09-11-2012, 11:38 AM
CDMA Technology
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Access Network
Access network, the network between local exchange and subscriber, in the Telecom Network accounts for a major portion of resources both in terms of capital and manpower. So far, the subscriber loop has remained in the domain of the copper cable providing cost effective solution in past. Quick deployment of subscriber loop, coverage of inaccessible and remote locations coupled with modern technology have led to the emergence of new Access Technologies. The various technological options available are as follows :
1. Multi Access Radio Relay
2. Wireless In Local Loop
3. Fibre In the Local Loop
Wireless in Local Loop (WILL)
Fixed Wireless telephony in the subscriber access network also known as Wireless in Local Loop (WLL) is one of the hottest emerging market segments in global telecommunications today. WLL is generally used as “the last mile solution” to deliver basic phone service expeditiously where none has existed before. Flexibility and expediency are becoming the key driving factors behind the deployment of WILL.
WLL shall facilitate cordless telephony for residential as well as commercial complexes where people are highly mobile. It is also used in remote areas where it is uneconomical to lay cables and for rapid development of telephone services. The technology employed shall depend upon various radio access techniques, like FDMA, TDMA and CDMA.
Different technologies have been developed by the different countries like CT2 from France, PHS from Japan, DECT from Europe and DAMPS & CDMA from USA. Let us discuss CDMA technology in WLL application as it has a potential ability to tolerate a fair amount of interference as compared to other conventional radios. This leads to a considerable advantage from a system point of view.
SPREAD SPECTRUM PRINCIPLE
Originally Spread spectrum radio technology was developed for military use to counter the interference by hostile jamming. The broad spectrum of the transmitted signal gives rise to “ Spread Spectrum”. A Spread Spectrum signal is generated by modulating the radio frequency (RF) signal with a code consisting of different pseudo random binary sequences, which is inherently resistant to noisy signal environment.
A number of Spread spectrum RF signals thus generated share the same frequency spectrum and thus the entire bandwidth available in the band is used by each of the users using same frequency at the same time.
Vocoder and variable data rates:
As the telephone quality speech is band limited to 4 Khz when it is digitized with PCM its bit rate rises to 64Kb/s vocoding compress it to a lower bit rate to reduce bandwidth. The transmitting vocoder takes voice samples and generates an encoded speech/packet for transmission to the receiving vocoder. The receiving vocoder decodes the received speech packet into voice samples. One of the important feature of the variable rate vocoder is the use of adaptive threshold to determine the required data rate. Vocoders are variable rate vocoders. By operating the vocoder at half rate on some of the frames the capacity of the system can be enhanced without noticeable degradation in the quality of the speech. This phenomenon helps to absorb the occasional heavy requirement of traffic apart from suppression of background noise. Thus the capacity advantage makes spread spectrum an ideal choice for use in areas where the frequency spectrum is congested.
Less (Optimum) Power per cell:
Power Control Methods: As we have already seen that in CDMA the entire bandwidth of 1.25Mhz is used by all the subscribers served in that area. Hence they all will be transmitting on the same frequency using the entire bandwidth but separated by different codes. At the receiving end the noise contributed by all the subscribers is added up.
Capacity Considerations
Let us discuss a typical CDMA wireless in local loop system consisting of a single base station located at the telephone exchange itself, serving a single “cell”. In order to increase the number of subscribers served the cell is further divided into “sectors”. These sectors are served by directional antennas.
The capacity of a cellular system is claimed to be 20-40 active lines per sector per 1.25 MHz for a single CDMA Radio Channel. In WLL environment assuming an average busy hour traffic of 0.1 Erlang, 400 subscribers can be served per sector over a single 1.25 MHz channel.
Assuming typically six sectors in a cell the total capacity of a CDMA network consisting of 1.25 MHz duplex channels is 2400 (400x6) subscribers.
Capacity can further be increased if we use another frequency on the same base station covering the same geographical area (overlapping cell). Thus in 10 Mhz in the bandwidth we can utilize 5 MHz of bandwidth in the forward link and 5 Mhz in the reverse link. Hence if we have 4 RF carriers in 5 Mhz bandwidth, the network can support 12000 (5x400x6) subscribers per cell.
Introduction to CDMA 2000-1X
Network entity description
Base station subsystem (BSS) Base station subsystem is the general term for the wireless devices and wireless channel control devices that serve one or several cells. Generally, a BSS contains one more base station controllers (BSC) and base transmitter stations (BTS).
Home location register (HLR)
The HLR provides subscriber information storage and management functions for the mobile network, including mobile subscriber subscription and cancellation and service authorization and cancellation. At the same time, it helps in the implementation of subscriber’s call and service operations. A CDMA can contain one or more HLRs based on the number of subscribers, equipment capacity and network organization mode, with multi-HLR mode realized in the form of virtual HLRs. The subscriber information stored in the HLR includes the following two types in information:
1. Subscription information
2. Subscriber-related information stored in the HLR
Authentication center (AUC)
Authentication center is a function entity for the management of authentication information related to the mobile station. It implement mobile subscriber authentication, stores the mobile subscriber authentication parameters, and is able to generate and transmit the corresponding authentication parameters based on the request from MSC/VLR. The authentication parameters in the AUC can be stored in the encrypted form.
Physical and Logical Channels:
In IS 95A, in the forward link Pilot, Sync, Paging and Traffic Channels exist where as in reverse link Access and traffic channel are available. All overhead information is carried on the Paging Channel. During conversation or in dedicated mode the signaling info is exchanged by either fully or partially clearing the traffic. CDMA2000 technology defines new Physical and Logical Channels for the transport of user data and signaling information.
A Physical Channel is a communication path between the mobile and the Base Station, described in terms of the digital coding and RF characteristics.
A Logical Channel is a communication path within the protocol layers of either the Base Station or the mobile.