Monthly Archives: October 2013

OFDM

Orthogonal Frequency Division Multiplexing(OFDM) is a multi-carrier transmission technique that has been recently recognized
as an excellent method for high-speed bi-directional wireless data communications. Fundamentally, Frequency Division
Multiplexing(FDM) uses multiples frequencies to simultaneously transmit multiple signals in parallel. While each sub-carrier is
separated by a guard band to ensure that they do not overlap in the ordinary FDM, the sub-carriers in the OFDM are squeezed
tightly together in order to reduce the required bandwidth. In fact the neighboring sub-channels are overlapped in OFDM. However,
the sub-carriers are orthogonal to each other such that there is no inter-carrier interference (ICI).
In mobile communication environment with the moving vehicle, there is a Doppler shift effect which causes each sub-channel
to be shifted and orthogonality to be broken. To prevent this phenomena, cyclic prefix to each symbol is added. The cyclic prefix
is for implementing the FFT when one needs to use circular convolution to emulate linear convolution. Instead of just sending
blank during guard time, repeating the end of the symbol provides ”guard time” (instead of the ”guard band”) to make sure all
the sub-carriers are orthogonal. If the end interferes with the next symbol, we can subtract it out because we already received the
prefix, which is the same.
There is another benefit of OFDM. Since each subcarrier of OFDM uses a narrow bandwidth, in the time domain each symbol
is transmitted during a relatively long symbol time. This guarantees all the significant multipaths arrive within a symbol time
minimizing Inter-symbol Interference(ISI). This is good news. Because it removes the need for an equalizer at a transmitter and
a receiver side, reducing complexity and the cost.

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Eid Mubarak

Eid Ul azha has been celebrated on 16th October,2013 . I wish all a happy Eid

Name Description Note
IEEE 802.1 Bridging (networking) and Network Management  
IEEE 802.2 LLC inactive
IEEE 802.3 Ethernet  
IEEE 802.4 Token bus disbanded
IEEE 802.5 Defines the MAC layer for a Token Ring inactive
IEEE 802.6 MANs (DQDB) disbanded
IEEE 802.7 Broadband LAN using Coaxial Cable disbanded
IEEE 802.8 Fiber Optic TAG disbanded
IEEE 802.9 Integrated Services LAN (ISLAN or isoEthernet) disbanded
IEEE 802.10 Interoperable LAN Security disbanded
IEEE 802.11 a/b/g/n Wireless LAN (WLAN) & Mesh (Wi-Fi certification)  
IEEE 802.12 100BaseVG disbanded
IEEE 802.13 Unused[2]  
IEEE 802.14 Cable modems disbanded
IEEE 802.15 Wireless PAN  
IEEE 802.15.1 Bluetooth certification  
IEEE 802.15.2 IEEE 802.15 and IEEE 802.11 coexistence  
IEEE 802.15.3 High-Rate wireless PAN (e.g., UWB, etc)  
IEEE 802.15.4 Low-Rate wireless PAN (e.g., ZigBee, WirelessHART, MiWi, etc.)  
IEEE 802.15.5 Mesh networking for WPAN  
IEEE 802.15.6 Body area network  
IEEE 802.16 Broadband Wireless Access (WiMAX certification)  
IEEE 802.16.1 Local Multipoint Distribution Service  
IEEE 802.17 Resilient packet ring  
IEEE 802.18 Radio Regulatory TAG  
IEEE 802.19 Coexistence TAG  
IEEE 802.20 Mobile Broadband Wireless Access  
IEEE 802.21 Media Independent Handoff  
IEEE 802.22 Wireless Regional Area Network  
IEEE 802.23 Emergency Services Working Group  
IEEE 802.24 Smart Grid TAG New (November, 2012)
IEEE 802.25 Omni-Range Area Network Not yet ratified

Views from my friends

Guys! We have finally finished the formal classes and exam in the campus at Institute of Science and Technology.Maybe we will meet after a long time like this . But who knows! maybe it is so near! however, I am making this post as I feel that i need some views from my friends regarding whatever it is. Feel free to say your words regarding me both in Bengali and English. Maybe it will be a matter of reminisce after a long long time.

 

 

Thanks

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