How the Mobile(Cellular) Communication takes place, and overview of different Generations of technologies

©2012 Ganti Sree Rajiv

These days cell phone has become integral part of daily life. You dial a number, within seconds it connects to the person who is hundreds/thousands of miles away. But within these few seconds, lot of things happen wirelessly to connect you to the destination.

The very first observation: You simply press the mobile number of your friend in your phone and click on the ‘green’ call button. Did you ever think what actually happens inside your mobile phone when you press some buttons (which the cell phone doesn’t actually understand that you pressed digits from 0…9) but connects you to that number?

Here is the answer:

Generally the keypad of the mobile phone is standardized with DTMF (Dual-Tone Multi Frequency) system. It is a standard 4*3 grid (4 rows and 3 columns) of buttons. As you can see from the above diagram, each row and each column in the grid has set to a particular frequency. So when you press telephone number of your friend on this keypad, every single key pressed will produce a pitch(music/sound) consisting of sinusoidal frequencies.

For example the mobile number you pressed starts something like this: 98*******6. The first digit here is 9. So when you press the button 9 on your phone keypad, according to the table above, 2 frequencies are generated by the mobile circuit. 1477 Hz representing the higher frequency and 852 Hz representing the lower frequency. So pressing 9 will result in a pitch (sound) composed of 2 frequencies, the 1477 Hz(higher frequency) and 852 Hz (lower frequency). So, in this way, the mobile transmits signal when you actually press the mobile number of your friend.

This is the case of old landline telephones. But in present day cell phones, you dial the entire phone number, and only when you press the ‘green’ call button, the number gets dialed. So for this, the circuit remembers the pattern that you pressed, and once you accept by pressing the ‘green’ button, the mobile then transmits signals with same principle as above. 

Try this:

(When you press buttons on your keypad, try to hear the sound produced by keeping your mobile near your ear. You can clearly observe the difference in sounds produced when you press different numbers). This is due to the above mentioned principle.

A brief History:

Radio was invented in 1920. During invention of radio only AM (Amplitude Modulation) was used. Later Edwin Armstrong invented FM (Frequency Modulation). Quality of signal is good in FM compared to AM (however there are also many other reasons to go for FM).

The first mobile network available for commercial use is by ‘ANetz’ in Germany in 1958 which used 160 MHz frequency. Then in 1987 came the cordless telephone.

The mobile phone transmits signals in all directions. If the receiver/antenna is in the coverage area then it receives this signal (irrespective of direction).

There are 3 most famous systems: AMPS (introduced in 1983 in chicago), GSM (introduced in Europe), and CDMA (commissioned in America).

For the purpose of study/research to be convenient, the coverage area of a cellular tower was assumed to be Circular, and then divided into small circular areas.

But as you can see, there are lots of gaps left which are not covered with these circular area (blue color) and also there are overlaps in several areas(brown).

So, this was then not used, and currently hexagonal representation is used.

You can now observe that the problem with the circular coverage is now resolved with hexagonal coverage area.

Uplink and Downlink:

These are the 2 common terms that you come across whenever you hear about wireless communication. In simple terms, uplink (uploading) is sending information from mobile to antenna/tower. Downlink (download) is the opposite, which connects antenna/tower to mobile. Seems to be confusing? See the illustration:

Why is the 2^nd generation(2G) most successful and is in existence for a long time??

Before knowing about this, it would be better to know the disadvantages of 1^st generation(1G) that made us to move to 2G.

1.       Quality of signal(speech) is poor as it is analog.

2.       Coverage area is limited due to FM (which would not have been sufficient for current generation).

3.       Illegal tapping is very easy. (Just take a receiver and keep it in the path of signal source, and you can hear the conversation).

4.       Most importantly, there is no authentication and security.

5.       No Inter operability to other network

6.       No Roaming

7.       Idea to increase Bandwidth, Channels

8.       To bill customers in different ways (outgoing tariff, roaming tariff, SMS tariff, etc).

    

    With all these requirements, then came the most successful 2G systems:  GSM and CDMA.

   GSM (Global System for Mobile):

   GSM is the most successful and is in existence for a long time. 

     This architecture illustrates GSM. When you call a person from your mobile (MS), It transmits signals. These signals are received by the nearest antenna/cellular tower in your coverage area. This antenna/tower is technically called BTS (Base Transceiver Station). Several of such towers (BTS) - all may be from one area or many areas together, are connected to a BSC ( Base station Controller). The number of towers connected to single BSC depends on several factors like geographical area(plain land or hills etc), population in that area etc.

      Several of such BSC’s are then connected to a MSC (Main station controller) and we call it ‘exchange’ as our daily life term. 

      

      The MSC (Main station) is the main centre. It’s main function Is Radio resource allocation (i.e., Frequency allocation). Apart from this, It provides several integrated functions to customers. As you can see from above figure, several functions like HLR, VLC, OMC etc are connected to MSC. Knowing these in detail is not necessary, but I would like to describe in short what they offer.

HLR (Home Location register):  stores the profile of every customer.

VLC (Visiting Location Register): Suppose if you are from some location. You are roaming to another location and you are connected via some other operator. So, in this case, the Main station of your roaming area gets your information via the VLC of your home town Main station.

Ex: When a Aircel customer is in coverage of Airtel when he is in roaming.

OMC (Operator Maintenance Centre), as the name describes it is to handle faults in the network.

Main Station also plays important role in receiving text messages apart from voice call, and sends it to destination customer database. Suppose if the destination person phone is switched off, even then the text message is stored in his database for some amount of time. Within this time if he turns on the mobile, he will receive the message. (Everyone might have experienced this).

GSM uses 890-915 MHz frequency range for Uplink, and 935-960 MHz range for Downlink.

CDMA (Code Division Multiple Access):

CDMA is also one of the most successful 2G system.

The main features of it are: Unlike GSM, same Bandwidth/ channel is used by all customers, and the data is coded in different way and sent through same channel. So in this way, CDMA saves lot of bandwidth. It assigns unique code for every customer. No frequency planning is required for CDMA.

It uses 824-849 MHz for uplink, and 869-894 MHz for downlink.

Even though CDMA seems to be more advantageous, the biggest disadvantage is that as all the information is coded and sent through same channel, there are more chances of getting Jammed. 

     

(Interesting Additional information):

1. 

When you look at cellular tower, you can find that every tower has antennas (whatever might be the shape of antenna) and also very thick wires flowing from top of the tower to bottom. The existence of these antenna means that the cellular tower (BTS) is connected to its BSC by Microwave (wireless) transmission link. The thick wires are also another way of connecting to BSC by optical fiber link (through the ground/earth).

2. Do you know one fact that, every mobile phone searches for signal from the tower for every 46 milli-seconds ???

3. One of the most commonly used methods to enhance coverage area with the currently available small bandwidth is the technique of “Frequency Re-Use”.

The same frequency used in one cluster/coverage area can be also given to another area (black colored in the figure), provided that there is a safe distance between these areas in order not to cause any interference.

4.  Suppose you have both DTH Tv (Direct to Home-satellite Television) and Cable Tv at your home and try watching cricket match. You can see that there will be a small delay and both of them don’t show at same time, even though you are watching the same cricket match. The reason behind that is: even though both DTH operator and Cable operator receive the signal from satellite at same time, after receiving it, the cable tv operator sends this information to you via cable(physical medium) where as DTH operator sends it again via satellite. So DTH signal is delayed compared to Cable Tv as the signal has to travel more distance and hence delayed propagation.

5. Practically the mobile is never in ‘line of sight’ directly with tower. There are several obstacles like huge buildings, reflectors etc which delay the propagation of signal, which is technically termed as ‘Fading’.

Overview about 3G:

Currently we are in the 3G (high speed internet, voice with video, mobile TV, etc) age however 3G is still not widespread in all areas like 2G. The following standards are typically branded as 3G:

1.       EDGE:  same like GPRS (which is used in 2G), but more efficient modulation schemes are used. EDGE is also called as ‘EGPRS’ which is not completely 3G, but often termed as 2.5G which allows data peak rates of 200kbit/sec. 

2. W-CDMA: commonly operated on the 2100 MHz band.

3.   HSPA: is an amalgamation of several upgrades to the original W-CDMA standard and offers speeds of 14.4Mbit/s downlink and 5.76MBit/s uplink.

4. HSDPA+ along with MIMO can provide theoretical peak data rates up to 168 Mbit/s in the downlink and 22 Mbit/s in the uplink.

©2012 Ganti Sree Rajiv