Follow Up - Understanding 2G systems

Now that we have reviewed the second generation systems, lets understand all the concepts that play a role in making these systems work!

In the last blog, I used words like FDD/TDD/FDMA etc.. Lets go into details of all these words..

In this blog we will dig deeper into Frequency division duplexing and time division duplexing

To begin with, lets see what is a duplex system.. A duplex system can send and receive functionality. Depending upon whether send and receive functions are done simultaneously, a system can be FULL duplex or half duplex. For point to multipoint networks like cellular systems , it is necessary to separate inward and outward signals. 

1. Time division duplexing : It is a method of transmitting and receiving at the same frequency but having synchronized switches at both the ends. Although it is intuitive that TDM systems are synchronous, statistical time division multiplexing has been successfully implemented. The advantages include lower spectrum usage, lower cost of implementation than FDD, asymmetrical or dynamic UL/DL allocation.

2. Frequency Division duplexing : In this case there are two separate frequency channels are required. The cons include higher spectrum usage , no dynamic allocation. Advantages include lower(theoretically zero) latency.

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The second One!

The second generation of cellular system was introduced after a decade of first generation. The last decade of 19th century started off with this digital generation. 2G systems were primarily aimed towards the voice market.

There were several shortcomings of the 1G systems, like limited coverage and  lower capacity. As the popularity of cellular phones started increasing , the capacity of cellular systems was a challenge for the service providers.
Another shortcoming of 1G systems was the limited ability of the controller and the phones to exchange information during the course of a call. The control information was passed through the voice channel in a "blank and burst" basis. That means speech was briefly( 100 ms or may be more) interrupted whenever  
the control message moves phone and the base station. I am sure this was a major issue as QoS was degraded. 

The shift from analog to digital enhanced the performance of cellular systems. 
Secondly to meet the increasing demands following techniques were used
1. The reuse of spectrum efficient digital speech codecs.
2. multiplexing techniques (frequency / time)
3. higher(tighter) frequency reuse by better performance of digital modulation and coding

There were several major 2G standards like GSM, IS-95 CDMA, IS- 135 TDMA.

1. GSM : This system originated in Europe in 1990. The frequency bands used are 850/900 MHz, 1.8/1.9 GHz with channel bandwidth of 200kHz. These systems were either TDMA or FDMA. FDD duplexing technique was implemented. The peak data rates supported by the system was 107 kbps(GPRS) and 384 kbps(EDGE). Typically user data rates were 20-40 kbps(GPRS) and 80-120 kbps(EDGE). The permissible latency was 600- 700 ms.

2. IS-95 : This was the first CDMA system using 850MHz/1.9 GHz frequency bands with 1.25 MHz channel bandwidth. The peak data rate was 115 kbps but typically a user could get a data rate upto 64 kbps. The user plane latency was greater than 600ms. (FYI : for LTE the latency is less than 100ms).

3. IS-54 : This system was later modified to IS-136. The frequency used was 850MHz/1.9 GHz with 30kHz channel bandwidth. TDMA/FDMA were the multiple access techniques. The data rate supported was very low, a peak of 12 kbps and typical user data rate was around 9 kbps.


The 2G systems not only provided improved voice quality but also supported some new applications like SMS , updates, alerts, bill payments. Later these systems were equipped to provide low data rates as well.

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