Differences between WCDMA and Second Generation Air Interfaces

Main differences between the third and second generation air interfaces are described. GSM and IS-95 (the standard for cdmaOne systems) are the second generation air interfaces considered here. Other second generation air interfaces are PDC in Japan and US-TDMA mainly in the Americas; these are based on TDMA (time division multiple access) and have more similarities with GSM than with IS-95. The second generation systems were built mainly to provide speech services in macro cells. To understand the background to the differences between second and third generation systems, we need to look at the new requirements of the third generation systems which are listed below:

       ·         Bit rates up to 2 Mbps;

·         Variable bit rate to offer bandwidth on demand;

·         Multiplexing of services with different quality requirements on a single connection, e.g. speech, video and packet data;

·         Delay requirements from delay-sensitive real time traffic to flexible best-effort packet data;

·         Quality requirements from 10 % frame error rate to 10_6 bit error rate;

·         Co-existence of second and third generation systems and inter-system handovers for coverage enhancements and load balancing;

·         Support of asymmetric uplink and downlink traffic, e.g. web browsing causes more loading to downlink than to uplink;

·         High spectrum efficiency;

·         Co-existence of FDD and TDD modes.

GSM also covers services and core network aspects, and this GSM platform will be used together with the WCDMA air interface: see the next section regarding core networks.

 

 

 

 

 

 

GSM PHASES

In the late 1980s, the groups involved in developing the GSM standard realized that within the given time-frame they could not complete the specifications for the entire range of GSM services and features as originally planned. Because of this, it was decided that GSM would be released in phases with phase 1 consisting of a limited set of services and features. Each new phase builds on the services offered by existing phases.

Phase 1

Phase 1 contains the most common services including:

·         Voice telephony

·         International roaming

·         Basic fax/data services (up to 9.6 kbits/s)

·         Call forwarding

·         Call barring

·         Short Message Service (SMS)

Phase 1 also incorporated features such as ciphering and Subscriber Identity Module (SIM) cards. Phase 1 specifications were then closed and cannot be modified.

Phase 2

Additional features were introduced in GSM phase 2 including:

·         Advice of charge

·         Calling line identification

·         Call waiting

·         Call hold

·         Conference calling

·         Closed user groups

·         Additional data communications capabilities

Phase 2+

The standardization groups have already defined the next phase, 2+. This program covers multiple subscriber numbers and a variety of business oriented features. Some of the enhancements offered by Phase 2+ include:

·         Multiple service profiles

·         Private numbering plans

·         Access to Centrex services

·         Interworking with GSM 1800, GSM 1900 and the Digital

Enhanced Cordless Telecommunications (DECT) standard Priorities and time schedules for new features and functions depend primarily on the interest shown by operating companies and manufacturers and technical developments in related areas.

Phase 2++ This phase includes sophisticated enhancements to the radio interface including:

·         Enhanced Datarates for Global Evolution (EDGE), a new modulation method which increases capacity on the air interface.

·         Customized Application for Mobile Enhanced Logic (CAMEL), a standard, governing IN service access while roaming internationally.

·         High Speed Circuit Switched Data (HSCSD), a method of delivering higher data rates per subscriber by allocating an increased number of time-slots per call.

Remote OMT and Remote OMT over IP

The features Remote OMT (Operation and Maintenance Terminal) and Remote OMT over IP are updated to support the new RBS 6000 DUG-20/RUS-01 configurations. In MCPA backwards compatible mode, having a BTS G11A or newer in a BSS 07B-G10B network, the configuration of an MCPA is made using OMT. Configuration in MCPA single mode (BTS G11B with BSS G10B or newer) is made from the BSC, refer to Section 5.7 on page 43.

All TRXs in a DUG are connected to one or several RUSs. It is the connections between RUSs and antennas that will decide which MCPAs to use for which antenna sectors (cells).

Each antenna sector is configured in the OMT with the default configuration of 3*20W (3*43.0 dBm) per MCPA. If desired it is possible to choose a different number of TRXs per MCPA, and it is possible to choose some configurations where the total MCPA mean power will end up on less than 60 W (47.8 dBm). Also the levels 40W (46.0 dBm) and 20W (43.0 dBm) are available.

The chosen number of TRXs and MCPA maximum mean power corresponds to different GSM RUS HW activation codes, although there is no licensing mechnism involved in the OMT based configuration.

Automatic FLP - Frequency Load Planning

Automatic FLP will enable operators to run Frequency Load Planning (FLP) networks, including Synchronized Radio Networks, with minimum effort and maximum performance. The feature performs daily downlink interference matrix measurements and creates synchronization clusters for synchronization status monitoring. FLP parameters are continuously supervised and automatically adjusted to network changes when needed due to for instance lost synchronization, addition of TRX HW, or changes in hopping frequency sets. The parameters that are put under direct BSC control by Automatic FLP activation are, HSN, FNOFFSET, MAIOs, TSC and FSOFFSET.

By using Automatic FLP an operator will get the following benefits:

·         Maximum capacity gain from FLP (best parameter configuration always used. Parameter settings can be kept continuously optimized)

·         Maximizes performance in all types of FLP networks (for example lower drop call rate, better speech quality)

·         Reduced O&M cost for FLP networks (Ease of use). Parameter settings will be optimized made without user intervention.

·         Necessary parameter changes in response to unplanned network changes can be very fast

·         Possibility to monitor synchronization status for each cell. Enables immediate FLP reconfiguration after loss of synchronization.

GSM - LTE Cell Reselection

 Broadcasts LTE system information in the GSM network to enable idle and packet transfer  mode cell reselection from GSM to LTE networks.

Each GSM cell broadcasts information about:

• Neighboring cells (WCDMA and LTE)

• Thresholds for IRAT

• Priority between GSM, WCDMA and LTE cells

The information is broadcasted in the GSM network via the system information message SI2quater.

The main purpose with priority based cell reselection is to allow reselection to LTE, but at the same time it also introduces cell reselection based on priority towards WCDMA. This is an alternative to the existing non priority based cell reselection to WCDMA. When cell reselection to LTE is used, cell reselection to WCDMA will be priority based as well. In MSs not supporting priority based cell reselection, the non-priority based cell reselection to WCDMA is used if the feature "GSM-UMTS Cell Reselection and Handover" is activated.

Commands and Printouts

• RLSRI: Radio Control Cell, System Information RAT Priority, Initiate - This new command is used to inform that priority based cell reselection shall be used.

• RLSRC: Radio Control Cell, System Information RAT Priority Data, Change - This new command have three formats, one each for GSM, WCDMA and LTE. The purpose with the commands are to configure different priorities between GSM, WCDMA and LTE in the cells.

• RLSRE: Radio Control Cell, System Information RAT Priority, End - This new command is used to inform that priority based cell reselection shall be deactivated.

• RLSRP: Radio Control Cell, System Information RAT Priority Data, Print This new command prints the state of the priority based cell reselection.

• RLSEI: Radio Control Cell, System Information E-UTRAN Restriction, Initiate - This new command is used to black list individual E-UTRAN (LTE) cells for cell reselection per LTE frequency in each GSM cell.

• RLSEC: Radio Control Cell, System Information E-UTRAN Restriction, Change - This new command is used to black list specific LTE cell groups for cell reselection per LTE frequency in each GSM cell.

Time Division Multiple Access (TDMA)

 GSM uses Time Division Multiple Access (TDMA) as its access scheme. This is how the MS interfaces with the network. TDMA is the protocol used on the Air (Um) Link. GSM uses Gaussian Minimum-Shift Keying (GMSK) as its modulation methods.

Time Division means that the frequency is divided up into blocks of time and only certain logical channels are transmitted at certain times. Logical channels will be introduced in the next lesson. The time divisions in TDMA are known as Time Slots.

Time Slots : A frequency is divided up into 8 time slots, numbered 0 to 7.

Absolute Radio Frequency Channel Number (ARFCN)

The ARFCN is a number that describes a pair of frequencies, one uplink and one downlink. The uplink and downlink frequencies each have a bandwidth of 200 kHz. The uplink and downlink have a specific offset that varies for each band. The offset is the frequency separation of the uplink from the downlink. Every time the ARFCN increases, the uplink will increase by 200 khz and the downlink also increases by 200 khz.

The given below table summarizes the frequency ranges, offsets, and ARFCNs for several popular bands.

Calculating Uplink/Downlink Frequencies

The following is a way to calculate the uplink and downlink frequencies for some of the bands, given the band, the ARFCN, and the offset.

GSM 900

Uplink = 890.0 + (ARFCN * .2) & Downlink = Up + 45.0

Example:

 Given the ARFCN 72, and we know the offset is 45MHz for the GSM900 band: Up = 890.0 + (72 * .2) Up = 890.0 + (14.4) Up = 904.40 MHz

Down = Up + Offset Down = 904.40 + 45.0 Down = 949.40 MHz

The Network and Switching Subsystem

Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below. 

The Mobile services Switching Center (MSC)
It is the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.

  The Gateway Mobile services Switching Center (GMSC)

A gateway is a node interconnecting two networks. The GMSC is the interface between the mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.

Home Location Register (HLR)

The HLR is considered as a very important database that stores information of the subscribers belonging to the covering area of a MSC. It also stores the current location of these subscribers and the services to which they have access. The location of the subscriber corresponds to the SS7 address of the Visitor Location Register (VLR) associated to the terminal.

Visitor Location Register (VLR)

The VLR contains information from a subscriber's HLR necessary in order to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough information in order to assure the subscribed services without needing to ask the HLR each time a communication is established.
The VLR is always implemented together with a MSC; so the area under control of the MSC is also the area under control of the VLR.

The Authentication Center (AuC)

The AuC register is used for security purposes. It provides the parameters needed for authentication and encryption functions. These parameters help to verify the user's identity.

The Equipment Identity Register (EIR)

The EIR is also used for security purposes. It is a register containing information about the mobile equipments. More particularly, it contains a list of all valid terminals. A terminal is identified by its International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g., a terminal which does not respect the specifications concerning the output RF power).

 The GSM Inter-working Unit (GIWU)

The GIWU corresponds to an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated. 

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)

Troubleshooting for GSM KPIs (SD Block & SD Drop)

KPIs to be monitored:

·            SD Blocking

·            SD Drop

SDCCH Channel:

·   SDCCH channel is a dedicated channel which is using for LAC updation, Call Setup, SMS in Ideal mode. It works in UL & DL

SD Blocking:

·   SD Blocking means that you are not getting SD resource for the call origination. When MS connects with Network then RACH and AGCH are provided. After AGCH, SDCCH is provided but if SDCCH is not provided at this time due to some problem or due to unavailable of SD by BSC.

KPI Formula in Ericsson:

·            SDCCH CONGESTION = (CCONGS / CCALLS) * 100

·            CCONGS - Congestion counter.

·            CCALLS - Channel allocation attempt counter (on SDCCH).

Reason for SD Blocking:

·            LAC Boundary

·            High Volume of SMS

·            SD utilization is high

·            Time Slot faulty

·            Adaptive configuration of logical channel switch off

·            Wrong SD Dimension

·            Incorrect CHAP Settings

·            Hardware Issue

Solution for Removal of SD Blocking:

·   Check the no of SD channels available, if less, then increase SD channel while TCH Blocking should be taken care.

·   Check LAC boundary, if location update is more, then change the LAC of that site and set C2 and HYS.

·   Use of dynamic SDCCH (it is a BSC parameter)

·   Shift SD to new time slot

·   Adaptive configuration of logical channel switch ON

·   Check for T3212 value

Need to check which parameter:

1. CHAP (Channel Allocation Profile): Its Immediate Assignment Process on TCH, It    provides different channel allocation strategies,

        CHAP 0: Immediate assignment on TCH is not permitted,

        CHAP 1: Immediate assignment on TCH is last preference, where in TCH is allocated at immediate assignment only when there is no Idle SDCCH is available

CHAP2: Immediate assignment on TCH is first preference where in SDCCH may only be allocated when there are no idle TCH is available.

2. Adaptive configuration of logical channel (ACLP): The purpose of this features dynamic reconfiguration of Idle TCH Channel to SDCCH Channel, when there is SDCCH High load

S LEVEL Defines: Reconfigure of an Idle TCH to an SDCCH will take place; Default 0 Congested rate for a cell is increase S LEVEL 2

S TIME Define Minimum Time Interval between SDCCH, Can be reconfigured back to TCH, Default value 20s, Range 15s to 3600s

3. T3212: Periodic update timer value:  High volume of LAC Border can cause SD Congestion so optimize the periodic registration timer.  Irrespective of the location, coverage, activity, the mobile has to update its location to the MSC after a defined time/period.

4. CRH (Cell Reselection Hysteresis) :Receiving Signal strength hysteresis for required cell reselection over location area border, In order to overcome the Ping-Pong effects in cell reselection across location area borders, CRO and PT can also used to delay reselection in LAC Borders.(Location area code is an identity number given to the site of a base station)

SD DROP:

                When SD is assigned for a mobile during call connection process and during this time due to any problem or any mismatch occurs by which SD loss occurs, It is between allocation of SD and before TCH allocation.

KPI Formula in Ericsson:

·   SDCCH Drop Rate = (CNDROP-CNRELCONG/CMSESTAB)*100

·   CNDROP- Total number of dropped SDCCH channels in a cell (for the measurement Period).

·   CNRELCONG- Total number dropped (released) connections on SDCCH due to TCH or Tran-coder congestion.

·   CMSESTAB - Total number of successful MS channel establishment on SDCCH.

Reason for SD Drop:

·            Overshooting

·            Shift the SD time slot

·            Interference

·            It may be uplink or downlink issue in which cells foe UL put TMA in that cell and DL provide tilt

·            HW Issue

·            Wrong parameter planning

·            Bad coverage

·            MAIO mismatch

·            High Pathless

·            High LAPD utilization

·            Wrong Power control settings

·            Check the Timer T 3101

·            Check the Timer T 200(20ms)

Solution for Removal of SD Drop:

Interference:

·            Check the BCCH Plan(C/I or C/A)

·            Co-BSIC & Co- BCCH

·            To find out proper frequency to reduce interference

Overshooting:

·            LAC Planning

·            If a cell is picking call from long distance, check the sample log according to TA

·            Cell orientation need to defined according to clutter

Bad Coverage:

·            If the drop call is due to low signal strength uplink, check the receive path of this particular TRX. Check receiver sensitivity, VSWR, feeder connection and etc. Drops due to Low Signal Strength.

·            If the drop call reason is due to low signal strength downlink, then, check the transmit path. Check cards, feeder and etc.

Hardware Fault:

·            Check Alarms.

·            TRX condition.

·            Check Path Imbalance.

·            VSWR of the Cell.

·            Connector Connection.

·            Sometimes you will find issues on BCCH TRX. In this case BCCH shift from one to other TRX will reduce SD drop

Need to check Which Parameter:

Drop Reason mainly Low signal strength (UL & DL), Bad Quality (UL & DL) and Excess Timing Advance

And High Interference (Co-BCCH & Co-BSIC), Wrong Power control Settings and Too High of CRH Can result in SDCCH drops

Power control settings: Lack of good power control settings for SDCCH can lead to excess drop. Two types of power control 1.MS Power control 2.BTS Power Control

SDCCHREG: Function of this parameter Enable (1)/Disable (0), Enable the power control to minimize the drops

INIDES (Initial desires signal strength): For the SDCCH UL and DL, Default value -70dbm, especially UL impact of drop rate, because extremely sensitive to interference, for INDIES from -70 dbm to -85dbm this will reduce the cumulative power emitted by mobile closer to base station (which need not transmit at very high power to communicate with the BS) and this will reduce the interference

Due to ICM Band (CDMA):

·            Some time SD drops takes place due to near sites of CDMA.

·            Check the ICM band value of that site.

·            Use BPF (Band pass filter).