4 is allocated.

Interference Reduction

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