SFH Basics

7/27/2019 SFH Basics

http://slidepdf.com/reader/full/sfh-basics 1/6

.

s Radio Planning

Synthesizer Frequency Hopping

Brief Ideology

Frequency hopping is a technique in which the information carrier changes the modulation

frequency within a specified band, this technique was use by the military to maintainconfidentiality over their transmission and prevent their signals from being intercepted by

the enemy.

There are broadly two types of frequency hopping, namely slow frequency hopping and

fast frequency hopping . If the frequency changes faster than the modulation rate then it istermed as fast frequency hopping and otherwise it is called the slow frequency hopping.

GSM applies only to the slow frequency hopping technique and this is further classified as

Baseband Frequency Hopping and Synthesizer frequency hopping . The difference in thetwo techniques is as follows.

Advantages of Frequency Hopping:

There are two basic advantages of frequency hopping.1) Frequency Hopping maintains confidentiality over the transmission, since the number

of frequencies in the hopping sequence are high therefore it is not possible to latch on

the frequencies and therefore maintain confidentiality.2) Frequency Hopping reduces the losses due to fading, multipath propagation and co-

channel interference

Baseband Frequency Hopping:

C1 C2 C3 C4 C5 C6 Combiner

PA PA PA PA PA PA

F0

TPU0

F3

TPU1

F6

TPU2

F9

TPU3

F12

TPU4

F15

TPU5

t1 t2 t3 t4 t5 t6

The figure above shows the hopping sequence as seen in the base-band hopping mode, the

t1….t6 are the sequences of the timeslots in different bursts, TPU is the transceiver processing unit, PA is the power amplifier, C1…..C6 are the resonant cavities in the

combiner. F0, F3, F6, F9, F12, F15 are the ARFCN the circuit is tuned to.

Page:1



.

s Radio Planning

The figure clearly shows that the TPU, the resonant cavity (C1..C6) and all the transceiver

circuits are always tuned to only one frequency only, so in order for a timeslot to hop the

timeslot is relayed to from one TPU to another on every burst basis.As per the example in the figure, the timeslot of a particular subscriber is at TPU 0 at the t1

instant of time. However in the next burst the same subscriber timeslot is at TPU 1 at t2

instant of time, and so on, so for this subscriber the timeslot is at different frequency ineach burst, hence for this subscriber the frequency is hopping.

Advantages:

1) BCCH can also participate in the hopping sequence.

2) Narrow band combiners such as Filter combiner (less combiner loss) can be used hence

saving the EIRP of transmission.3) (Siemens Advantage), no change in hardware required.

Disadvantages:

1) Hopping gain is negligible for less than 3 frequencies in the hopping sequence and

therefore is not suitable for low TCH configurations.

2) The numbers of total TRX in the cell limits the maximum frequencies in the hoppingsequence.

3) Siemens disadvantage (till BR 3.7) If a TRX fails, frequency hopping is disabled.

Synthesizer Frequency Hopping

C1 C2 C3 C4 C5 C6 Combiner

PA PA PA PA PA PA

F0,3

TPU

0

F0,3

TPU

1

F0,3

TPU

2

F0,3

TPU

3

TPU

4

F0,3

TPU

5

Page:2

Different colors signify different Burst, an

different bursts are transmitted on differe

fre uenc .TS7 TS6 TS5 TS4 TS3 TS2 TS1 TS0

Different lines

Signify different

Frequencies, the

total frequencies islimited by the No.

of frequencies

defined in thehopping sequence

and not by the No.

of TRX in the cell.TS 0- TS 7 mean different timeslots.

The TPU willchange frequency

on each Burst, the

total frequencies

will be as definedin the BSC

database.



.

s Radio Planning

Figure above shows a descriptive diagram of Synthesizer frequency hopping, the different

colors of the subscriber timeslot signify different bursts, it should be noticed here that theTPU in this case will change the frequency at each burst. Therefore after each burst is

transmitted the TPU should change the frequency to the new frequency in the hopping

sequence. This is achieved by having two frequency tuned circuits in the same TPU , OneRF circuit will prepare to change frequency as long as the other is already transmitting.

Advantages:

1) More frequencies than the total TRX in a cell are possible in the hopping sequence.

Therefore allowing more hopping gain in the system.

2) Lower C/Ic ratio are possible in the system without compromising speech quality, thisresults in a tighter frequency re-use and hence a higher capacity gain.

3) TRX expansions are very easy.

Disadvantages:

1) Since the cavity in the combiner will be required to change frequency very fast,

therefore the combiner such as FICOM cannot be used for Synthesizer hopping sincethese combiners need 2-3 seconds to tune to each frequency. This is a disadvantage

because in higher configuration the FICOMS have less combiner loss compared to

DUCOM or HYCOMs.2) BCCH cannot participate in the hopping sequence. Since the total number of

frequencies in the hopping sequence is more than the number of TRX required therefore

no fixed frequency allocated for each TRX. However BCCH must always betransmitted, therefore BCCH allocation is done separately as a separate frequency.

Hardware Requirements:

1) For SIEMENS Base Station Only: TPU 2 is mandatory to be used for synthesizer

hopping.

2) All the combiners in the base station with synthesizer hopping should be wide band

combiners, therefore FICOMS cannot be used in base station with synthesizer hopping.

Frequency Planning Ideology for Synthesizer hopping:

Many frequency planing ideologies are propounded for the frequency planning of thesynthesizer hopping, however nearly all of them state that although the frequency plan is

easy to generate but MAIO (Mobile Allocation Index Offset) planning is of crucial

importance. Also the synthesizer hopping requires in some configurations that the Base

station should be synchronized between them to avoid any frequency collisions.What is MAIO?

MRP:- 1 X 3 Reuse Pattern.

Page:3



.

s Radio Planning

A 1 X 3 reuse pattern is the frequency assignment in which all the three sectors of a site

have different frequency group, these frequency group may contain adjacent channelfrequencies but no co-channel frequency, the adjacent channel interference can be avoided

by intelligent allocation of MAIO.

Before we continue our discussion further, let us define a few new terms

MRP: (Multiple Reuse Pattern):a frequency re-use scheme in which the BCCH and TCH

allocation is done by reserving separate band of frequencies for BCCH and separate band of frequencies for TCH.

MAIO: (Mobile Allocation Index offset): The MAIO defines the start frequency of thehopping sequence. The maximum value of MAIO is determined by the total number of

frequencies in the hopping sequence and not by the total number of TRX in a cell.

HSN: Hopping sequence number: It defines the sequence of the frequencies while hopping. In Siemens base station it is possible to define 64 hopping sequences (0 - 63), where “0”

defines cyclic hopping and 1 - 63 define un-correlated pseudo random hopping sequences.

Channel Occupancy Rate: A term specifically used for synthesizer hopping which defines a

ratio of total number of TCH frequencies in a cell to total number of frequencies in the

hopping sequence, for a good network this ratio should not exceed 40%.

Mathematically: No. of TCH in a cell

No. Of Frequencies in the hopping sequence.

In a 1 X3 Re-Use pattern the frequency re- use pattern will look like the following,

Page:4

C C C

AB

C AB

C

AB

C AB

C

AB

C AB

C

AB

C AB

CAB

C AB

AB

C AB

AB

C AB

AB

C AB

C



.

s Radio Planning

Where A, B, C are mutually exclusive groups however these groups have adjacent channel

frequencies. A typical allocation of the frequencies may look like the following:

Group ARFCN Numbers allocated to the group.

A 1 4 7 10 13

B 2 5 8 11 14C 3 6 9 12 15

Therefore for a site the allocation may look like

In the above case we assume that the total site configuration is 3/3/3 and that BCCH

frequency planning is done separately using a dedicated band therefore the remaining

configuration is 2/2/2 for the TCH frequency allocation. Therefore if we have 5 frequenciesin the hopping sequence then the Channel occupancy rate would be 2/5 = 40%, which is

acceptable value.

Notice that in the above example, each sector has adjacent channel frequency within the

same site, this could pose a serious problem. This is problem can be solved by carefully

planning MAIO for the above site we propose the following MAIO allocations. (notice that

since there are 5 frequencies in the hopping sequence hence the Maximum number of MAIO can be 5 { MAIO= 0- 4})

(For reasons of simplicity we assume cyclic hopping in the above case.)

So consider the first stage of Hopping, in this case the first sector will transmit frequency 1

and 7 , sector 2 will transmit frequency 5 and 11 and sector 3 will transmit frequency 3 and

Page:5

(1,4,7,10,13)

(2,5,8,11,14)(3,6,9,12,15)

Stages Sector 1 Sector 2 Sector 3

MAIO 0,2 MAIO 1,3 MAIO 0,2

1 1,7 5,11 3,9

2 4,10 8,14 6,12

3 7,13 11,2 9,15

4 10,1 14,5 12,35 13,4 2,11 15,6



.

s Radio Planning

9, hence even-though we have adjacent channel frequencies in the same site still we can

avoid adjacent channel interference by planning MAIO.

For the case of neighboring sites with the same frequency allocation, to avoid the co-

channel collision we must plan different Hopping Sequence Number (HSN).

MRP:- 1 X 1 Reuse Pattern.

For low configuration networks with limited frequency band, another idea can be proposed

is that of a 1 X 1 Re-use pattern. In such a case a separate band can be reserved for BCCHalone and the other band can be used for a TCH in the hopping sequence, usually the

allocation is such that the number of frequencies in the BCCH band is sufficient to avoid

any possibility of collision.

It should be pointed out that the 1X 1 configuration requires that the BTS be synchronize

between all the cells in one site, that mean with the existing BS60 the maximum

configuration possible with 1 X 1 hopping is 2/2/2, because the BS60 is unable tosynchronize beyond one rack. As a suggestion for configuration like 3/3/3 what can be

done is to allocate a second band for this third cell which is in the extension BS60 rack, this

will result in a 1 X 2 re-use pattern. However the BS240 is capable of synchronizing between racks and therefore 1 X 1 can be implemented more easily in BS240.

One can configure the network in such a fashion that all the cells in one site have the samehopping sequence number, however differ only in the MAIO allocation. This argument is

valid both for 1 X 3 re-use pattern and also for 1X 1 re-use pattern also.

Impact on the Network:

Since the frequency re-use is very tight the effective BER ( Bit error rate, a measure of RX-Quality in GSM ) will be very high, however the quality as perceived by the subscriber is

not a measure of BER but a measure of F.E.R (Frame Erasure Rate), in a good network the

FER should not exceed 2 %.

One important point to notice is that since the emergency (quality Based) hand-over

parameters is the based on the quality as perceived by BER, therefore the value of the

parameters related to quality handovers should be adjusted to avoid unnecessary handoversin the network.

Page:6

Documents

SFH Basics