October 21, 2011 Category : Higher Studies
Code Division Multiple Access (CDMA) is a radically new concept in wireless communications. It has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. It has likewise been chosen for deployment by the majority of the winners of the United States Personal Communications System spectrum auctions. It may seem, however, mysterious for those who aren't familiar with it. This site is provided in an effort to dispel some of the mystery and to disseminate at least a basic level of knowledge about the technology.CDMA is a form of spread-spectrum , a family of digital communication techniques that have been used in military applications for many years. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). It has a history that goes back to the early days of World War II.
The use of CDMA for civilian mobile radio applications is novel. It was proposed theoretically in the late 1940's, but the practical application in the civilian marketplace did not take place until 40 years later. Commercial applications became possible because of two evolutionary developments. One was the availability of very low cost, high density digital integrated circuits, which reduce the size, weight, and cost of the subscriber stations to an acceptably low level. The other was the realization that optimal multiple access communication requires that all user stations regulate their transmitter powers to the lowest that will achieve adequate signal quality
October 19, 2011 Category : Higher Studies
Global system for mobile communication (GSM) is a globally accepted standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz. It is estimated that many countries outside of Europe will join the GSM partnership.
Cellular is one of the fastest growing and most demanding telecommunications applications. Throughout the evolution of cellular telecommunications, various systems have been developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard is intended to address these problems.
From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen.
GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system (OSS).
GSM System Architecture:
l System Architecture
Mobile Station (MS)
The Mobile Station is made up of two entities:
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
- Portable,vehicle mounted, hand held device
- Uniquely identified by an IMEI (International Mobile Equipment Identity)
- Voice and data transmission
- Monitoring power and signal quality of surrounding cells for optimum handover
- Power level : 0.8W – 20 W
- 160 character long SMS.
Subscriber Identity Module (SIM)
- Smart card contains the International Mobile Subscriber Identity (IMSI)
- Allows user to send and receive calls and receive other subscribed services
- Encoded network identification details
a. Key Ki,Kc and A3,A5 and A8 algorithms
- Protected by a password or PIN
- Can be moved from phone to phone – contains key information to activate the phone
Base Station Subsystem (BSS)
Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing operation between components made by different suppliers
1. Base Transceiver Station (BTS)
2. Base Station Controller (BSC)
Base Transceiver Station (BTS):
- Encodes,encrypts,multiplexes,modulates and feeds the RF signals to the antenna.
- Frequency hopping
- Communicates with Mobile station and BSC
- Consists of Transceivers (TRX) units
Base Station Controller (BSC)
- Manages Radio resources for BTS
- Assigns Frequency and time slots for all MS’s in its area
- Handles call set up
- Transcoding and rate adaptation functionality
- Handover for each MS
- Radio Power control
- It communicates with MSC and BTS
Network Switching Subsystem(NSS)
Mobile Switching Center (MSC)
- Heart of the network
- Manages communication between GSM and other networks
- Call setup function and basic switching
- Call routing
- Billing information and collection
- Mobility management
- Location Updating
- Inter BSS and inter MSC call handoff
- MSC does gateway function while its customer roams to other network by using HLR/VLR.
Home Location Registers (HLR)
1. permanent database about mobile subscribers in a large service area(generally one per GSM network operator)
database contains IMSI,MSISDN,prepaid/postpaid,roaming restrictions,supplementary services.
Visitor Location Registers (VLR)
1. Temporary database which updates whenever new MS enters its area, by HLR database
2. Controls those mobiles roaming in its area
3. Reduces number of queries to HLR
4. Database contains IMSI,TMSI,MSISDN,MSRN,Location Area,authentication key
Authentication Center (AUC)
1. Protects against intruders in air interface
2. Maintains authentication keys and algorithms and provides security triplets ( RAND,SRES,Kc)
3. Generally associated with HLR
Equipment Identity Register (EIR)
1. Database that is used to track handsets using the IMEI (International Mobile Equipment Identity) Made up of three sub-classes: The White List, The Black List and the Gray List Only one EIR per PLMN