Johny Learns

What is WiMAX?


Johny: What is WiMAX?

Dad :WiMAX is a wireless broadband  mainly used for High speed Internet Connection..  Have a look at this Youtube video. In this video, a farmer has installed WiMAX, a school is using WiMAX, and   persons travelling in buses and traines are using WiMAX. You will get atleast 1mbps speed even if you are travelling in bus with a speed of 80kph. 

 See this video. Here an office is having wi-fi access-point. A person is trying to get connectivity from outside his office building. The signal is lost when the distance goes beyond 150m. The same office is also having WiMAX access-point. His signal is ok even when he is 16KM away from his office and he gets a speed of 6mbps.


Johny: Well, Tell me some  important points I should remember about WiMAX?

Dad : 1) WiMAX stands for  Worldwide Interoperability for Microwave Access.

2) It is basically a wireless Metropolitan Area network (Wireless MAN).

3) The transmission occurs between Base Station and Subscriber Station

4) The Subscriber Station  can be called as Receiver. The Receiver can be inbuilt in a laptop, It can be a small PCB that can be inserted in the mother of a PC. It is can be a small box.

5) Base station is just like any cellphone base station. One tower can cover upto 50 km.

6) The speed is 2 Mbps to 75 Mbps. Yeah. It is broadband.

7) About IEEE 802.16 :

  •    This is  for Fixed WiMAX
  •    Speed is 32 to 124 Mbps
  •    RF channel bandwidth is 20,25,28 MHz
  •    Cell radius is 1 to 3 miles
  •    Access techniques is   Request/Grant
  •    Modulation is Adaptive from 64QAM to QPSK 

7) IEEE 802.16e for Mobile

8) IEEE 802.16-2004 is for Portable

8) When higher frequencies are used (11 to 66 Ghz), you should use line of sight antennaes.

9) For 2 to 11 GHz you can use non-line-of-sight antennaes.



Johny: Mobile WMAX means, even when I am travelling in car, I will get the connectivity?

Dad: Yes.You are right. See this video demonstration. A person is  using wimax while travelling in a bus




Johny : What is Adaptive Modulation?

Dad : See this image. QAM64 is high speed. QAM16 is medium speed. QPSK gives low speed. When the climate is sunny, the QAM64 reaches large distance. But when the climate worsens, the users at the distant places automatically gets lower speed.

Johny: What is Convolutional encoding?

Dad: Suppose You are transmitting the data 101. The receiver will find if there is any error  in the received data, with the help of checksum and ask the transmitter to retransmit the data. This method is Backward Error Correction. There is another technique called as  FORWARD ERROR CORRECTION(FEC). In this method, if the receiver knows tht there is an error in the data, the receiver will automatically correct the error. It works like this : Suppose You want to transmit the data : 101.  The transmitter will transmit each bit 3 times (like this > 111 000 111),  the receiver will automatically correct the error and retrieve the data as 101 because the transmitter is adding redundant information. The convolutional Encoding is a FEC technique. :

 Carefully observe the green background portion. It is  a three bit shift register. Initially, it has 000 (not shown). Now 1 is moving from left side. Register value becomes 100. XOR of all the three bits in 100=1. In this image, this value is coming out from the top. XOR of first and third bits of 100=1. This value is coming out from the bottom. So the resultant output is 11. The final  output of 101 is 11 10 00 10 11. This  output is transmitted over the medium. And the receiver is able to retrieve the original data even if there is some error in the received bits.

Johny: What is QPSK?

 Dad: Compare the signal at C and 01. 01 is 90 degree forward. Compare B and 10. 01 is 180 degree forward.  So there are Four combinations and each symbol represents 2 bits. If you use 64QAM, then each symbol (baud) will represent 6 bits.