Introduction to Orthogonal Frequency Division Multiplexing:
The use of movable communication system was increased tremendously like mobiles PDA’S etc. In this communication only low bit data rate voices is delivered to the destination. The radio channel of mobile is characterized by multi channel reception.
The received signal by receiver is not only LOS(line of sight) radio waves besides a lot of reflected radio waves can be received at different time intervals. The delay in signals is due to reflection of waves from hills, terrains, buildings etc.The waves which are reflected will cause interference that means they reflect each other which causes significant degradation of network performance. The wireless system such as mobiles and Tabs should be minimized from these effects. All these effects are overcome by orthogonal frequency division multiplexing system.
Working of OFDM:
- The transmitter converts the input data from serial stream to parallel sets. It contains symbol Xk for each carrier. A set of 4 data will be 1 1 1 1 -1 -1.
- If the transmission takes 4 seconds then than each piece of data has 1 second duration. OFDM sends the data one after another if the duration of the data is 4 seconds.
- If the communication length is high the transfer speed and quality become shorter. The delay caused in the multi path remains constant.
- OFDM is highly suitable for high speed communication because of IST restrictions. Because it avoids low speed data transmissions.
- OFDM channel has 128 to 2048 sub-carriers and can occupy bandwidth from 1.25 MHz to 20MHz.
| Channel bandwidth | 1.25MHz 2.5 MHz 5MHz 10MHz 20Mhz |
| No. of sub-carriers | 128 256 512 1024 2048 |
| Sub-carrier spacing
Symbol Time Guard Time |
11.16 KHz
100.8μs 11.2μs |
Transmitted signal x(n) :
Where N= Total no. of sub-carrier
K= Subcarrier index with K=0, 1, 2…. ….. …… …… N-1.
Download Orthogonal Frequency Division Multiplexing Electronics Seminar Report for ECE Students.