CHANNEL CODING

CHANNEL CODING:

The Different methods of channel coding used in GSM are,

· Block coding

· Convolutional coding

Block coding:

When the block coding is used, one or several check bits are added to the information block. The check bits only depend on the bits in that every block. A simple form of block coding is using is parity coding. Block coding is mainly used for detecting errors.

Convolutional coding:

The Convolutional coder consists of a shift register into which the information bits are shifted one by one. Convolutional coding is not only good for detecting errors, but also for correcting them.

INTERLEAVING:

Interleaving is a method spreading the potential losses, so that they can be taken care of by “channel coding” thus minimizing the harm burst.

In GSM the channel coder produces a total of 456 bits for every 20 ms segment of speech. These are in blocks of 57 bits interleaved over the burst.

EQUALISER:

The equalizer will mainly address the problems of Inter Symbol interference, desired earlier. The problem is that the air interface affects the signals in some way that causes bit error in the receiving side.

In a normal burst, used for traffic, there is a 26 bits training sequence in the middle of the burst.

TIMING ADVANCE:

The radio signals take finite period of time to travel from the mobile station to the base station. it is called propagation delay.

The system will repeatedly send “timing advance” orders to the MS. The system will simply tell the MS how many bit times earlier, or later, to send. These decisions are based on an analysis of how the bursts are received in the base station.

• BSS calculates access delay from RACH in terms of bits.
• Informs mobile to delay its in terms of bits
• Maximum Timing advance of 63 bits.

AREAS in GSM

1) PLMN SERVICE AREA

The Public Land Mobile network (PLMN) is a geographical area served by one network operator and is defined as the area in which an operator offers radio coverage and possibility to access its network.

2) MSC/VLR (SERVICE AREA)

If the system has more than one MSC,the PLMN is subdivided into several MSC/VLR service areas. To be able to route calls to the right MSC and eventually to the right MS, it is necessary to know in which MSC/VLR service area the MS is.

The HLR stores the data ,about which MSC/VLR service area the MS is in. The VLR contains detailed information about all the MS:s in the MSC/VLR service area.

3) LOCATION AREA

Location area is an identity, which specifies a group of cells as defined by the technicians is called as location area.

Each MSC/VLR service area is subdivided into a number of location area (LA). Information about, in which location area an MS is registered, is also stored in the VLR together with subscriber data of all the visiting subscribers in that MSC/VLR service area.

If an MS move within the location area ,the system does not need to change the information in the subscriber register i.e.VLR and HLR.If the MS crosses over into a cell belonging to a new location area however, the system must be informed. This report done by the MS to the system is called “location updating”.

One LA may include cells from different BSC: s, and cells belonging to one BSC may be subdivided into several LA: s.

CLUSTER

A group of neighboring cells using all the frequencies available in the system frequency band is called a cluster of cells.

4) CELLS

Location area subdivided into a number of cells. A cell is the geographical area covered by one Base Transceiver Station. A cell is the smallest geographical entity in a PLMN.A cell could be any size, from a radius of tens of kilometers down to a radius of tens or hundreds of meters.

GSM SERVICE AREA

The GSM service area is the total geographical area in which subscriber can access the network. The more operators who sign contracts agreeing to work together, the more this area will increase.

MOC and MTC

CALL TO A MOBILE STATION

The difference between making a call to a mobile subscriber and a PSTN network subscriber is that the mobile subscriber’s location is unknown. Therefore, the mobile station must be paged before a connection can be made. The steps in the call setup procedure from a PSTN subscriber to a mobile station are listed below. The numbers refer to figure

1. The PSTN subscriber dials the mobile subscriber’s number. the Gateway MSC receives the call.

2. The Gateway MSC requires the HLR for the information needed to route the call to the serving MSC/VLR.

3. The GMSC routes the call to the MSC.

4. MSC checks VLR for the Location Area of the mobile station.

5. MSC contacts the mobile station via BSC and BTS by sending a page request.

6. The mobile station responds.

7. BSC selects a traffic channel and orders the mobile station to tune to this traffic channel. The mobile station generates a ringing signal and when the subscriber answers the speech-connection is established.

CALL FROM A MOBILE STATION

When a mobile station wishes to establish a speech call, the following steps are performed: the numbers refer to figure.

1. Mobile subscriber dials the number.
2. MSC /VLR receives a message requesting access.
3. MSC /VLR checks if the mobile station is authorized to initiates a call set up to the PSTN network.
4. the dialed number is analyzed by MSC /VLR , which in turn initiates a call set-up to the PSTN network.
5. MSC / VLR asks bsc to allocate a free traffic channel. This information is forwarded to BTS and the mobile station.
6. the person receiving the call answer and a connection is established.

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 gain

1/5 interference: 6 dB gain

2/4 interference: 0 dB gain

2/5 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 number

MA: 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.

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 Burst

As 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 Bit

As 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 Delay

Now, 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 Cell

In 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 Advance

When 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.