Modulation methods are designed to be efficient in transmitting as many bits per hertz as possible. Here the goal is in the given bandwidth these modulation methods have to transmit the data at the highest speed and should minimize the use of spectrum space. Other Modulation methods that use bandwidth greater than the information bandwidth are called wideband modulation techniques. The two widely used wideband Modulation methods are Direct Sequence spread spectrum (DSSS )and orthogonal frequency-division multiplexing (OFDM).CDMA and OFDM both are wide-band wireless communication systems.
What is CDMA?
CDMA stands for Code Division Multiple Access. Direct Sequence Spread Spectrum is also referred to as CDMA. Here, “Multiple Access” means it is used for multiplexing many signals on a single communication channel. CDMA is widely used in cellular telephone systems. Compared to other methods, it allows more users to occupy a given band. Additionally, it facilitates communication with different codes.
Types of CDMA
There are two types of CDMA:
- DS- CDMA ( Direct Sequence Code Division Multiple Access): It is used to spread the signals from different users with different codes. Each user’s signal is multiplied by a unique code sequence, allowing multiple users to transmit simultaneously over the same frequency band without interfering with each other.
- FH – CDMA (Frequency Hopping Code Division Multiple Access): It is used to avoid interference of the signals by changing the carrier frequency and occupying a large spectral band. By changing frequencies in a pseudo-random pattern, FH-CDMA spreads the signal energy across the frequency band, making it more resistant to interference and jamming.
CDMA Working
Given below is the block diagram for CDMA transmitter and receiver
CDMA Transmitter and Receiver
- The input signal is given to spreading modulator which is used to prevent the interference of the signal by spreading the spectrum (bandwidth) of the signal.
- Raised cosine filter is used to shape the pulse, So that to minimize inter symbol interference (ISI).
- QPSK (Quadrature Phase Shift Keying) is used to decompose one signal to two independent signals.
- Further, the Up converter converts the increase the frequency of a signal from a lower frequency to higher frequency.
- The Encoded signal is Transmitted through channel and along with some Noise which is given to down converter.
- In the Receiver side, Down converter is used to convert high frequency signal to low frequency signal.
- This signal is given to QPSK Demodulator used to demodulate the signal that was modulated using the quadrature phase shift keying method in the transmitter part.
- De spreading is used to combines the independent signal to one signal. This signal is given to output.
What is OFDM ?
OFDM Stands for Orthogonal Frequency-Division Multiplexing. OFDM is also known as Multi Carrier Modulation (MCM). OFDM transmits data by simultaneously modulating segments of the high-speed serial bit stream onto multiple carriers spaced throughout the channel bandwidth.
There are different types of OFDM, Most used one is COFDM (Coded Orthogonal Frequency-Division Multiplexing), for the transmitted signal the Forward Error correction is applied.
Another types of OFDM are Vector OFDM (VOFDM), Flash OFDM (F-OFDM), Cyclic prefix ( CP-OFDM).
OFDM Working
Given below is the block diagram for OFDM transmitter and receiver
OFDM Transmitter and Receiver
The main idea of OFDM is to divide a broadband frequency-selective channel into a number of narrowband at sub channels using Fast Fourier transform (FFT). The frequency responses of the subcarriers are overlapping and orthogonal, to get the high spectral efficiency, so it is named as OFDM.
- Input data is given to Modulator, as it modulates or transmits the signals from one location to another location with faster speed.
- Serial to parallel converter : It converts the series of input data to parallel output data.
- IFFT means Inverse Fast Fourier Transform is used at transmitter to convert frequency domain to time domain.
- Parallel to Serial Converter is used to convert Parallel input data to series of output data.
- Cyclic prefix is used to extend the length of the symbol, while maintains the orthogonality of the waveform or data.
- Digital to analog Converter used to convert digital signal into analog or electrical signal.
- Parallel data must be converted to serial data form before transmitting to the outside devices.
- Demodulator is used to demodulates the signal and given to output.
Difference Between CDMA and OFDM
|
Specification |
CDMA |
OFDM |
|---|---|---|
|
Transmission Bandwidth |
CDMA transmission bandwidth is Equal to the system bandwidth. |
OFDM Transmission Bandwidth is Up to the system Bandwidth and is variable. |
|
Symbol period |
Symbol period is Very short and it is opposite to the system bandwidth. |
Symbol period is Very long and it depends on the spacing of subcarrier and it does not depend on system Bandwidth. |
|
Equalization |
Using of CDMA is very difficult above 5MHz. |
OFDM is used for all the bandwidths because of frequency domain representation. |
|
Domain |
Signal is represented in time domain. |
Signal is represented in frequency domain. |
|
Synchronization |
There is a need of time synchronization. |
No need of any time synchronization. |
|
Structure |
Little complex structure. |
More complex structure. |
|
Cost |
Cost is low compared to OFDM method. |
Overall cost is high. |
|
MIMO |
Not Ideal for Multiple Input and Multiple Output(MIMO) system. |
OFDM is Ideal for Multiple Input and Multiple Output(MIMO) system. |
Advantages of CDMA and OFDM
Given below are the Advantages of CDMA and OFDM
Advantages of CDMA
- CDMA is highly efficient and scalable.
- Due to different code words assigned to each user, Interference is decreased.
- Improved security, resistance to interference and jamming, and efficient use of bandwidth.
- Number of users can use the same bandwidth by splitting.
- Uses Fixed frequency spectrum Efficiently.
- CDMA is more protected from hackers.
Advantages of OFDM
- Used to control different parts of the signal in an easy manner.
- Uses Equalization strategy so that there is less interference of the antennas.
- Integrated with MIMO(Multiple Input and Multiple Output) system, so that Multiple samples of same signal is transmitted.
- Used for Avoiding Intercell Interference.
- Simple to use.
- Lost data can be recovered using sufficient channel coding.
Disadvantages of CDMA and OFDM
Given below are the Disadvantages of CDMA and OFDM
Disadvantages of CDMA
- Overall quality will be decreased, As the number of users Increases.
- Need of time synchronization.
- Data transmission rate is low.
- This method produce large Interference.
- Hard to improve to increase the performance.
Disadvantages of OFDM
- Peak to Average ratio is increased: Using of Inverse Fast Fourier Transform (IFFT) which combines other signals having different frequency produces large signal distortion which is termed as peak to Average Ratio. This will cause interference between the different channels.
- Synchronization: Frequency synchronization is difficult at Receiver than the Time Synchronization.
- Using of cyclic prefix makes the circuit loss the efficiency.
- Compared to single frequency this method is more sensitive to carrier frequency.
- There is a required of linear transmitter circuit which is low efficient in power.
Applications of CDMA and OFDM
Given below are the Applications of CDMA and OFDM
Applications of CDMA
- CDMA is immune to noise, it is used in military purpose.
- Used in digital telephone services.
- CDMA used for Wireless communication systems.
- Used in satellite systems.
- For simultaneous transmission, CDMA is used to combine several data signals.
Applications of OFDM
- Used in narrow band power line communications and broad band power line communications.
- Used in Digital Audio Broadcasting and digital radio.
- Used in Wired and wireless Data Transmission.
- 4G (fourth Generation) and LTE (Long Term Evolution) Networks use OFDM.
- Used in Digital Television and Video broadcasting.
Conclusion
As the future technologies May lead the current technologies like CDMA , OFDM. There is no particular answer to select which one method is widely used in the communication systems. Based on the requirements of the technology at a particular place or time will decide the method of technology used for. CDMA, OFDM will play an important roles in communications systems. Making use of their Advantages and in specific application they serve a great purpose in the wired and wireless communication system.
Difference Between CDMA and OFDM – FAQs
Is CDMA is used in wireless networks?
Yes, CDMA is used in wireless local area networks (WLAN’s) and also used in 3G, 4G telephone networks. CDMA also used in satellite communications where large number of signals are transmitted simultaneously.
What is the Bandwidth of OFDM?
The total channel bandwidth of OFDM is 20 MHz(Mega Hertz), with an occupied bandwidth of 16.6 MHz(Mega Hertz).
State the reason why OFDM is called Orthogonal?
In the time of signal transmission the Sub Carriers are placed in such a way, so that they partially overlap with each other. The peak of the sub carrier intersects with the zero of the next or preceding sub carrier. This condition is called Orthogonality.