Interleaving

Interleaving is a simple, but powerful, method of reducing the effects of burst errors and recovering bits when burst errors occur. The symbols (output of Forward Error Correction Coder) from each group are interleaved in a pattern that the receiver knows. The interleaver is located at the BTS and in the phone.

Duplexing

Duplexing :

Duplexing is the technique by which the send and receive paths are separated over the medium, since transmission entities (modulator, amplifiers, demodulators) are involved.

There are two types of duplexing.\

1. Frequency Division Duplexing FDD

2. Time Division Duplexing TDD 

      Frequency Division Duplexing FDD

Different Frequencies are used for send and receive paths and hence there will be a forward band and reverse band. Duplexer is needed if simultaneous transmission (send) and reception (receive) methodology is adopted .Frequency separation between forward band and reverse band is constant

Time Division Duplexing (TDD)

TDD uses different time slots for transmission and reception paths. Single radio frequency can be used in both the directions instead of two as in FDD. No duplexer is required. Only a fast switching synthesizer, RF filter path and fast antenna switch are needed. It increases the battery life of mobile phones.

GSM and CDMA systems use Frequency Division Duplexing and corDECT uses Time Division Duplexing.

Cell Load Sharing

–The purpose of the Cell Load Sharing feature is to distribute some of a cells traffic load to surrounding cells during peaks in traffic.

−This is achieved by moving established connections to neighboring cells that have idle resources.

−Cell Load Sharing increases the number of handovers in the part of the network where the traffic load is unevenly distributed

-Cell Load Sharing is activated on the BSC level via parameter LSSTATE (Active/Inactive) and activated on cell level via parameter CLSSTATE (Active/Inactive)

–The traffic load (amount of idle full rate TCHs) on each cell is examined by the BSC every CLS time Interval defined by a parameter CLSTIMEINTERVAL (default=100msec)

−If the percentage of idle full rate traffic channels is ≤ parameter CLSLEVEL, then this cell will try to get rid of some traffic by initiating cell load sharing handovers to neighbors.

−For a neighbor cell to accept HOs due to cell load sharing then parameter HOCLSACC should be set to “ON”

−The traffic load on the neighbor cells should also be examined so handovers due to cell load sharing will only be done to neighbors having enough idle full rate TCHs ( percentage of idle full rate TCHs > CLSACC inorder to accept HO due to CLS)

–CLS evaluation is performed after normal locating evaluation for neighboring cells.

–The normal Basic ranking evaluation was done as follows:

Rankservingcell = SS_DLservingcell

Rankneighbor= SS_DLneighbor – OFFSETneighbor – HYSTneighbor

−Now when the % idle full rate TCHs < CLSLEVEL, then the HYST for neighbors will be recalculated with reduced values based on parameter RHYST

−Rankneighbor= SS_DLneighbor – OFFSETneighbor – HYSTnew neighbor

Where HYSTnew neighbor = HYSTneighbor [1-2 (RHYST/100)]

Multi Band Cells ( MBC ) in GSM

Multi Band Cells (MBC)

A multi band network consists of cells from different frequency bands for example: 900/1800 MHz

−By combining these frequencies in the same cell with 1 common BCCH, the radio performance and traffic capacity are improved where the no. of cells and

-Using MBC concept with only 1 BCCH, this will reduce the no. of defined neighbors to 50% leading to better accuracy for the measurement reports coz there will be more time available for measurements for each neighbor.

-The Dynamic OL/UL subcells (Concentric cells) is a prerequisite feature for the Multi Band Cells.

−Mostly the frequency band with “Better coverage” (i.e. lower frequency band) is configured as the Underlaid subcell while the other frequency band with “Worse coverage” (i.e. higher frequency band) is configured as the Overlaid Subcell.

−Ex: 900MHz frequency band UL, while 1800MHz frequency band OL

−It is recommended to select the BCCH frequency to lie in the “Better Coverage” i.e. UL subcell.

−for the previous example then BCCH frequency will belong to the 900MHz band

−A parameter CSYSTYPE defines the band of the used BCCH frequency in a multi band cell.

−A parameter BAND defines the band of the Channel Group, where the channel group consists of no. of frequencies as will be seen later.

−As mentioned before, the path loss/Distance to cell border/time advance criteria will define the coverage limit of the frequency band used in the OL subcell vs. UL subcell, (In this case the OL&UL will belong to two different bands)

−Also the traffic load can be maintained between the two subcells (that belong to two different bands) using the subcell load distribution feature where the SCLDSC parameter will define which subcell is preferred first.

Overlaid Underlaid Subcells in GSM

−Traffic Capacity of a cellular network can be increased by either adding more frequencies or reducing the frequency reuse distance.

−One approach is to apply a second frequency re-use pattern with a tighter frequency reuse (Overlay) on the existing pattern.

−These cells should be restricted in size, so shorter reuse distance can be accomplished without causing Co-channel/Adjacent channel interference.

−They are termed Overlaid (OL) Subcells, whereas the original cells will be called Underlaid (UL) Subcells.

−Now by having more frequencies per cell, then Network capacity is increased

-The fundamental idea behind the OL/UL subcells is to let the traffic close to the site to be moved to the OL subcell, while traffic close to the cell border to be moved to the UL subcell.

−In that way of treading the traffic, the frequencies in the OL subcell can have tighter frequency reuse.

-Using the OL/UL concept we can solve the case as follows:

-f4 will be used in the OL subcell and it will be restricted to serve in a small area only near to the site so interference from the neighbor cell will be minimized and a good C/I can be enjoyed.

To maintain the service area of the OL subcell restricted to a certain region we have three thresholds we can play with:

A.Path Loss Threshold

B.Timing Advance Threshold

C.Distance to Cell Border Threshold

−With the ordinary OL/UL subcells, the MS near the cell will camp on the overlaid subcell but even if the OL subcell got high utilized there is no way to push traffic to the UL subcell.

−Using Subcell Load Distribution (SCLD) Concept, we can configure the cell to use the OL as the preferred subcell initially and when traffic on the OL increased beyond certain load, any extra traffic will be offloaded to the UL subcell.

Monitor the Incoming Paging in GSM

Paging Groups

−The MS will monitor the incoming paging in only specific times, and the rest of the time it will remain in sleeping mode.

−In this way we save the MS battery and we decrease the UL interference on the system.

−The MS will monitor the incoming paging when the “Paging Group” assigned for this MS is transmitted only.

−The CCCH block can be used by either PCH or AGCH.

−When the CCCH block is used for paging it will be called “Paging Block”

−The Paging Block consists of 4 consecutive Time slots lie in 4 consecutive frames.

−The Paging Block can be used to page 4/3/2 users according to IMSI or TMSI is used when paging the MS ( Length IMSI = 2 TS, Length TMSI = 1 TS)

−The group of users belong to the same paging block will be called “Paging Group” 

IMSI, MSISDN

MCC	MNC	MSIN
( 3Digits)	( 2Digits)	( 10Digits)

IMSI : International Mobile Subscriber Identity IMSI = MCC + MNC + MSINMCC= Mobile Country Code (3 digits)MNC= Mobile Network Code (2 digit )MSIN= Mobile Subscriber Identification Number (10 digits)Ex: IMSI = MCC-MNC-MSIN = 404-22-1234567890 where,602 -  India Country Code22  -   Bharti Airtel  Network Code1234567890 -  Mobile Subscriber Identification Number
MSISDN : Mobile Station Integrated Services Digital NetworkMSISDN = CC + NDC + SNCC= Country Code (2-3 digits)NDC= Network Destination Code (2-3 digit )SN= Subscriber Number ( max 10 digits)