Topic: GSM AND CDMA Channel

TYPES           

The are two types of logical channels, called traffic channels (TCHs) and control channels (CCHs). Traffic channels carry digitally encoded user speech or user data and have identical functions and formats on both the forward and reverse link. Control channels carry signaling and synchronizing commands between the base station and the mobile station. Certain types of control channels are defined for just the forward or reverse link.

GSM TRAFFIC CHANNELS (TCHs)
            GSM traffic channels may be either full-rate or half-rate and may carry either digitized speech or data. When transmitted as full-rate, user data is contained within one TS per frame. When transmitted as half-rate, user date is mapped onto the same time slot, but is sent in alternate frames. That is, two half-rate channel users would share the same time slot, but would alternately transmit during every other frame.

GSM CONTROL CHANNELS (CCHs)
            There are three control channels in the GSM system. These are the broadcast channel (BCH), the common control channel (CCCH), and the dedicated control channel (DDCH). Each control channel consists of several logical channels, which are distributed in time to provide the necessary GSM control functions.

Broadcast Channels (BCHs)
            The broadcast channel operated on the forward link of a certain ARFCN (Absolute Radio Frequency Channel Number) within each cell, and transmits data only in the first time slot (TS0) of certain GSM frames. Unlike TCHs which are duplex, BCHs only use the forward link. The BCH provides synchronization for all mobiles within the cell and is occasionally monitored by mobiles in neighboring cells so that received power and MAHO decisions may be made by out-of-cell users.

            The three types of BCH are Broadcast Control Channel (BCCH), Frequency Correction Channel (FCCH) and Synchronization Channel (SCH).

COMMON CONTROL CHANNELS (CCHs)
            On the broadcast ARFCN, the common control channels occupy TS0 of every GSM frame that is not otherwise used by the BCH or the Idle frame. CCCH consists of three different channels, the Paging Channel (PCH), which is a forward link channel, the Random Access Channel (RACH) which is a reverse link channel, and Access Grant Channel (AGCH), which is a forward link channel.

PAGING CHANNEL (PCH)
            The PCH provides paging signals from the base station to all mobiles in the cell, and notifies a specific mobile of an incoming call, which originates from the PSTN. The PCH transmits the IMSI of the target subscriber, along with a request for acknowledgement from the mobile unit on the RACH. Alternatively, the PCH may be used to provide cell broadcast ASCII text messages to all subscribers, as part of the SMS feature of GSM.

FAQs in GSM & CDMA

           
Q : How do Mobile Station associate itself with a Base Station?

A: Every MS has got an unique identification namely, Subscriber Identification Module (SIM) card, which is programmed to search for or scan the GSM radio spectrum for maximum signal strength, when it is powered on. Then it will send request to the strongest BS for association. MSC has got the data base which will contain the subscriber information and location Information ( not the exact coordinates but only the group of BTS’ with which it could be associated with), if the MS is an authorized user then the base station sends acceptance signal to the MS and association is complete.

Q : Discuss about Dynamic Channel Allocation.

A : In DCA, the Mobile Switching Centre maintains a band of available channels, and as and when the need arises, it allocates channels to each cell with due consideration to co channel and adjacent channel Interference, according to some algorithm. In DCA the key advantages are, there is no need for frequency planning and utilization of resources is optimized. The frequency reuse factor is one here, which intuitively explains higher capacity or efficient usage of resources.

Q : How do CDMA support Multiple Access?

A : Multiple Access of any resource requires that users are separated in some domain, either time or frequency or other. In CDMA systems, all the users communicate at the same time and over same frequency band, but the separation is in coding. Each user is given a unique code, used for spreading the spectrum of the signal typically to 1.25 MHz. It is possible to extract information only using that particular code.

Q : What is spread spectrum?

A :  Spread Spectrum is a transmission technique, which spreads the narrow band spectrum of the signal into a wider band using  some algorithms. Spread Spectrum may be Direct Sequence (DSSS) or Frequency Hopping (FHSS). In Direct Sequence Spread Spectrum, the information bit is multiplied by a high bit rate code or sequence to induce spectrum spread. In Frequency Hopping system the carrier frequency over which the information bit modulated is changed according to a pattern.  Commercial cellular CDMA systems use the direct sequence method.  The popular CDMA cellular standard is the IS-95 and the 3G standards coming up are WCDMA and CDMA 2000.  Reliance
Has deployed CDMA 2000, which enables high-speed data delivery as well.

Q : What is the difference in  handoff strategies between the GSM and CDMA systems?

A : Unlike in GSM system, that assign different radio channels during a handoff (hard handoff), spread spectrum mobiles share the same RF channel in every cell except when CDMA+FDMA is employed. Thus, the term handoff does not mean a physical change in the assigned channel, but rather that a different base station handles the radio communication link. By simultaneously evaluating the received signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in time. This technique exploits macroscopic space diversity provided by the different physical locations of the base stations and allows the MSC to make a soft decision as to which version of the user’s signal to pass along to the PSTN at any instance. The ability to select between the instantaneous received signal from a variety of base stations is a feature of the CDMA system.  Soft handoff refers to the fact that there is no change in the RF channel and consequently, the analog RF circuits do not have to retune; only the codes employed for despreading need to change and this is usually done using digital operations.