|
In OFDM modem with 48 channels,each channel uses 16-QAM modulation.if the over all transmission rate is 10Mbps.what is the symbol transmission rate per channel? vinoth 17-April-2008 12:05:34 AMComments Lesser than 10Mbps Posted by waqqas1 OFDM uses IFFT at the transmitter to speed up modulation and FFT at the receiver.The complex exponential tones on which data are modultaed are orthogonal over one FFT frame duration.This makes them easy to recover at the receiver. However the channel thrugh which data are sent may have more than one path.This causes interference from one FFT frame to another. To avoid this and to have certain computational advantage at the receiver last few samples of the FFT frame is added in the front in such way that the added part is longer thanthe channel. Since there are 48Channels and each carry 16 QAM the number of data per FFT frame is 48*log2(16)=192 bits. Sine the required transmission rate is 10 Mbps the OFDM symbol rate must be 10^7/192. OFDM symbol period=FFT period+Guard(Cyclic Prefix). In actual OFDM not all the carrier will be used.Some are reserved for guard band against interference and in some carriers pilot tones which are used for synchronization and channel estimation are used.Besides there will be coding. Hence for the symbol rate and number of tones actual payload data rate will be much lesser than 10Mbps Posted by Mannai_Murali |
Posted: 21-April-2008 03:03:40 AM By: Mannai_Murali OFDM uses IFFT at the transmitter to speed up modulation and FFT at the receiver.The complex exponential tones on which data are modultaed are orthogonal over one FFT frame duration.This makes them easy to recover at the receiver. However the channel thrugh which data are sent may have more than one path.This causes interference from one FFT frame to another. To avoid this and to have certain computational advantage at the receiver last few samples of the FFT frame is added in the front in such way that the added part is longer thanthe channel. Since there are 48Channels and each carry 16 QAM the number of data per FFT frame is 48*log2(16)=192 bits. Sine the required transmission rate is 10 Mbps the OFDM symbol rate must be 10^7/192. OFDM symbol period=FFT period+Guard(Cyclic Prefix). In actual OFDM not all the carrier will be used.Some are reserved for guard band against interference and in some carriers pilot tones which are used for synchronization and channel estimation are used.Besides there will be coding. Hence for the symbol rate and number of tones actual payload data rate will be much lesser than 10Mbps | |
Posted: 31-December-2008 01:09:02 PM By: waqqas1 Lesser than 10Mbps |