Explain forward link features of CDMA 2000 and also explain basic services provided by upper layer of CDMA 2000
1 Answer

Forward link features of CDMA 2000 are as given below:

  • Pilot channel (PC): The pilot channel is transmitted as a reference by the base station to provide timing and phase reference for the mobiles, and carries no real data. The "data" carried by the channel is a continuous stream of zeros which is spread by Walsh code zero, which itself a stream of zeros. This is further spread by a pair of quadrature PN sequences. This means that the pilot channel is effectively the PN sequence with its associated offset. A measurement of the signal-to-noise ratio of the pilot channel also gives the mobile an indication of which is the strongest serving sector.
  • Paging channels (PCH): This IS-95 channel is used to carry information to enable mobiles to be paged. Data carried by this IS-95 channel includes system parameters, voice pages, SMS and other broadcast messages. It occupies Walsh codes 1 - 7 dependent upon the system requirements. As with other channels there are a number of stages taken to produce the final channel. First the baseband information is error protected. After this the data is repeated if it is at a rate of 4.8 kbps, otherwise it is left as it is. Following this the data is interleaved and then scrambled by the decimated long PN sequence, and finally spread by the Walsh code for the particular channel assignment. In this process the long PN code is itself masked with a code which is specific to the channel being used. In this way the long PN code for Paging Channel 1 (using Walsh Code 1) is different to Paging Channel 4 (using Walsh Code 4).
  • Synchronisation channel (SC): This IS-95 channel is used to provide the timing reference to access the cell. This IS-95 channel always uses Walsh code 32. Each base station has a fixed timing offset to reduce the interference between adjacent base-stations.
    The Sync channel incorporates an 80 mSsuperframe structure. This is divided into three 26.667 mS frames which correspond to the same length as the short PN sequences. This means that they align with the timing on the Pilot channel.
    This IS-95 channel is allocated the least power of the overhead channels in the overall CDMA transmission. The data that is transmitted on this channel includes the system time, pilot PN of the base station, long code state, system ID, and the network ID.

Forward Traffic Channel (FTC):  

It is used to carry voice, data and signaling channels. When carrying voice, the coded voice data does not require a constant bit rate and IS-95 allows the rate of the frames to change dynamically (every 20 ms). When the rate is reduced it reduces the level of interference to other users. The original vocoder specification used a set of rates based on divisions of 9.6 kbps. This is reflected in IS-95A. Later the vocoder was improved to give better voice quality and in IS-95B a vocoder was introduced with a rate set based on 14.4 kbps. The 9.6 kbps rate set was termed RS1 and the second based on 14.4 kbps was termed RS2. However data is always carried at full rate.

Basic services provided by the upper layers of CDMA2000 are as below:

Protocol Architecture of CDMA 2000

  1. Medium access control layer

The medium access control (MAC) layer establishes, maintains, and releases channels for higher layers by activating and deactivating physical channels. MACmultiplexes several logical channels onto physical channels. Logical channelsexist for signaling network control, user data transmission, paging, or sendingbroadcast messages. Additional services offered include segmentation/reassemblyof packets and error control/error correction.

2.  Data link control layer

The data link control (DLC) layer creates and maintains reliable connectionsbetween the mobile terminal and the base station. Two services have beendefined for the C-Plane: a connectionless broadcast service for paging (calledLb) and a point-to-point protocol similar to LAPD in ISDN, but adapted to theunderlying MAC (called LAPC+Lc).Several services exist for the U-Plane, e.g., a transparent unprotected service(basically a null service), a forward error correction service, rate adaptation services,and services for future enhancements. If services are used, e.g., to transferISDN data at 64 kbit/s, then DECT also tries to transfer 64 kbit/s. However, incase of errors, DECT raises the transfer rate to 72 kbit/s, and includes FEC and abuffer for up to eight blocks to perform ARQ. This buffer then introduces anadditional delay of up to 80 ms.

3.  Network layer

The network layer of DECT is similar to those in ISDN and GSM and only existsfor the C-Plane. This layer provides services to request, check, reserve, control,and release resources at the fixed station (connection to the fixed network, wirelessconnection) and the mobile terminal (wireless connection). The mobilitymanagement (MM) within the network layer is responsible for identity management,authentication, and the management of the location data bases. Callcontrol (CC) handles connection setup, release, and negotiation. Two messageservices, the connection oriented message service (COMS) and the connectionlessmessage service (CLMS) transfer data to and from the interworkingunit that connects the DECT system with the outside world.

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