CCK vs DSSS vs OFDM: Modulation Techniques Compared

This page compares CCK vs DSSS vs OFDM and describes the differences between CCK, DSSS, and OFDM modulation techniques. Various versions of the WLAN standard have been developed to address different data rate and coverage requirements. IEEE 802.11b supports four data rates: 1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps.

DSSS is used to provide support for 1 Mbps and 2 Mbps data rates. CCK is used 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 higher data rates, namely 5.5 Mbps and 11 Mbps.

Figure 1: CCK transmitter for 5.5 Mbps rate as defined in WLAN 802.11b

CCK stands for Complementary Code Keying.

For 5.5 Mbps transmission in WLAN 11b, information bits are first grouped into blocks of 4 bits each. The first 2 bits are mapped as per Table 1, and the rest of the two bits are mapped as per the CCK sequence shown in Table 2. CCK uses a code word to carry information signals. In other words, it spreads the data signal. Several phase angles are typically used to generate a complex code word of 8 bits.

Figure 2: CCK transmitter for 11 Mbps as mentioned in IEEE 802.11b

For 11Mbps transmission in WLAN 11b, information bits are first grouped into blocks of 8 bits each. Then, out of these 8 bits, 2 bits are encoded by the phase shift of the transmitted symbol relative to the previous symbol. The rest of the 6 bits are encoded using CCK. One out of 64 code words is mapped to these 6 bits each.

Table 3 is used to map the appropriate phase as per dibits in the information for the 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

Figure 3: DSSS modulation transmitter and receiver

DSSS stands for Direct Sequence Spread Spectrum. DSSS uses a pseudo-random binary sequence (PRBS) for spreading the information bits. It is called a PN (Pseudo Noise) sequence as its spectrum approaches random noise. Figure 3 depicts the transmitter and receiver part of the DSSS modulation technique.

When the DSSS transmitted signal passes through the channel, interference gets added to it due to nearby transmitters and a noisy environment. Knowledge of the PN sequence is needed at the receiver to recover the information signal by eliminating the interference signal as shown.

Figure 4: DSSS spectrum

Figure 4 depicts the DSSS spectrum. CCK will also have a similar spectrum due to spreading by high-rate code words. The spreading concept is the same as used in CDMA technique. The clock used to generate the PN sequence should be the same at the DSSS transmitter and receiver.

OFDM Modulation

Figure 5: OFDM modulation transmitter with spectrums

OFDM stands for Orthogonal Frequency Division Multiplexing. It uses IFFT and FFT equations to transform a frequency domain vector to a time domain vector and vice versa.

The idea of OFDM is to map complex data onto multiple narrowband subcarriers so that a higher data rate can be achieved. This is shown in the figure. As shown, a 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 subcarriers. This bunch of subcarriers, as per IFFT size, are combined and given as input to the IFFT block.

This block converts frequency domain complex mapper data into a time domain data vector. This vector is converted to analog form before being provided as input to an RF converter before transmission into the air using an antenna.

Figure 6: OFDM receiver with spectrums

The reverse actions will take place at the OFDM receiver. Initially, after front-end synchronization is carried out, the OFDM time domain samples are recovered. They are passed through the FFT block, which converts the time domain samples to frequency domain samples. These vectors are provided as input to the de-mapper block.

The demapper block converts complex symbols into binary bits. If 16-QAM is used at the transmitter, then at the receiver, the same is being used, and hence each symbol produces 4 bits at the output of the demapper. Figure 6 depicts the OFDM receiver with signal spectrums.

Summary

The following points highlight the comparison between CCK, DSSS, and OFDM techniques:

• Both CCK and DSSS use a single carrier, while OFDM uses multi-carrier for transmission.
• CCK and DSSS are spread spectrum modulation techniques, which provide high security during transmission due to the presence of information below the noise level. OFDM achieves spreading of data by transmitting a large number of carriers, each at a 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 up to 11 Mbps, while OFDM is used for high data rate applications such as 50 to 100 Mbps. OFDM can be used in combination with MIMO to achieve a very high data rate as per the number of antennas used in the transmit and receive chain.