A comparison between CDMA and OFDM

Spectrum utilization, supporting high speed multimedia services, system capacity, multipath channel interference and other factors are the main starting points for the majority of fixed broadband wireless access devices to select CDMA (Code Division multiplex) or OFDM (Orthogonal Frequency Division multiplexing) as the key technology for point to Point (PMP). And these two technologies have their own strengths in these areas, so the equipment business needs to weigh the pros and cons of the actual situation, the comprehensive analysis, so as to make the best choice.

CDMA technology is a technique of modulation and multiple access connection based on spread spectrum communication theory. OFDM technology belongs to Multicarrier modulation technology, its basic idea is to divide the channel into many orthogonal sub channels, use a subcarrier to modulate each sub channel, and transmit each subcarrier in parallel. OFDM and CDMA technology have their own advantages and disadvantages. CDMA has a well-known advantage, and OFDM with a variety of new technologies also shows good network structure scalability, higher spectrum utilization, more flexible modulation and multipath jamming ability. The following mainly from the modulation technology, peak average power ratio, the ability to resist narrowband interference and other aspects of the performance of the two technologies in the specific differences.

Modulation technology. Generally speaking, the spectrum efficiency of wireless system can be improved by using 16QAM (orthogonal amplitude modulation), 64QAM and even higher order modulation mode, and a good communication system should get the best balance between spectral efficiency and BER.

In CDMA system, downlink link can support a variety of modulation, but each link must be the same symbol modulation mode, and uplink does not support a variety of modulation, which makes the CDMA system lost some flexibility. Moreover, in this kind of non orthogonal link, the user who uses the High-order modulation method will make a great noise disturbance to the user with low order modulation. (Computer science)

In OFDM systems, each link can be modulated independently, so the system can easily accommodate a variety of mixed modulation modes both upstream and downstream. This can introduce the concept of "adaptive modulation". It increases the flexibility of the system, for example, the terminal can adopt higher order such as 64QAM modulation to obtain maximum spectral efficiency in the condition of good channel, and can choose QPSK (four phase shift keying) modulation and other low order modulation to ensure signal-to-noise ratio when the channel condition becomes worse. In this way, the system can achieve the best balance between spectrum utilization and BER. In addition, although the interference between channels restricts the modulation of a particular link, it can be solved by means of network frequency planning and wireless resource management.

Peak average power ratio (PAPR). This is an important factor that the equipment business should consider. Because the PAPR is too high, the transmitter has a high linearity requirement for the power amplifier, which means that it can increase the cost of the base station and the user's equipment by providing additional wattage, battery backup and enlarging the size of the equipment.

The PAPR of CDMA system is generally 5~11db, and will increase with the increase of data rate and usage code number. At present, there are many techniques to reduce the papr of CDMA.

In OFDM system, because of the instability of signal envelope, the system is very sensitive to nonlinearity. Without measures to improve non-linear sensitivity, OFDM technology will not be used in battery transmission systems and mobile phones. At present, there are many techniques to reduce OFDM papr.

Ability to resist narrowband interference. The biggest advantage of CDMA is the ability of resisting narrowband interference. Because the interference affects only a small part of the whole spread spectrum signal, the narrowband interference in OFDM only affects a small part of the frequency band, and the system can not use some interference frequency bands, or use the method of forward error correction and lower order modulation to solve.

Anti-multipath jamming capability. In the wireless channel, the multipath propagation effect causes the receiving signals to overlap each other, resulting in mutual interference between the signal waveforms, which makes the receiver's judgment error. This can seriously affect the quality of the signal transmission.

In order to offset the signal self interference, the CDMA receiver uses rake diversity receiving technology to differentiate and bind the multi-channel signal energy. In order to reduce the jamming source, rake receiver provides some diversity gain. However, because of the unequal signal energy, the experiment proves that if the path number exceeds 7 or 8, the dispersion of the signal energy will make the channel estimation precision lower, and Rake's receiving performance will decrease quickly.

OFDM technology is different from rake reception, it is to send the information code element through the string and transform, reduce the rate, thereby increasing the code element cycle, to weaken the impact of multipath interference. At the same time, it uses the cyclic prefix (CP) as the protection interval, which greatly reduces and even eliminates the interference between the codes, and ensures the orthogonality between the channels, thus greatly reducing the interference between the channels. Of course, this also pays the cost of bandwidth, and brings the energy loss: the longer the CP, the greater the energy loss.

Power control technology. In the CDMA system, the power control technology is an important method to solve the near-far effect, and the power control effectiveness determines the capacity of the network. Relatively speaking, power control is not the basic requirement of OFDM system. The purpose of introducing power control in OFDM system is to minimize interference between channels.

Network planning. Because of the technical characteristics of CDMA, the frequency planning problem of CDMA system is not very prominent, but it faces the problem of code design and planning. The most basic purpose of OFDM system network planning is to reduce the interference between channels. Since this program is based on frequency allocation, designers can solve the problem of cell division by reserving some frequency bands.

Equalization techniques. The equalization technique can compensate the ISI generated by multipath effect in the time channel. In CDMA system, the channel bandwidth is much larger than the flat fading bandwidth of the channel. Because of the good autocorrelation of the spread code, the delay propagation in the wireless channel transmission can be regarded as just the transmission of the transmitted signal. If the multipath signals are longer than the length of a single piece of code, the rake receiver is considered to be a irrelevant noise, and no balance is needed.

For OFDM systems, equalization is not an effective method to improve system performance in general fading environment, because the essence of equilibrium is to compensate multipath channel characteristics. OFDM technology has already utilized the diversity of Multipath channel, so the system does not need to be balanced.