Frequency Hopping, MAIO Management & Synchronized Radio
NetworkThis chapter contains the features Frequency Hopping, MAIO
Management and Synchronized Radio Networks.
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Frequency Hopping
HFS – Hopping Frequency Set
GSM Radio Network Features
Principle of frequency hopping: Instead of transmitting and
receiving bursts on a fixed frequency, the bursts will be sent on
different frequencies.
A frequency change can be performed for every new burst sent, that
is 217 times per second.
The hopping can be performed on all the frequencies allocated to a
channel group. The hopping over the frequencies can be made in
several ways. This slide shows so called cyclic frequency
hopping.
Note that the BCCH (TS 0 on the BCCH carrier) does not hop, but is
fixed.
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Effect on the Air Interface
GSM Radio Network Features
Frequency diversity
Frequency hopping can reduce the influence of signal strength
variations caused by multipath fading.
Multipath fading is frequency dependent. This implies that the
fading dips appear at different locations for different
frequencies. Thus a mobile utilizing frequency hopping will not
remain in a specific fading dip for a longer time than one single
burst. Thereby signal strength variations are broken up into pieces
of a duration short enough for the interleaving and speech coding
process to correct for errors. Multipath fading dips, causing low
signal strength, are thus apparently leveled out, and slowly moving
mobiles (and cars stuck at a red light) will perceive a more even
radio environment. Frequency hopping makes most of the fading dips
appear more shallow.
Interference averaging
Frequency hopping can also break up persistent interference into
periodic occasions of single burst interference. The cell planning
margin against situations of bad radio conditions can thus be
decreased, since the probability of encountering these conditions
decreases.
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Frequency Diversity
GSM Radio Network Features
This figure shows the received signal strength as a function of
distance for two different frequencies (dashed & thin solid).
The Rayleigh fading dips appear in different places for different
frequencies. The thick solid line corresponds to the apparent
signal strength obtained by frequency hopping, known as frequency
diversity.
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Interference Diversity
GSM Radio Network Features
This slide illustrates schematically the effect on the
C/I-distribution depending on what features are switched on. FH and
BTSPC are using a Robin Hood principle, whereas DTX give a net gain
in C/I (at least if the other features are active).
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Achievements
GSM Radio Network Features
The benefits of frequency hopping are noticeable already when
hopping over two frequencies.
The wider band the better.
A high traffic load will decrease the number of idle time slots and
will increase the number of interfering bursts. In a situation
where almost all time slots on all frequencies suffer from
interference, there will be no gain from interference
averaging.
Usage of DTX and dynamic power control will decrease the number of
interfering bursts. This will increase the gain of interference
averaging.
From a subscriber point of view, frequency hopping gives an
improved speech quality in many situations. From an operator point
of view, the benefits are:
tighter frequency reuse and increase in capacity,
a more robust radio environment,
a possibility to give subscribers a more uniform speech
quality.
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A higher RxQual can be tolerated
GSM Radio Network Features
When frequency hopping is used, a higher RxQual value can be
tolerated. This is because the “bad” bursts are distributed more
evenly so that the GSM coding and interleaving is utilized
efficiently.
When the combination of frequency hopping and DTX is used an even
higher rxqual value can be tolerated. The reason for this is not
clear.
(note: with no FH and no DTX sometimes maximum acceptable rxqual
for acceptable speech quality is 4.5)
RxQual is seldom a very good way of describing quality.
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Capabilities
Up to 128 frequencies can be assigned per cell
Note: maximum of 32 frequencies per Channel Group (CHGR)
Frequencies can be reused (except the BCCH frequency) in other
CHGRs within the cell
Note: Use MAIO planning to avoid co-channel interference
GSM Radio Network Features
Hopping Sequences - Cyclic
GSM Radio Network Features
There are two types of hopping sequences - cyclic and random. In a
cyclic sequence the frequencies are used consecutively. This
sequence gives a slightly better frequency diversity than the
random sequences.
A cyclic sequence is specified by setting parameter HSN (hopping
sequence number) to 0. There is only one cyclic sequence defined in
the GSM specifications. The sequence of frequencies goes from the
lowest absolute frequency number in the set of frequencies
specified for that channel group, to the highest, and over
again.
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Hopping Sequences - Random
GSM Radio Network Features
A random hopping sequence is implemented as a pseudo-random
sequence. The sequence is stored in a look-up table in the mobile
as well as in the base stations. 63 independent sequences are
defined. Which of the 63 sequences to be used is specified with
parameter HSN.
The actual frequency to be used at each instant is obtained by an
algorithm with the available frequencies, see 3GPP Technical
Specification 45.002.
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Two Hopping Modes
GSM Radio Network Features
In baseband hopping, each transmitter is assigned with a fixed
frequency.
Synthesizer hopping means that one transmitter handles all bursts
that belong to a specific connection.
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Baseband Hopping
transmitter
filter
Baseband hopping : Each transmitter transmits on a fixed
frequency.
The bursts from the transceiver controller are routed to the
different
transmitters by a bus.
+
A narrow-band filter combiner can be used. To this combiner it
is
possible to connect up to 6 TRX:s without more than 3dB
combiner
loss.
-
It is impossible to hop on more frequencies than there are
TX:s.
GSM Radio Network Features
The bursts are routed to the appropriate transmitter by a
bus.
The benefit of baseband hopping is that low loss narrow band filter
combiners with up to six inputs and only 3 dB loss can be used.
This is because each TX transmits on a fixed frequency.
The negative is that it is impossible to have more frequencies than
there are TXs.
Minimum carrier separation for filter combiners is 600 kHz.
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Synthesizer hopping
+
-
Hybrid combiners must be used. When connecting many transmitters
the
loss will be big.
GSM Radio Network Features
In this case the bursts of a connection are transmitted by one and
the same TX, and the TX changes frequency for each burst. Since the
transmitter transmits over many frequencies wide hybrid combiners
must be used. These transmitters have only two inputs and about 3
dB loss.
The advantage of synthesizer hopping is that it is possible to hop
on more frequencies than there are transmitters.
Minimum carrier separation for hybrid combiners is 400 kHz.
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Co Filling at Baseband Hopping
•
controller
controller
controller
controller
f
0
f
1
transmitter
f
2
transmitter
f
3
transmitter
TRX1
TRX2
TRX3
TRX4
combiner
transmitter
filter
TS
0
TS
1
TS
2
TS
3
TS
4
TS
5
TS
6
TS
7
f
0
f
1
f
2
f
3
c
0
filling
c
0
filling
c
0
filling
c
0
filling
c
0
filling
c
0
filling
c
0
filling
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
SDCCH
TCH
BCCH
Co i.e. the BCCH carrier is special:
1. It contains the Broadcast Control Channel which must not be
hopping.
2. It must always be on air since all mobiles measure SS on that
frequency, i.e. the transmitter must send dummy bursts when nothing
arrives from the controller.
The slide shows a configuration of 4 TRXs with 30 TCHs and hopping
on the BCCH frequency
TS0 may cause alarms since it will normally not be used a lot if
you choose TCH for maximum number of frequencies to hop on.
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Co Filling at Synthesizer Hopping With an Extra Transmitter
•
•
•
All traffic bursts that are to be sent on Co are routed to Co
transmitter.
TS
0
TS
1
TS
2
TS
3
TS
4
TS
5
TS
6
7
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
SDCCH
f
n
f
0
f
1
f
2
f
3
GSM Radio Network Features
A transmitter configured for synthesizer hopping can not perform Co
filling (i.e. transmitting dummy bursts when nothing else arrives
from the controller). There are different ways to solve this
problem, one way is to equip the BTS with an extra fixed
transmitter, which only transmits on Co. A bus routes the bursts to
be sent on Co to this transmitter. When nothing arrives from the
controller, the transmitter will send dummy bursts.
11/03813-LZU 108 3704 Uae Rev G
•
•
One transmitter acting only as Co filler and one “BCCH
controller”
•
All traffic bursts that are to be sent on Co are routed to
this
transmitter.
TS
0
TS
1
TS
2
TS
3
TS
4
TS
5
TS
6
7
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
TCH
SDCCH
f
n
f
0
f
1
f
2
f
3
Co Filling at Synthesizer Hopping
With Two Channel Groups
•
If utilization of hardware is more important than it is to hop on
the
BCCH frequency.
Prior configurations can be seen as waist of hardware.
If it is not so important that the TCHs on the BCCH carrier are
hopping the frequencies can be split into two channel groups, one
hopping and one non-hopping.
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MAIO MANAGEMENT
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Purpose of MAIO Management
The MAIO Management feature provides increased control over
synthesized frequency hopping to minimize channel interference
within a site (or between sites if synchronized network is
used).
This is beneficial in a network with tight re-use of frequencies,
such as 1/1 & 1/3.
GSM Radio Network Features
MAIO Management provides increased control over synthesizer
frequency hopping to avoid co- and adjacent channel interference
within a cell as well as in co-sited or Synchronized cells. This is
beneficial in a network with tight re-use of frequencies such as
1/1 & 1/3.
Note that MAIO Management only increases control over the
interference between cells if the cells are synchronized, i.e.
cells within a site using one TG or a site using Transceiver Group
Synchronization or Synchronized Radio Networks.
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Algorithm
At frequency hopping MAIO values are used (together with the HSN
and the current FN) to point out the frequencies to be used from
the HFS at an instant in time.
Cyclic hopping
Random hopping
"pointer" = (MAIO+random value) modulo (number of frequencies in
HFS)
GSM Radio Network Features
FN Frame Number
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Example
Cyclic hopping, 3 TRX:s in a cell, nine frequencies in the HFS. The
current FN is 1.
The first TRX use frequency number:
(FN+MAIO) mod (# of frequencies in HFS) = (1+0) mod 9 = 1 (which
will relate the pointer to the second frequency in the HFS
The next time FN=2 and the pointers will be shifted downwards one
step.
0, 2, 4
Default MAIO list:
Example: Cyclic hopping, 3 TRXs, 9 frequencies and FN=1.
The default MAIO-list is 0, 2, 4. We can say that each TRX is
assigned a MAIO number of its own.
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Default MAIO list
The number of MAIO values in the default list are the same as the
number of frequencies in the HFS.
The order of the MAIO values in the default list are arranged in a
"first even then odd MAIO values" manner.
The actual MAIO values to be used for a CHGR depends on the number
of TRXs for the CHGR.
GSM Radio Network Features
The number of MAIO values in the default list are the same as the
number of frequencies in the HFS. The values themselves stretch
from 0 up to one less than the number of frequencies in the HFS.
E.g. If there are 15 frequencies in the HFS, the MAIO list will
contain the values 0-14.
The order of the MAIO values in the default list are arranged in a
"first even then odd MAIO values" manner. This means that the
beginning of the list will consist of all even MAIO values in
ascending order. After these even values all the odd values are
arranged in ascending order. E.g. for a HG with HFS containing 7
frequencies the default list will be 0, 2, 4, 6, 1, 3, 5.
The actual MAIO values to be used for a CHGR depends on the number
of TRXs for the CHGR. If e.g. three TRXs are used for a CHGR, only
the first three MAIO values in the MAIO list will be used. With 7
frequencies in the HFS (as in the previous example), the used
default MAIO values would be 0, 2, 4. The remaining values, i.e. 6,
1, 3, 5, will not be used unless additional TRXs are added.
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Example of Default MAIO list
If there are seven frequencies in the HFS, the MAIO list will
contain the values 0-6.
MAIO=0, 2, 4, 6, 1, 3, 5
If three TRXs are used for a CHGR, only the first three MAIO values
in the MAIO list will be used, i.e. 0, 2, 4.
GSM Radio Network Features
Manual MAIO list
A manual MAIO list for a CHGR can be created by specifying up to 32
values for the parameter MAIO.
If the manual MAIO list is too short then random MAIO values will
be added on to the end of the list.
If there is an invalid MAIO value in the manual MAIO list it will
be skipped in favor of the next MAIO value in the list.
GSM Radio Network Features
A manual MAIO list for a CHGR can be created by specifying up to
sixteen values for the parameter MAIO .
If the manual MAIO list is too short (i.e. the length of the MAIO
list is less than the number of TRXs for the CHGR), then random
MAIO values will be added on to the end of the list. This process
will be randomized as much as is reasonable whilst minimizing the
risk of having consecutive MAIOs in the list. This means that at
installation of an additional TRX for a cell, additional MAIO
values will be allocated.
If there is an invalid MAIO value (a value that is equal to or
higher than the number of frequencies in the HFS) in the manual
MAIO list it will be skipped in favor of the next MAIO value in the
list. This means that any invalid MAIO values that are specified by
the operator are not allocated and a randomized valid MAIO value
will be allocated to the last BPC in the HG.
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Recommendations
Manual MAIO Planning is beneficial to use when using a 1/1 re-use
or when frequencies are repeated in different CHGRs in a cell
If there are adjacent frequencies within the HFSs, only even (or
odd) MAIO values should be used within the site
When reusing frequencies in a cell, it is important that the
frequency is not used at the same time in both the CHGRs.
By having different MAIO values for each CHGR, e.g. odd values in
one and even in the other, collisions are avoided
GSM Radio Network Features
SYNCHRONIZED RADIO NETWORK
Synchronized Radio Network
Frame Synchronization between cells belonging to different sites is
possible
MAIO Planning is not restricted to the site
ICDM (Inter Cell Dependency Matrix) can be used to identify
interfering cells
Hopping Sequence Number
- All sites synchronized
GSM Radio Network Features
Synchronized Radio Network offers the possibility to frame
synchronize cells located on different sites with each other. This
gives opportunities to enhance performance even more for FLP
networks, since interfering cells can be handled no matter on which
site they are located. In a synchronized radio network interference
is not only managed by planning of MAIO but also appropriate
handling of for example HSN, TSC and FN Offset is needed. As a
basis for the planning it is necessary to identify which cells are
interfering each other, this is described in the ICDM (Inter-Cell
Dependency Matrix) which can be assembled with the help of the
Frequency Allocation Support (FAS) tool in OSS.
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Increased frequency utilization
GSM Radio Network Features
Synchronization of cells allows a more efficient spectrum
utilization for frequency hopping channels when combined with MAIO
Management and tight reuse of frequencies, such as 1/1 and 1/3. The
benefit is that traffic capacity can be increased within a radio
network without requiring additional frequencies. This is valuable
for operators with a limited set of available frequencies.
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Interference Rejection Combining (IRC)
IRC performance vs MRC
3-5 dB for 1 non-synchronized interferer
Equal coverage
TSC
TSC*
TSC*
GSM Radio Network Features
Interference Rejection Combining is a new receiver algorithm for
the transceiver which drastically improves interference robustness.
Simulations show that IRC can provide a C/I gain of up to 11 dB,
with a value in typical urban environments of around 5-6 dB,
compared to the currently used receive algorithm.
A prerequisite for IRC is that two receive antennas (receive
antenna diversity) are used. This means that there are two versions
of the signal available in the transceiver that are slightly
different due to the antenna diversity. IRC also uses the training
sequence (as defined by the Training Sequence Code, TSC), which is
a known bit pattern in the middle of each burst. By comparing the
received signal with the training sequence it is possible to
estimate the characteristics of the interfering signal. The IRC
algorithm can utilize this information to efficiently remove
interference from the wanted signal.
IRC performs best when the desired signal and the interfering
signal are synchronized in time, since then the interfering signal
is the same during the whole burst and the interference
characteristics estimated during the training sequence are more
likely to be valid for the whole burst.
In order to have synchronized interference between cells on the
same site, the features FAJ 122 854 RBS 2000 Synchronization or FAJ
122 855 RBS 200 and 2000 in the same Cell might be needed depending
on the site configuration and the RBS type. To also have
synchronized interference between cell located on different sites,
the feature FAJ 122 081 Synchronized Radio Networks is
needed.
The gain that IRC provides will solve interference problems that
are encountered on the uplink, this also means that radio network
capacity can increase in places where the uplink is the limiting
link. In all networks IRC will improve speech quality and data
throughput in the uplink, thereby increasing subscriber
satisfaction.
In radio environments not limited by interference, IRC will perform
as well as the currently available receiver diversity
algorithm.
IRC is available for all dTRU, EDGE sTRU and RBS 2308. Older
transceivers cannot be supported due to the increased processing
capacity required by the IRC algorithm.
(Today’s MRC* diversity gives ~3-5 dB in both C/N and C/I)
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Synchronization areas
A Synchronized Radio Network is possible to deploy on a small scale
within a limited area or on a grand scale over a larger area
Synchronization of cells in a GSM radio network is realized
per:
TG
Radio Network
Parameters (I)
HOP is the switch for tuning frequency hopping on or off, defined
per channel group. HOP defines whether all channels except the
broadcast channel hop ( HOP = ON), or no channels at all hop (HOP =
OFF).
HSN is the hopping sequence number, defined per CHGR. This
parameter specifies which hopping sequence to be used. All
timeslots in one channel group are configured with the same HSN.
HSN = 0 yields a cyclic sequence. HSN = 1 to 63 yields
pseudo-random sequences.
GSM Radio Network Features
Parameters (II)
FHOP selects which hopping method to be used, baseband (FHOP = BB)
or synthesizer ( FHOP = SY) hopping. It is defined per TG.
COMB specifies which combiner type that has been connected, a
wide-band hybrid combiner (COMB = HYB) or a narrow-band filter
combiner (COMB = FLT). It is defined per TG. If a filter combiner
is connected, only baseband hopping can be used.
GSM Radio Network Features
Parameters (III)
MAIO This parameter allows operators to specify a MAIO list of, up
to 16 MAIO values (with a range of 0-31), in the order of
allocation, to a channel group or specify the channel group to use
default MAIO list (MAIO = DEFAULT).
BCCD Defines if the channel group frequencies are allowed (YES) or
not (NO) for Immediate Assignment. It might not be possible to set
BCCD=YES for all channel groups in the cell. This is due to
restrictions on the maximum number of hopping frequencies allowed
for Immediate Assignment and their maximum ranges for different
frequency bands.
GSM Radio Network Features
Value Ranges and Default Values
GSM Radio Network Features
Parameter name
Default value
Recommended value
Value range
0-63
and frequency
and the bandwidth
•
9702689
C/I
consecutive fading dips.
6 minutes long.
averaging (if the network is planned using
frequency groups.)
frequencies.
9702816