Topic: OVERVIEW OF WIRELESS COMMUNICATION

BASIC CELLULAR CONCEPTS

Recent years have seen a tremendous increase in use of mobile services by people all around the world. The key advantage of going for mobile system is that it offers a great deal of flexibility to its users, i.e. the people can always stay connected to the rest of the world, it doesn’t matter where they are. We can use wireless as a medium of communication for a variety of services, say voice, audio, data, Internet etc. The principle objective of this lecture is to introduce the basic concepts underlying cellular communication, which is primarily used for handling voice traffic. 

CELLULAR COMMUNICATION BASICS
            At the onset, let us try to examine the case of typical voice communication scenario, as we progress, we will gradually land ourselves in to the cellular world.
The following processes are involved in successful voice communication.

• The acoustic waves of speech are converted into analog voice signal by acoustic transducer inside the phone,
• The analog signal is converted in to digital form using a Analog to Digital Converter (ADC),
• This digitized voice is coded using PCM / DPCM etc,
• The digitally coded message is modulated using any one of modulating techniques such as ASK, FSK, PSK etc. for transmission over the wireless channel.

The above mentioned processes must be done at the user-phone (we would call it as handset or more formally a Mobile Station(MS)) itself. For reaching the intended destination we need to connect our MS to the existing backbone infrastructure. This is done, by having a fixed infrastructure called Base Station (BS). Base Station acts as a gateway or interface between MS/cellular network and backbone infrastructure network called PSTN (Public Switched Telephone Network – nothing but our common telephone network). Figure 1.1 shows the Cellular Network Architecture.

            The transmission of information in the form of coded bits between Base Station and Mobile Station is the responsibility of physical layer of OSI Protocol Stack. The physical layer takes care of air interface operations. Achieving fast, reliable and secure transformation of information by the physical layer is the primary concern of wireless system designer.

                                    Fig 1.1 – Cellular Network Architecture

           
            There are different standards existing in the wireless industry, to regulate and coordinate operations by different service providers. Standards ensure that products manufactured by different manufactures are interoperable. GSM is one of the standards that is in operation in most part of the world and in India. GSM stands for Global System for Mobile communications.

            Base Stations are connected to PSTN through Base Station Controller (BSC) and Mobile Switching Center (MSC). Base Station Controller manages the resources and services available to the MS (through appropriate BSs) such as bandwidth, handoff etc.
BSC informs MSs to select a particular band of available bandwidth via BSs.
In GSM Architecture, simultaneous access of channel by more than one user is accomplished by Frequency Division Multiple Access (FDMA). Typical frequency of operation is at 900MHz and 1.8GHz.

WHY CELLULAR ARCHITECTURE
            Wireless Voice communication involves splitting of coverage area in to smaller zones called cells. Let us first get some insight in to, why we are going for cellular architecture, instead of large area centralized broadcast architecture, as in the case of radio broadcast. It is because of the fact that the MSs are battery operated hence they cannot reach the central base station if they are at the verge of the coverage area. The other reason is that with cellular architecture we can increase the capacity of the system by frequency reuse concept.

            The key design challenges in cellular services are due to the air-interface and location of Mobile Stations. Normally the BSs and BSC are connected using wired medium, it is because of two reasons they are,

• the wired link (especially optical link) is highly reliable and fast and
• both BSs and BSC are stationary.

FREQUENCY REUSE CONCEPT
            In wireless communication, frequency spectrum is the key resource. Dividing the coverage area in to small zones called cells does the conservation of this resource. The key issue lying behind the frequency reuse is interference management, i.e. assigning frequency to different cells is done in such a way as to minimize interference between cells. Figure 1.2 shows the concept of frequency reuse.

                                    Fig 1.2 – Frequency Reuse Concept

FREQUENCY REUSE CALCULATIONS
            A group of cells having different frequency band forms a cluster. As there  are three clusters in our example we have the total capacity of 3*500=1500 channels instead of 500 as in the case of large area broadcast. In fig 1.1, a cluster is formed of  7 cells. Thus the same frequency can be reused only every 8th cell, thus the reuse factor for this example is 7.

          Let us assume that the total bandwidth available is 25MHz and each channel requires a bandwidth of 25KHz, i.e. 50KHz(2*25) for a duplex channel.

          Hence no. of  channels supported by this system is  (25M/50K)=500.    
Since in our example reuse factor is 7,
              No. of  channels accommodated in a cell is  500/7 = 71.

If the reuse factor is 4, then the channels per cell is 500/4=125.