CCK vs DSSS vs OFDM | Difference between CCK,DSSS,OFDM
This page compares CCK vs DSSS vs OFDM and describes difference between CCK, DSSS and OFDM modulation techniques.
There are various versions of WLAN standard developed to address different data rate and coverage requirements. IEEE 802.11b supports four data rates viz. 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps. DSSS is used to provide support for 1 Mbps and 2 Mbps data rate. CCK for 5.5 and 11 Mbps while OFDM is used for higher data rate applications. OFDM is used in IEEE 802.11a, 11g, 11n, 11ac and 11ad versions. OFDM is employed along with MIMO to increase the data rate further.
CCK Modulation
CCK modulation is employed to achieve the last two data rates viz. 5.5 Mbps and 11 Mbps.
The figure-1 depicts CCK transmitter for 5.5 Mbps rate as defined in WLAN 802.11b. CCK stands for Complementary Code Keying.
| Dibits | Phase |
|---|---|
| 00 | O degree |
| 01 | π/2 |
| 10 | π |
| 11 | 3*(π/2) |
For 5.5 Mbps transmission in WLAN 11b, information bits are first grouped to blocks of 4 bits each. First 2 bits are mapped as per table-1 and rest of the two bits are mapped as per CCK sequence shown in the following table-2. CCK uses code word to carry information signals. In other words it spreads the data signal. several phase angles are typically used to generate complex code word of 8 bits.
| Bit sequence | CCK code word |
|---|---|
| 00 | +i, +1, +i, -1, +i, +1, -i, +1 |
| 01 | -i, -1, -i, +1, +1, +1, -i, +1 |
| 10 | -i, +1, -i, -1, -i, 1, +i, 1 |
| 11 | +i, -1, +i, 1, -i, +1, i, 1 |
The figure-2 depicts CCK transmitter for 11 Mbps as mentioned in IEEE 802.11b. For 11Mbps transmission in WLAN 11b, information bits are first grouped to blocks of 8 bits each. Then out of these 8 bits, 2 bits are encoded by phase shift of transmitted symbol relative to previous symbol. Rest of the 6 bits are encoded using CCK. One out of 64 code words is mapped to this 6 bits each.
| Dibits | Phase (even symbol) | Phase (odd symbol) |
|---|---|---|
| 00 | O degree | π |
| 01 | π/2 | 3*(π/2) |
| 10 | π | 0 |
| 11 | 3*(π/2) | π/2 |
This table-3 is used to map appropriate phase as per dibits in the information for 11 MBps rate as per odd and even symbols in the transmitted data. The first symbol in the frame is taken as even.
DSSS Modulation
DSSS stands for Direct Sequence Spread Spectrum. DSSS uses pseudo random binary sequence (PRBS) for spreading the information bits. It is called PN (Pseudo Noise) sequence as its spectrum approaches random noise.
The figure-3 depicts transmitter and receiver part of DSSS modulation technique.
When the DSSS transmitted signal passes through the channel, interference gets added to it due to
nearby transmitters and noisy environment.
Knowledge of PN sequence is needed at the receiver to recover the information signal by
eliminating the interference signal as shown.
DSSS Modulation>>
The figure-4 depicts DSSS spectrum. CCK will also have similar spectrum due to spreading by high rate code words. The spreading concept is same as used in CDMA technique. The clock used to generate PN sequence should be same at the DSSS transmitter and receiver.
OFDM Modulation
The figure-5 depicts OFDM modulation transmitter with signal spectrums. OFDM stands for Orthogonal Frequency Division Multiplexing. It uses IFFT and FFT equations>> in order to transform frequency domain vector to time domain vector and vice versa.
The idea of OFDM is to map complex data on to multiple narrow band subcarriers so that higher data rate can be achieved. The same is shown in the figure. As shown complex modulation scheme such as 16-QAM is first used to map binary data information into complex frequency domain vector form. 16-QAM maps 4 bits on each of the subcarrier. This bunch of subcarriers as per IFFT size are combined and given as input to IFFT block. This block converts frequency domain complex mapper data into time domain data vector. This vector is converted to analog form before being provided as input to RF converter before transmission into the air using antenna.
The reverse actions will take place at the OFDM receiver. Initially after front end synchronization is carried out and OFDM time domain samples are recovered. They are passed through the FFT block which converts time domain samples to frequency domain samples. This vectors are provided as input to de-mapper block. The demapper block converts complex symbols into binary bits. If 16-QAM is used at the transmitter then at the receiver also same is being used and hence each symbol produces 4 bits at the output of the demapper. The figure-6 depicts OFDM receiver with signal spectrums. Also Refer SC vs OFDM>> OFDMA vs OFDM>> CDMA vs OFDM>> FBMC vs OFDM>> OFDM MATLAB>>
Following points highlight comparison between CCK, DSSS and OFDM techniques:
➨ Both CCK and DSSS use single carrier while OFDM uses multi-carrier for transmission.
➨ CCK and DSSS are spread spectrum modulation techniques which provides high security during
transmission due to presence of information below noise level. OFDM achieves spreading
of data by transmitting large number of carriers, each at low data rate. Here carriers are orthogonal to
each other by choosing appropriate frequency spacing between them.
Hence OFDM offers many benefits for alleviating problems encountered in single carrier systems.
This is done by spreading out frequency selective fading over many symbols in OFDM.
➨CCK and DSSS are used for low data rates upto 11 Mbps while OFDM is used for high data rate
applications such as 50 to 100 Mbps. OFDM can be used in combination of MIMO to achieve very high
data rate as per number of antennas used in transmit and receive chain.
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