Cell Search procedure in WCDMA

Cell Search procedure:

During the cell search, the mobile station searches for a cell and determines the downlink scrambling code and common channel frame synchronization of that cell. The cell search is typically carried out in three steps: slot synchronization; frame synchronization and code-group identification; and scrambling-code identification.

Step 1: Slot synchronization.

During the first step of the cell search procedure, the mobile station uses the SCH’s primary synchronization code to acquire slot synchronization to a cell. This can be done with a single matched filter matched to the primary synchronization code that is common to all cells.

Step 2: Frame synchronization and code-group identification.

During the second step of the cell search procedure, the mobile station uses the SCH’s secondary synchronization code to find frame synchronization and identify the code group of the cell found in the first step. This is done by correlating the received signal with all possible secondary synchronization code sequences and identifying the maximum correlation value. Because the cyclic shifts of the sequences are unique, the code group and the frame synchronization are determined.

Step 3: Scrambling-code identification.

During the third and last step of the cell search procedure, the mobile station determines the exact primary scrambling code used by the found cell. The primary scrambling code is typically identified through symbol-by-symbol correlation over the CPICH with all codes within thecode group identified in the second step. Afterthe primary scrambling code has been identified; the primary CCPCH can be detected. And the system- and cell specific BCH information can be read.

WCDMA Air interface channel

Briefly describe UMTS air interface channel types and their functions.

There are 3 types of channels across air interface – physical channel, transport channel and logical channel:

·         Physical Channel: carries data between physical layers of UE and NodeB.

·         Transport Channel: carries data between physical layer and MAC layer.

·         Logical Channel: carries data between MAC layer and RRC layer.

1.      Logical Channel:

·         Control channel:   BCCH, PCCH, CCCH, DCCH.

·         Traffic channel:    DTCH, CTCH.

2. Transport Channel:

·         Common control channel:  BCH, FACH, PCH, RACH, CPCH.

·         Dedicated channel: DCH, DSCH.

3.      Physical Channel:

·         Common control channel: P-CCPCH, S-CCPCH, P-SCH, S-SCH, CPICH,        AICH, PICH, PDSCH, PRACH, PCPCH, CD/CA-ICH.

·         Dedicated channel:         DPDCH, DPCCH.

Broadcast Control Channel (BCCH):

 It broadcast system control information for all mobiles in a cell like Code values in the cell, neighbor information, allowed power levels in downlink direction.

Paging Control Channel (PCCH):

   It transfers paging information in downlink direction.  Network page the mobile to discover the UE location or UE in a cell connected state.

Common Control Channel (CCCH):

Network may have certain tasks which are or may be common to UE in the cell. It’s being used in both direction DL & UL. The CCCH is also used when UE is accessing a new cell after cell reselection.

Dedicated Control Channel (DCCH):

When there is dedicated/active connection means RRC connection between the network and the mobile, the control information transferred using DCCH. It’s a bi-directional channel.

Dedicated Traffic Channel (DTCH):

It is used transfer the user data between the network and the UE in both uplink and downlink directions.

Common Traffic Channel (CTCH):

It is used to transfer the data from one point to all mobiles or a specified group of mobiles.

Dedicated Channel (DCH):

 It is the channel used to send dedicated control and user data between UE and the network in both uplink & downlink directions

Random Access Channel (RACH):

It is used to send control information from UE in the uplink direction. Also may carry short user packets.

Forward Access Channel (FACH):

It is a downlink common channel used to send small amounts of control and user data.

Broadcast Channel (BCH):

It broadcast system information in the downlink direction for all mobile stations in a cell.

Paging Channel (PCH):

It’s a downlink common channel used to send paging notification messages.

Dedicated Channel (DCH):

It is a channel that is used to send dedicated control and user data between the UE and the network in both directions

(Paging Indicator Channel) PICH:

 It is used to carry paging and notification message to all UEs and it is always associated with SCCPCH

 (Acquisition Indicator channel) AICH:

It is used in random access procedure by network to indicate that the RACH preamble was detected

(Synchronization channel) SCH:

It is downlink channel. It is used to cell search procedure and synchronization with network. There is two type of SCH primary & secondary

 (Physical Random access channel) PRACH:

It is uplink channel and Initial message when UE want gain access to network and transfer small amount of data

(common Pilot channel) CPICH:

It is downlink channel and used to continuously sending scrambling code to cell. It is also aids channel estimation for cell re-selection and Handover for UEs. There are two types of CPICH- Primary and secondary.

 (Primary-common control physical channel) P-CCPCH:

It is downlink channel and used to carry Synchronization channel (SCH) and BCH. It has fixed rate of 30 Kbps.

 (Secondary- common control physical channel) S-CCPCH:

It is downlink channel and used to carry PCH and FACH. It has variable bit rate from 30kbps to 1,920 kbps.

Dedicated Channels:

Dedicated Physical Data Channel (DPDCH)

Dedicated Physical Control Channel (DPCCH)

Dedicated Physical Control Channel (DPCCH)

For each radio link, there is one DPCCH, it carries control information like

• Pilot bits (used to support channel estimation at rake receiver)
• Transmit Power Control (TPC) commands (Used power control)
• Feedback information (FBI) needed with transmit diversity
• Transport Format Combination Indicator (TFCI)

Spreading factor is always 256.

Dedicated Physical Data Channel (DPDCH)

 Each radio link may have no DPDCHs or several DPDCHs.

• Spreading factor for the DPDCH can be between 256 and 4.

DPCCH & DPDCH are time – multiplexed in downlink but uplink these are I/Q modulation.

What is Erlang ?

Erlang :   An Erlang is a unit of telecommunications traffic measurement.  Strictly speaking, an Erlang represents the continuous use of one voice path.  In practice, it is used to describe the total traffic volume of one hour.  in other words " a one ckt  can one hour of traffic is known as 1Erlang"

For example, if a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows:

Minutes of traffic in the hour	=	number of calls x duration
Minutes of traffic in the hour	=	30 x 5
Minutes of traffic in the hour	=	150
Hours of traffic in the hour	=	150 / 60
Hours of traffic in the hour	=	2.5
Traffic figure	=	2.5 Erlangs

The main Erlang traffic model are listed below

  Erlang BThis is the most commonly used traffic model, and is used to work out how many lines are required if the traffic figure (in Erlangs) during the busiest hour is known.   The model assumes that all blocked calls are immediately cleared.

  Extended Erlang BThis model is similar to Erlang B, but takes into account that a percentage of calls are immediately represented to the system if they encounter blocking (a busy signal).  The retry percentage can be specified.

  Erlang CThis model assumes that all blocked calls stay in the system until they can be handled.  This model can be applied to the design of call center staffing arrangements where, if calls cannot be immediately answered, they enter a queue.  

LTE Architecture

WCDMA Architecture

GSM Architecture

GSM Architecture

GSM channel concept

Channel concepts:

There are two types of channel in GSM Network - Control channel and Traffic channel: 

The traffic Channels:  are used to transport encoded speech and data information. Full rate traffic channels TCH/F are defined using a group of 26 TDMA frames called a 26 frame multi-frame. The 26 frame multi-frame lasts 120ms and the traffic channels for the downlink and uplink are separated by three bursts. The TCH/F consists of one time slot in each TDMA frame i.e., one slot every 4.615ms.

The control Channels: are used for network management messages (call set up, control signaling etc.) and some channel maintenance tasks. These can be subdivided into BCH (Broadcast Channel), CCCH (Common Control Channel), and DCH (Dedicated Channel)

BCH: BCCH, FCCH, SCH / CCCH: RACH, AGCH, PCH / DCH: SDCCH, SACCH, FACCH, CBCH

BCCH (Broadcast channel): Its downlink channel is used for transmitting of system information unique BCCH define for entire cell and it is transmitted on Timeslot zero of BCCH carrier and Read only by idle mobile at least once every 30 secs

SCH (Synchronization Channel): It’s a Downlink channel and Carries information for frame synchronization and Contains TDMA frame number and BSIC.

FCCH (Frequency Correction Channel) it is a Downlink only and Enables MS to synchronize to the frequency and also helps mobiles of the ncells to locate TS 0 of BCCH carrier

RACH (Random Access Channel) It is Uplink only and Used by the MS to access the Network.

AGCH (Access Grant Channel) It is Downlink only and Used by the network to assign a signaling channel upon successful decoding of access bursts.

PCH (Paging Channel): Downlink only and Used by the Network to contact the MS.

SDCCH (Standalone Dedicated Control Channel): Its Uplink and Downlink and Used for call setup, location update and SMS.

SACCH (Slow Associated Control Channel): Used on Uplink and Downlink only in dedicated mode and Uplink SACCH messages - Measurement reports. Downlink SACCH messages - control info.

FACCH (Fast Associated Control Channel): Uplink and Downlink and Associated with TCH only and it are used to send fast messages like handover messages. Works by stealing traffic bursts

CBCH (Cell broadcast channel): It is used transmitting of broadcasting msg

TCH/f and TCH/H : gross rate 22.8kbps, speech 13kbps, data 9.6kbps     (gross rate = 57*2*24/120ms = 22.8KBPS)