CDMA - Network

CDMA Network Overview

A base station is an essential element of the CDMA network. A base station covers a small geographical area called a cell. A cell may be omnidirectional or sectoral. Each base station has a transmitting antenna and two receiving antennas for each cell. Two receiving antennas are used per cell for the purpose of spatial diversity. In many applications, it is a BSC (Base Station Controller), which controls several base stations.

As the rate of the mobile phone data is either 13kbps or 8kbps, which is nonISDN, but the switches which are the mobile switching center (MSC) are generally switched to 64 kbps. Therefore, before it is switched, it is necessary to convert this mobile data rates to 64 kbps. This is accomplished by a member, which is the transcoder. The transcoder may be a separate element or it can be collocated in each base station or MSC.

All base stations are connected to the MSC, which is the mobile switching center. MSC is the entity that manages the establishment, connection, maintenance, and disposal of calls within the network and also with the outside world.

MSC also has a database called HLR/AC, which is a home location register/authentication center. HLR is the database, which maintains the database of all network subscribers. AC Authentication Centre is the part of the security of the HLR, which some algorithms to examine mobile phones.

The MSC is connected to the outside world, i.e. the fixed line network. MSC can also be connected to several other MSCs.

CDMA Identities

Network Identities −

SID (System Identity)
NID (Network Identity)

Mobile Station Identities −

ESN (Electronic Serial Number)
Permuted ESN
IMSI (International Mobile Station Identity)
IMSI_S
IMSI_11_12
Station Class Mark

System and Network Identity

A base station is a member of a cellular system and a network. A network is a subset of a system. The systems are installed with an identification called Identification System (CIS). The networks with a system receiving is Network identification (NID). It is a uniquely identified network pair of (SID, NID). The mobile station has a list of one or more home (non-roaming) pairs (SID, NID).

SID

A system identification indicator 15 bits (SID) is stored in a mobile station. It is used to determine the host system of the mobile stations. The bit allocation of the system identification indicator is shown below.

The distribution of international codes (INTL) (bits 14 and 13) is also shown in the table. Bits 12-0 is assigned to each US system by the FCC for non-US countries. The bit allocation will be made by local regulatory authorities.

NID

NID has a range of 0-65535 reserved values. Value of 65535 in a SID means, NID pair is to indicate that the Mobile Station considers the entire SID as home.

Systems and Networks

A mobile station has a list of one or more home (non-roaming) pairs (SID, NID). A mobile station is roaming when the base station broadcast (SID, NID) pair does not match with one of the non-roaming mobile stations (SID, NID) pairs.

A mobile station is a foreign NID roamer −

if the mobile station is roaming and there are some (SID, NID) pair in the mobile stations (SID, NID) list that corresponds to SID.
if the mobile station is roaming and there are some (SID, NID) pair in the mobile stations (SID, NID) list for which no matching SID is available (means a mobile station has roaming customer foreign SID).

Electronic Serial Number (ESN)

ESN is a 32-bit binary number that uniquely identifies the mobile station in a CDMA cellular system. It should be set at the factory and cannot be easily changed in the field. Changing the ESN will require special equipment, not normally available to subscribers. The bit allocation of ESN is shown below −

The circuit that provides the ESN must be isolated so that no one can contact and tamper. Attempts to change the ESN circuit should make the mobile station inoperative. At the time of the issuance of the initial acceptance, the manufacturer must be assigned a code Manufacturers (MFR) in the eight most significant bits (bits 31-24 bits) 32-bit serial number. Bits 23-18 are reserved (initially zero). And, every manufacturer only allocates 17 bits to 0. When a manufacturer has used almost all possible combinations of serial numbers in bits 17-0, the manufacturer may submit a notification to the FCC. The FCC will assign the next sequential binary number in the reserve block (bits 23 through).

Permuted ESN

CDMA is a spread spectrum technique where multiple users to access the system at the same example in a cell, and of course on the same frequency. Therefore, it discriminates the users on the reverse link (i.e. information from MS to the base station). It spreads information using codes that are unique to the mobile station in all the CDMA cellular systems. This code has an element that is the ESN, but it doesn’t use the ESN in the same format instead, it uses an ESN swapped.

If there are two mobiles in a cell of the same brand and have consecutive serial numbers and for the receiver of the base station, it becomes difficult to connect them. Therefore, to avoid a strong correlation between the long codes corresponding to successive ESN, we use permuted ESNs.

International Mobile Station Identity (IMSI)

Mobile stations are identified by the identity of the international mobile station Identity (IMSI). The IMSI consists of up to 10 to 15 numeric digits. The first three digits of the IMSI are the country code of the mobile (MCC), the remaining digits are the National NMSI mobile station identity. The NMSI consists of the mobile network code (MNC) and the mobile station identification number (SIDS).

IMSI ≤15 digits
MCC	MSN	MSIN
	NMSI

MCC: Mobile Country Code
MNC: Mobile Network Code
MSIN: Mobile Station Identification
NMSI: National Mobile Station Identity

An IMSI that is 15 digits in length is called a class 0 IMSI (NMSI is the 12 digits in length). IMSI, which is less than 15 digits in length, is called a class 1 IMSI (NMSI the length is less than 12 counts). For CDMA operation, the same IMSI may be registered in multiple mobile stations. Individual systems may or may not allow these capabilities. The management of these functions is a function of the base station and the system operator.

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