GSM Timers-Network side

Timers on the network side

T3101: This timer is started when a channel is allocated with an IMMEDIATE ASSIGNMENT message. It is stopped when the MS has correctly seized the channels. Its value is network dependent. NOTE: It could be higher than the maximum time for a L2 establishment attempt.T3103: This timer is started by the sending of a HANDOVER message and is normally stopped when the MS has correctly seized the new channel. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the HANDOVER COMMAND, plus the value of T3124, plus the maximum duration of an attempt to establish a data link in multiframe mode.)T3105: This timer is used for the repetition of the PHYSICAL INFORMATION message during the hand-over procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted.T3107: This timer is started by the sending of an ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly seized the new channels. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the ASSIGNMENT COMMAND message plus twice the maximum duration of an attempt to establish a data link multiframe mode.T3109: This timer is started when a lower layer failure is detected by the network, when it is not engaged in a RF procedure. It is also used in the channel release procedure. Its purpose is to release the channels in case of loss of communication. Its value is network dependent. NOTE: Its value should be large enough to ensure that the MS detects a radio link failure.T3111: This timer is used to delay the channel deactivation after disconnection of the main signalling link. Its purpose is to let some time for possible repetition of the disconnection. Its value is equal to the value of T3110.T3113: This timer is started when the network has sent a PAGING REQUEST message and is stopped when the network has received the PAGING RESPONSE message. Its value is network dependent. NOTE: The value could allow for repetitions of the Channel Request message and the requirements associated with T3101.T3115: This timer is used for the repetition of the VGCS UPLINK GRANT message during the uplink access procedure. Its value is network dependent. NOTE: This timer may be set to such a low value that the message is in fact continuously transmitted.

T3117: This timer is started by the sending of a PDCH ASSIGNMENT COMMAND message and is normally stopped when the MS has correctly accessed the target TBF. Its purpose is to keep the old channel sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the PDCH ASSIGNMENT COMMAND message plus T3132 plus the maximum duration of an attempt to establish a data link in multiframe mode.T3119: This timer is started by the sending of a RR-CELL CHANGE ORDER message and is normally stopped when the MS has correctly accessed the new cell. Its purpose is to keep the old channels sufficiently long for the MS to be able to return to the old channels, and to release the channels if the MS is lost. Its value is network dependent. NOTE: It could be higher than the maximum transmission time of the RR_CELL CHANGE ORDER, plus T3134, plus the maximum duration of an attempt to establish a data link in multiframe mode.T3141: This timer is started when a temporary block flow is allocated with an IMMEDIATE ASSIGNMENT message during a packet access procedure. It is stopped when the mobile station has correctly seized the temporary block flow. Its value is network dependent.

GSM Timers-MS side

GSM Timers

Timers on the mobile station side

T3122: This timer is used during random access, after the receipt of an IMMEDIATE ASSIGN REJECT message. Its value is given by the network in the IMMEDIATE ASSIGN REJECT message.

T3124: This timer is used in the seizure procedure during a hand-over, when the two cells are not synchronized. Its purpose is to detect the lack of answer from the network to the special signal. Its value is set to 675 ms if the channel type of the channel allocated in the HANDOVER COMMAND is an SDCCH (+ SACCH); otherwise its value is set to 320 ms.

T3126:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during an immediate assignment procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the immediate assignment procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station's RACH. S and T. The maximum value of this timer is 5 seconds.

T3128: This timer is started when the mobile station starts the uplink investigation procedure and the uplink is busy. It is stopped at receipt of the first UPLINK FREE message. At its expiry, the uplink investigation procedure is aborted. The value of this timer is set to 1 second.

T3130: This timer is started after sending the first UPLINK ACCESS message during a VGCS uplink access procedure. It is stopped at receipt of a VGCS ACCESS GRANT message. At its expiry, the uplink access procedure is aborted. The value of this timer is set to 5 seconds.

T3110: This timer is used to delay the channel deactivation after the receipt of a (full) CHANNEL RELEASE. Its purpose is to let some time for disconnection of the main signalling link. Its value is set to such that the DISC frame is sent twice in case of no answer from the network. (It should be chosen to obtain a good probability of normal termination (i.e. no time out of T3109) of the channel release procedure.)

T3134:This timer is used in the seizure procedure during an RR network commanded cell change order procedure. Its purpose is to detect the lack of answer from the network or the lack of availability of the target cell. Its value is set to 5 seconds.

T3142: The timer is used during packet access on CCCH, after the receipt of an IMMEDIATE ASSIGNMENT REJECT message. Its value is given by the network in the IMMEDIATE ASSIGNMENT REJECT message.

T3146:This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during a packet access procedure. Or on receipt of an IMMEDIATE ASSIGNMENT REJECT message during a packet access procedure, whichever occurs first. It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message. At its expiry, the packet access procedure is aborted. The minimum value of this timer is equal to the time taken by T+2S slots of the mobile station's RACH. S and T are defined in section 3.3.1.2. The maximum value of this timer is 5 seconds.

T3164: This timer is used during packet access using CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message. It is stopped at the transmission of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expire, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.

T3190: The timer is used during packet downlink assignment on CCCH. It is started at the receipt of an IMMEDIATE ASSIGNMENT message or of an PDCH ASSIGNMENT COMMAND message when in dedicated mode.It is stopped at the receipt of a RLC/MAC block on the assigned temporary block flow, see GSM 04.60. At expiry, the mobile station returns to the packet idle mode. The value of the timer is 5 seconds.

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GSM Timers

Timer Name Description Value

T100 RADIO-LINK-TIMEOUT

Detects the presence of the radio link by detecting SACCH frames every 480 ms.

4 SACCH multiframes. That is 1.92 seconds if the SACCH is completely absent.

T200 Data link timer

Used for re-transmission on the data link. The value varies depending on the message type.

155 ms for FACCH

T301 Alerting (ringing) timer

Timer used to limit the amount of time a user has to answer a call.

20 seconds

T303 Mobility Management connection timer

Time the network waits after sending a CM SERVICE REQUEST until receiving a response. This occurs before initiating call clearing procedures towards the MS.

10 seconds

T305 Release timer

Time the network waits after transmitting a DISCONNECT message until receiving a RELEASE message.

10 seconds

T306 In-band tones release timer

Time the network waits after transmitting a DISCONNECT message while in-band tones/announcements are provided, until receiving a RELEASE message.

10 seconds

T308 Release timer

Time the network waits after sending a RELEASE message until receiving a RELEASE COMPLETE message. This occurs before re-transmitting the RELEASE or releasing the Mobility Management connection.

10 seconds

T310 Call proceeding timer

Time the network waits after receiving a CALL CONFIRMED message until receiving a ALERTING, CONNECT, or DISCONNECT message before initiating clearing procedures towards the MS.

10 seconds

T313 Connect acknowledge timer

Time the network waits after transmitting a CONNECT message until receiving the CONNECT ACKNOWLEDGE message before performing clearing procedures with the MS.

10 seconds

T323 Modify complete timer

Time the network waits after sending a MODIFY message during call mode changes, until receiving a MODIFY COMPLETE or MODIFY REJECT message before initiating call clearing procedures.

10 seconds

T3101 Immediate assignment timer

Time the network waits after sending the IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED message until the main signalling link is established before releasing the newly allocated channels.

1 second

T3103 Handover timer

Time the network waits after transmitting a HANDOVER COMMAND message until receiving HANDOVER COMPLETE or HANDOVER FAILURE or the MS re-establishes the call before the old channels are released. If the timer expires and the network has not received a correctly decoded L2 (format A or B) or TCH frame, then

2 seconds

Timer Name Description Value

the newly allocated channels are released.

T3105 Physical information repetition timer

Time the network waits after sending the PHYSICAL INFORMATION message until receiving a correctly decoded L2 (format A or B) or TCH frame. This occur before re-transmitting the PHYSICAL INFORMATION message or releasing the newly allocated channels.

50 ms

T3107 Channel assignment timer

Time the network waits after transmitting an ASSIGNMENT COMMAND message until receiving the ASSESSMENT FAILURE message or the MS re-establishes the call before releasing the old and the new channels.

3 seconds

T3109 Signaling disconnection timer

Time the network waits after sending the CHANNEL RELEASE message before disconnecting the signalling link.

5 seconds

T3111 Channel deactivation after disconnection timer

Time the network waits after disconnecting the signalling link before deactivating the channel.

500 ms

T3113 Paging timer

Time the network waits after transmitting the PAGING REQUEST message until receiving the PAGING RESPONSE message. This occurs before re-transmitting the PAGING REQUEST (if the maximum number of re-transmissions have not been exceeded).

5 seconds

T3212 Location update timer

The location update timer is set to zero, periodic location update by the MS are disabled. If the MS camps to the BCH and decodes a new MCC or MNC from the one it last camped on, it should perform a location update.

zero = infinite time

T3250 TMSI reallocation timer

Time the network waits after sending the TMSI REALLOCATION COMMAND until receiving TMSI REALLOCATION COMPLETE. This occurs before aborting the procedure and releasing the Radio Resource connection.

5 seconds

T3260 Authentication response timer

Time the network waits after an AUTHENTICATION REQUEST until receiving AUTHENTICATION RESPONSE. This occurs before aborting the procedure and releasing the Radio Resource connection.

5 seconds

BASIC DEFINITIONS

Isotropic RF Source ·A point source that radiates RF energy uniformly in all directions (I.e.: in the shape of a sphere) ·Theoretical only: does not physically exist. ·Has a power gain of unity I.e. 0dBi. Effective Radiated Power (ERP) ·Has a power gain of unity i.e. 0dBi ·The radiated power from a half-wave dipole. ·A lossless half-wave dipole antenna has a power gain of 0dBd or 2.15dBi. · Effective Isotropic Radiated Power (EIRP) ·The radiated power from an isotropic source       EIRP = ERP + 2.15 dB 

•Radio signals travel through space at the Speed of Light C = 3 * 108 meters / second •Frequency (F) is the number of waves per second (unit: Hertz) •Wavelength () (length of one wave) =  (distance traveled in one second)          (waves in one second)          = C / F If frequency is 900MHZ then

wavelength =  ( 3 * 108)/(900 * 106)         =  0.333 meters 

dB •*dB is a a relative unit of measurement used to describe power gain or loss. •*The dB value is calculated by taking the log of the ratio of the measured or calculated power (P2) with respect to a reference power (P1). This result is then multiplied by 10 to obtain the value in dB.      dB = 10 * log10(P1/P2)

•*The powers P1 ad P2 must be in the same units. If the units are not compatible, then they should be transformed. •Equal power corresponds to 0dB. •A factor of 2 corresponds to 3dB If P1 = 30W and P2 = 15 W then 10 * log10(P1/P2) = 10 * 10 * log10(30/15)  = 2 

dBm •*The most common "defined reference" use of the decibel is the dBm, or decibel relative to one milliwatt. •*It is different from the dB because it uses the same specific, measurable power level as a reference in all cases, whereas the dB is relative to either whatever reference a particular user chooses or to no reference at all. •*A dB has no particular defined reference while a dBm is referenced to a specific quantity: the milliwatt (1/1000 of a watt). •*The IEEE definition of dBm is "a unit for expression of power level in decibels with reference to a power of 1 milliwatt." •*The dBm is merely an expression of power present in a circuit relative to a known fixed amount (i.e., 1 milliwatt) and the circuit impedance is irrelevant.}  

dBm •dBm = 10 log (P) (1000 mW/watt) where  dBm = Power in dB referenced to 1 milliwatt P = Power in watts •If power level is 1 milliwatt: Power(dBm) = 10 log (0.001 watt) (1000 mW/watt) = 10 log (1) = 10 (0) = 0 •Thus a power level of 1 milliwatt is 0 dBm. •If the power level is 1 watt 1 watt Power in dBm = 10 log (1 watt) (1000 mW/watt)    = 10 (3)    = 30

dBm •dBm = 10 log (P) (1000 mW/watt) •The dBm can also be negative value.

•If power level is 1 microwatt Power in dBm = 10 log (1 x 10E-6 watt) (1000 mW/watt)        = -30 dBm •Since the dBm has a defined reference it can be converted back to watts if desired. •Since it is in logarithmic form it may also be conveniently

combined with other dB terms.

dB v/m •To convert  field strength in dbv/m to received power in dBm with a 50 optimum terminal impedance and effective length of a half wave dipole / 0dBu = 10 log[(10-6)2(1000)(/)2/(4*50)] dBm   At 850MHZ 0dBu = -132 dBm       39dBu = -93 dBm

GSM Frame Erasure Rate (FER) Measurement DescriptionThis section is only applicable to the lab applications and is not applicable to GPRS or EGPRS.You can use the GSM Frame Erasure Rate (FER) measurement to verify the mobile station's reference sensitivity for control channels.

How is the FER Measurement Made?The test set measures FER by sending a Layer 3 message that does not require a Layer 3 response from the mobile station. It does require acknowledgment in the form of an RR frame from the mobile station. When the test set does not receive the RR frame in acknowledgment, it retransmits the Layer 2 message. The test set counts the number of times it resends Layer 2 messages.The test set uses an MM Information message with all the optional fields omitted for the Layer 3 message.You can make the Frame Erasure Rate Measurement on a full-rate FACCH channel (FACCH/F) or a half-rate FACCH channel (FACCH/H).

Operating ConsiderationsThe FER measurement can only be performed in Active Cell Operating Mode.The connection type must be Auto.

FER Measurement Parameters Samples to Test - The number of samples to be taken by the

measurement. Minimum Frame Interval (FACCH/F)- The minimum interval

between FACCH frames (full rate) being sent to the mobile station.

Minimum Frame Interval (FACCH/H)- The minimum interval between FACCH frames (half rate) being sent to the mobile station.

Trigger Arm Measurement Timeout

FER Measurement Results Integrity Indicator - Frames Sampled - The count of samples tested. Frames Erased - The count of frames requiring retransmission

by the test set.

Frame Erasure Rate - The ratio of Frames Erased to Frames Sampled

Beamwidth - Defined by –3dB power points on both vertical and horizontal planes. - Usually affects the physical size of the antenna.

Gain

- Defined as the power output relative to an isotropic antenna (Gain 0dB) or Dipole antenna (Gain 2.2dB).

Front-to-Back Ratio - Defined as the amount of power in Front direction relative to Back direction. - Usually approximately 20-25dB.

Polarization - Electromagnetic wave consists of both an E Field and H Field. Polarisation usually refers to the direction of the Electric field relative to the intended direction of use for the antenna.

Downtilt - Downtilt is required to focus max.power where signal is desired (Coverage). -Downtilt is required to prevent interference to other coverage areas (Interference). 0 comments Links to this post

Timing Advance With Calculation

A Timing Advance (TA) is used to compensate for the propagation delay as the signal travels between the Mobile Station (MS) and Base Transceiver Station (BTS). The Base Station System (BSS) assigns the TA to the MS based on how far away it perceives the MS to be. Determination of the TA is a normally a function of the Base Station Controller (BSC), bit this function can be handled anywhere in the BSS, depending on the manufacturer.

Time Division Multiple Access (TDMA) requires precise timing of both the MS and BTS systems. When a MS wants to gain access to the network, it sends an access burst on the RACH. The further away the MS is from the BTS, the longer it will take the access burst to arrive at the BTS, due to propagation delay. Eventually there comes a certain point where the access burst would arrive so late that it would occur outside its designated timeslot and would interfere with the next time slot.

Access BurstAs you recall from the TDMA Tutorial, an access burst has 68.25 guard bits at the end of it.

This guard time is to compensate for propagation delay due to the unknown distance of the MS from the BTS. It allows an access burst to arrive up to 68.25 bits later than it is supposed to without interfering with the next time slot.

68.25 bits doesnt mean much to us in the sense of time, so we must convert 68.25 bits into a frame of time. To do this, it is necessary to calculate the duration of a single bit, the duration is the amount of time it would take to transmit a single bit.

Duration of a Single BitAs you recall, GSM uses Gaussian Minimum Shift Keying (GMSK) as its modulation method, which has a data throughput of 270.833 kilobits/second (kb/s).

Calculate duration of a bit.

So now we know that it takes 3.69µs to transmit a single bit.

Propagation DelayNow, if an access burst has a guard period of 68.25 bits this results in a maximum delay time of approximately 252µs (3.69µs × 68.25 bits). This means that a signal from the MS could arrive up to 252µs after it is expected and it would not interfere with the next time slot.

The next step is to calculate how far away a mobile station would have to be for a radio wave to take 252µs to arrive at the BTS, this would be the theoretical maximum distance that a MS could transmit and still arrive within the correct time slot. 

Using the speed of light, we can calculate the distance that a radio wave would travel in a given time frame. The speed of light (c) is 300,000 km/s.

So, we can determine that a MS could theoretically be up to 75.6km away from a BTS when it transmits its access burst and still not interfere with the next time slot.

However, we must take into account that the MS synchronizes with the signal it receives from the BTS. We must account for the time it takes for the synchronization signal to travel from the BTS to the MS. When the MS receives the synchronization signal from the BTS, it has no way of determining how far away it is from the BTS. So, when the MS receives the syncronization signal on the SCH, it synchronizes its time with the timing of the system. However, by the time the signal arrives at the MS, the timing of the BTS has already progressed some. Therefore, the timing of the MS will now be behind the timing of the BTS for an amount of time equal to the travel time from the BTS to the MS.

For example, if a MS were exactly 75.6km away from the BTS, then it would take 252µs for the signal to travel from the BTS to the MS. 

The MS would then synchronize with this timing and send its access burst on the RACH. It would take 252µs for this signal to return to the BTS. The total round trip time would be 504µs. So, by the time the signal from the MS arrives at the BTS, it will be 504µs behind the timing of the BTS. 504µs equals about 136.5 bits. 

The 68.25 bits of guard time would absorb some of the delay of 136.5 bits, but the access burst would still cut into the next time slot a whopping 68.25bits. 

Maximum Size of a CellIn order to compensate for the two-way trip of the radio link, we must divide the maximum delay distance in half. So, dividing 75.6km in half, we get approximately 37.8 km. If a MS is further out than 37.8km and transmits an access burst it will most likely interfere with the following time slot. Any distance less than 37.8km and the access burst should arrive within the guard time allowed for an access burst and it will not interfere with the next time slot.In GSM, the maximum distance of a cell is standardized at 35km. This is due mainly to the number of timing advances allowed in GSM, which is explained below.

How a BSS Determines a Timing Advance

For each 3.69µs of propagation delay, the TA will be incremented by 1. If the delay is less than 3.69µs, no adjustment is used and this is known as TA0. For every TA, the MS will start its transmission 3.69µs (or one bit) early. Each TA really corresponds to a range of propagation delay. Each TA is essentially equal to a 1-bit delay detected in the synchronization sequence.

In order to determine the propagation delay between the MS and the BSS, the BSS uses the synchronization sequence within an access burst. The BSS examines the synchronization sequence and sees how long it arrived after the time that it expected it to arrive. As we learned from above, the duration of a single bit is approximately 3.69µs. So, if the BSS sees that the synchronization is late by a single bit, then it knows that the propagation delay is 3.69µs. This is how the BSS knows which TA to send to the MS.

The Distance of a Timing AdvanceWhen calculating the distances involved for each TA, we must remember that the total propagation delay accounts for a two-way trip of the radio wave. The first leg is the synchronization signal traveling from the BTS to the MS, and the second leg is the access burst traveling from the MS to the BTS. If we want to know the true distance of the MS from the BTS, we must divide the total propagation

delay in half. 

For example, if the BSS determines the total propagation delay to be 3.69µs, we can determine the distance of the MS from the BTS. 

We determined earlier that for each propagation delay of 3.69µs the TA is inceremented by one. We just learned that a propagation delay of 3.69µs equals a one-way distance of 553.5 meters. So, we see that each TA is equal to a distance of 553.5 meters from the tower. Starting from the BTS (0 meters) a new TA will start every 553.5m.

The TA becomes very important when the MS switches over to using a normal burst in order to transmit data. The normal burst does not have the 68.25 bits of guard time. The normal burst only has 8.25 bits of guard time, so the MS must transmit with more precise timing. With a guard time of 8.25 bits, the normal burst can only be received up to 30.44µs late and not interfere with the next time slot. Because of the two-way trip of the radio signal, if the MS transmits more than 15.22µs after it is supposed to then it will interfere with the next time slot.0 comments Links to this post

FREQUENCY HOPPING

What is Frequency Hopping?

Frequency Hopping is an old technique introduced firstly in military transmission system to ensure the secrecy of communications and combat jamming.  Frequency Hopping is mechanism in which the system changes the frequency (uplink and downlink) during transmission at regular intervals.  It allows the RF channel used for signaling channel (SDCCH) timeslot or traffic channel (TCH) timeslots, to change frequency every TDMA frame (4.615 ms).  The frequency is changed on a per burst basis, which means that all the bits in a burst are transmitted in the same frequency. 

  Advantages of Frequency Hopping

1. Frequency Diversity

In cellular urban environment, multipath propagation exists in most cases.  Due to Rayleigh fading, short-term variations in received level

are frequently observed.  This mainly affects stationary or quasi-stationary mobiles.  For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious.  Frequency Hopping is able to take the advantage due to frequency selective nature of fading to decrease the number of errors and at the same time they are temporally spread.  As a result, the decoding and de-interleaving processes can more effectively remove bit errors caused by bursts received whilst on fading frequencies (errors will be randomly distributed instead of having long bursts of errors).  This increase in effectiveness leads to a transmission quality improvement of the same proportion.

·        Frame Erasure Rate reduces due to 6 dB to 8 dB gain.·        Number of reports with rxqual 6 and 7 reduce.·        Reported values of rxlev are more concentrated around mean.

  2. Interference Averaging

Interference Averaging means spreading raw bit errors (BER caused by the interference) in order to have random distribution of errors instead of having burst of errors, and therefore, enhance the effectiveness of decoding and de-interleaving process to cope with the BER and lead to better value of FER. 

With hopping, the set of interfering calls will be continually changing and the effect is that all the calls experience average quality rather than extreme situations of either good or bad quality.  All the calls suffer from controlled interference but only for short and distant periods of time, not for all the duration of the call.

·        For the same capacity, Frequency Hopping improves quality and for a given average quality Frequency Hopping makes possible increase in capacity.

·        When more than 3 % of the reports have rxqual of 6 or 7 then voice quality disturbances start to appear.

·        Gains (reduction in the C/I value needed to satisfy the quality requirements involved in the criterion) from hopping relative to fixed frequency operation can be achieved.

1/3 interference: 1 dB gain  i.e. if 1 out of 3 frequencies are experiencing a continuous interference a gain of 1 dB in C/I requirement is obtained.

Similarly,1/4 interference: 4 dB gain1/5 interference: 6 dB gain

2/4 interference: 0 dB gain2/5 interference: 4 dB gain

The effective gain obtained with Frequency Hopping is due to the fact that the interference effect is minimized and it is easier to keep it under control.

Types of Frequency Hopping

There are two ways of implementing Frequency Hopping in a Base Station System, one referred as Base Band Frequency Hopping (BBH) and another as Synthesizer Frequency Hopping (SFH).  Their operation differs in the way they establish the Base to Mobile Station link (downlink), however there is not difference at all between Mobile Station to Base Station link in both types of hopping.  Motorola does not allow BBH and SFH to be used together on the same site

1. Base Band Frequency Hopping

This is accomplished by routing the traffic channel data through fixed frequency DRCUs via the TDM highway on a timeslot basis.  In this case, the DRCU would have fixed tuned transmitters combined either in low loss tuned combiners or hybrid combiners.

·        DRCU always transmits fixed frequency.·        The information for every call is moved among the available DRCUs

on a per burst basis. (Burst of 577 µs)·        Call hops between same timeslots of all DRCUs.·        Processing (coding and interleaving) is done by digital part

associated with DRCU on which call was initially assigned.·        For uplink – call is always processed by DRCU on which the call was

initially assigned.·        Number of DRCUs needed is equal to the number of frequencies in

the hopping sequence.·        BCCH frequency can be included in the hopping sequence.·        Power control does not apply to BCCH or bursts transmitting BCCH

frequency.·        BCCH, timeslot 0 will never hop.·        Any timeslot with CCCH will never hop.·        Timeslot carrying all SDCCHs can hop.

If a network running with fixed frequency plan is switched over to BBH (BCCH included in MA list) without any frequency changes, significant quality improvement can be observed in the network.  As a result drop call rate reduces in the network.  Alternatively, for the

existing network quality additional capacity can be provided.  FHI can be used effectively in BBH.  Further details regarding FHI planning are discussed later in the document.

  

2. Synthesizer Frequency Hopping

This is accomplished by high speed switching of the transmit and receive frequency synthesizers of the individual DRCUs.  As a result of dynamic nature of the transmit frequency, broadband (hybrid) combining of the transmitters is necessary.

·        DRCU changes transmitting frequency every burst.·        Call stays on the same DRCU where it started.·        Remote tune combiners (RTC) are not allowed.·        Number of DRCUs is not related to number of frequencies in hopping

sequence.·        BCCH can be included in the hopping sequence:

1. If BCCH is included in the hopping sequence, timeslots 1 to 7 can not be used to carry traffic.  They transmit dummy burst when BCCH frequency is not in the burst.  Whenever BCCH frequency is being transmitted in a burst by DRCU, it will be transmitted at full power.

2. BCCH DRCU will never hop.  It either carries traffic in timeslots 1 to 7 or it transmits dummy bursts.

·        Transmission and reception is done on the same timeslot and same DRCU.

Frequency Hopping Parameters

GSM defines the following set of parameters:

Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop over.  Maximum of 63 frequencies can be defined in the MA list.

Hopping Sequence Number (HSN): Determines the hopping order used in the cell.  It is possible to assign 64 different HSNs.  Setting

HSN = 0 provides cyclic hopping sequence and HSN = 1 to 63 provide various pseudorandom hopping sequences. 

Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence, which frequency the mobile starts to transmit on.  The value of MAIO ranges between 0 to (N-1) where N is the number of frequencies defined in the MA list.  MAIO is set on per carrier basis.

Motorola has defined an additional parameter, FHI.

Frequency Hopping Indicator (FHI): Defines a hopping system, made up by an associated set of frequencies (MA) to hop over and sequence of hopping (HSN).  The value of FHI varies between 0 to 3.  It is possible to define all 4 FHIs in a single cell. 

Motorola system allows to define the hopping system on a per timeslot basis.  So different hopping configurations are allowed for different timeslots.  This is very useful for interference averaging and to randomize the distribution of errors.

GSM algorithm

GSM has defined an algorithm for deciding hopping sequence.  The algorithm is used to generate Mobile Allocation Index (MAI) for a given set of parameters. 

ARFCN: absolute radio frequency channel numberMA: mobile allocation frequencies.MAIO: Mobile allocation offset (0 to N-1), where N is the number of frequencies defined in MA.HSN: Hopping sequence number (0-63)T1: Super frame number (0-2047)T2: TCH multiframe number (0-25)T3: Signaling multiframe number (0-50)

This algorithm generates a pseudorandom sequence of MAIs.  MAI along with MAIO and MA will decide the actual ARFCN to be used for the burst.

Planning for Frequency Hopping

1. Frequency Plan:

Frequency Hopping plan differs from the conventional fixed frequency plan.  The plan depends upon the type of Frequency Hopping system used.  In case of SFH including BCCH frequency in hopping sequence is not a practical option, as it results in loss of traffic channels on BCCH carrier.  A separate frequency plan is prepared for the BCCH carriers.  This planning is very much similar to the conventional fixed frequency plan with lesser number of frequencies.  This plan needs to be done very carefully as the system monitors cells based on the BCCH frequency only. Since BCCH carrier radiates continuously without downlink power control, frequencies used for BCCH on one cell should not be used as hopping frequencies on other cell.  The reason is to avoid continuous interference from BCCH carriers.  The benefits of hopping increase if more frequencies are available for hopping. Generally the frequency band is divided into two parts, one used for BCCH frequency plan and other for hopping frequencies.  The division of frequency band for allocation of BCCH and hopping carriers should be done to maintain reasonable C/I for BCCH carriers as well as to have enough frequencies for hopping.

e.g. consider a network with 31 frequencies, using 12 frequencies for BCCH and using 18 for hopping with 1 frequency as guard, is the ideal option.  But it may not be practically possible to plan BCCHs with 12 frequencies (4/12 reuse).  Using 15 for BCCH plan and 15 for hopping frequencies is more practical.  There always exists a trade-off between BCCH and hopping plans.  Using very less frequencies for BCCH plan might result in poor quality on BCCH carrier and the advantages of having quality improvement on hopping carriers may be lost.

In case of BBH, generally BCCH carrier is included in the hopping sequence.  The benefits of BBH can be obtained only when most of the sites in the network are having more than one NBCCH carriers.  Benefits of BBH comparable to SFH can only be obtained by equipping additional hardware in order to include more frequencies in hopping sequence.  However BBH without additional hardware will result in quality improvements and provide scope of additional capacity as compared to fixed frequency plan though the benefits may not be as significant as seen in SFH.

2. Planning of HSN:

HSN allocation to the cells is done in random fashion.  Various scenarios are explained below:

a.       MA list is same for all the cells of the site – In this case HSN is kept same for all the cells of the site.  MAIO is used on per carrier basis to provide offset for starting frequency in hopping sequence and avoid hits among carriers of the site.  Practically it is possible to achieve 0% hit rate within the site, as all the cells of the same site are synchronized.

b.      MA list is same for the cells of different sites – In this case HSN should be different for all such cells.  MAIO can be same or different in this case as HSN is different.

c.       MA list is different for the cells – In this case HSN planning is not important, as there can not be any hits between these cells. 

d.      HSN is set to 0 – This is the case of cyclic hopping.  The sequence for hopping remains same and is repeated continuously.  This is not recommended in the urban environment where frequency reuse is more. This is because the network is not synchronized so if there is any one hit it will result in continuous sequence of hits.  Cyclic hopping is preferred in rural environment as it provides the maximum benefits of frequency diversity.

3. Planning of MAIO:

The benefits of MAIO planning can be best achieved only in case when sectors having same MA list are synchronized.  For non-synchronized sectors MAIO can be the same.  In the present version (GSR2), Motorola does not provide manual MAIO setting.  It is set automatically by the system.  However from GSR3 onwards it will be possible to set MAIO manually.  It has to be changed on a case to case basis.  In cases where there are large numbers of hits, MAIO change can be very effective as it adds the offset in the hopping sequence and hitrate can be reduced.

  

4. Planning of FHI:

This parameter is not specified in GSM.  FHI is the Motorola defined hopping system.  It actually means an independent hopping system consisting of MA and HSN.  Total of 4 such hopping systems can be set in a cell.  FHI can be defined on a timeslot basis.  e.g.  consider a cell with 3 carriers i.e. 2 carriers are hopping.  It is then possible to define 4 different FHIs for 16 timeslots.  That means timeslot 0 to 3 of 1 carrier can have one FHI and so on. 

Benefits and Drawbacks of FHI

·        Separate FHI can be defined even for each carrier with separate MA list.

·        For a fully utilized cell, FHI can be used to control increase in hitrate during peak hours.  This can be done by defining different MA list associated with a FHI for one of the carriers.

·        Main benefits of FHI can be obtained in BBH.  Consider a cell with 2 carriers using BBH with BCCH included in the hopping sequence.  Timeslot 0 of BCCH will not hop.  A separate FHI (with MA list without BCCH frequency) has to be defined for timeslot 0 of NBCCH.

·        Different FHIs in the same cell is not used extensively in Motorola networks with SFH, where BCCH frequency is not included in hopping sequence.

·        One drawback of using FHI on timeslot basis is that it adds more complexity to the database.

5. Reuse pattern for hopping carriers:

Conventionally there are 3 main reuse patterns followed for hopping frequencies.

1 X 1: It means all the cells in the network use the same frequencies for hopping.e.g. If 15 frequencies are to be used for hopping, then every cell will have all 15 frequencies in the MA list.  This type of reuse is useful in urban areas, where capacity requirement is large.  However there is very less planning involved and so less control over quality problems.

3 X 9: Three hopping groups are used in 3 sites, one per site.  In this case all the sites should be considered as omni sites for planning frequency reuse.  The advantage of this scheme is it provides better isolation between sites using same hopping frequencies.  The problem with this method is that, addition of new site may require frequency replan for the area.

1 X 3: This scheme is very commonly used in Motorola networks.  Hopping frequencies are divided in 3 groups.  Each cell on a site uses one group and it is repeated on all sites.  e.g. consider a network with standard orientation, all V1 sectors will use the same group and so on.  It is very easy to add a site in the network.  This reuse scheme is suitable for homogeneous network with minimum overlapping areas.  The problem with this scheme is in peak hours there may be more hits.6. Effect of Frequency Hopping

Handovers:  When SFH is implemented, BCCH plan is done using lesser number of  frequencies as compared to fixed frequency plan.  This may result in quality degradation.  However quality of hopping carriers improves than before.  Also, quality threshold for handovers on hopping carrier should be increased as compared to fixed frequency plan. In the present version (GSR2), same quality threshold settings are set for both BCCH and NBCCH.  This may result on more drop calls on BCCH carriers.  However GSR 3 provides separate settings for BCCH and NBCCH carriers.  By setting lower quality thresholds for BCCH as compared to NBCCH, number of dropped calls can be controlled. 

Call setup: In call setup, SDCCH hopping is also possible.  There are no separate settings required for SDCCH hopping. b Since GSR3 allows control over SDCCH configuration (location of SDCCH on timeslot basis), SDCCH hopping depends on the location of SDCCH.  In case of SFH (with BCCH not included in MA list), if SDCCHs are on BCCH carrier they will not hop whereas SDCCHs on NBCCH carriers may hop.  Generally it is preferred to keep SDCCHs on hopping carriers as they have better C/I compared to BCCH carriers.  Call success rate will depend on the cleanliness of BCCH carriers.       

Frame Erasure Rate (FER):  FER indicates the number of TDMA frames that could not be decoded by the mobile due to interference.  This parameter gives the indication of hitrate.  FER improves (gain of 6 to 8 dB) after implementation of frequency hopping.     

7. Tools for simulation and drive test: Motorola uses a tool “Handsem” which can simulate SFH plan (different reuse patters and HSN plan).  Latest versions of plaNET and Golf are supposed to support Frequency Hopping simulation.  Drive test tools that display decoded layer 3 information are used for monitoring frequency hopping networks.  TEMS is one of the drive test tools that can be used for the purpose.