Technical comparison between CDMA and OFDM
Factors such as spectrum utilization rate of OFDM control channel insertion mode, support for high-rate multimedia services, system capacity, and resistance to multipath channel interference are currently the choices of most fixed broadband wireless access equipment manufacturers to choose CDMA (code division multiple access) or OFDM (orthogonal Frequency division multiplexing) is the main starting point when it is the key technology of point-to-multipoint (PMP). These two technologies have their own advantages in these aspects, so equipment manufacturers need to weigh the pros and cons according to the actual situation and conduct a comprehensive analysis to make the best choice.
OFDM system hardware structure CDMA technology is a modulation and multiple access technology based on spread spectrum communication theory. OFDM technology belongs to multi-carrier modulation technology, its basic idea is to divide the channel into many orthogonal sub-channels, use one sub-carrier for modulation on each sub-channel, and each sub-carrier is transmitted in parallel. OFDM and CDMA technologies have their pros and cons. CDMA has well-known advantages, and OFDM using a variety of new technologies also shows good network structure scalability, higher spectrum utilization, more flexible modulation methods and anti-multipath interference capabilities. The following mainly analyzes the specific differences in performance of these two technologies from the perspective of modulation technology, peak-to-average power ratio, and ability to resist narrow-band interference.
OFDM spectrum efficiency comparison-modulation technology. In general, the spectral efficiency in wireless systems can be improved by using 16QAM (quadrature amplitude modulation), 64QAM, and even higher-order modulation methods, and a good communication system should obtain the best between spectral efficiency and bit error rate balance.
In a CDMA system, the downlink can support multiple modulations, but the symbol modulation of each link must be the same, while the uplink does not support multiple modulations, which makes the CDMA system lose some flexibility. In addition, in this non-orthogonal link, users using high-order modulation will inevitably cause significant noise interference to users using low-order modulation.
In an OFDM system, each link can be independently modulated, so the system can easily accommodate multiple mixed modulation methods simultaneously on both the uplink and downlink. This can introduce the concept of "adaptive modulation". It increases the flexibility of the system. For example, under good channel conditions, the terminal can use higher-order modulation such as 64QAM to obtain maximum spectral efficiency, and can choose QPSK (Quadrature Phase Shift Keying) when the channel conditions become worse. Modulation and other low-order modulation to ensure the signal-to-noise ratio. In this way, the system can achieve the best balance between spectrum utilization and bit error rate. In addition, although inter-channel interference limits the modulation of a particular link, this can be solved by means of network frequency planning and radio resource management.
OFDM frequency deviation error-peak-to-average power ratio (PAPR). This is also an important factor that equipment manufacturers should consider. Because the PAPR is too high, the transmitter will have high linearity requirements for the power amplifier, which means providing extra power, battery backup, and expanding the size of the equipment, which will increase the cost of the base station and user equipment.
The PAPR of the CDMA system is generally 5 to 11 dB, and will increase with the increase of the data rate and the number of codes used. There are many technologies that can reduce the PAPR of CDMA.
In the OFDM system, due to the instability of the signal envelope, the system is very sensitive to nonlinearity. Without measures to improve non-linear sensitivity, OFDM technology will not be used for transmission systems and mobile phones that use batteries. There are many technologies that can reduce the PAPR of OFDM.
——Narrowband interference resistance. The biggest advantage of CDMA is its ability to resist narrow-band interference. Because interference affects only a small part of the entire spread spectrum signal; narrow-band interference in OFDM also affects only a small part of its frequency band, and the system can not use the interfered part of the frequency band, or use forward error correction and use lower order Modulation and other means to solve.
High peak-to-average power ratio-anti-multipath interference capability. In the wireless channel, the multipath propagation effect causes the received signals to overlap with each other, causing mutual interference between the signal waveforms, which makes the receiving end judge wrong. This will seriously affect the quality of signal transmission.
In order to counteract such signal self-interference, the CDMA receiver uses RAKE diversity reception technology to distinguish and bind the energy of multiple signals. To reduce interference sources, RAKE receivers provide some diversity gain. However, due to the unequal energy of multiple signals, the experiment proves that if the number of paths exceeds 7 or 8, the dispersion of this signal energy will reduce the accuracy of channel estimation, and the reception performance of RAKE will decrease quickly.
OFDM technology is different from the idea of ​​RAKE receiving. It is to convert the information symbols to be transmitted through serial-parallel conversion to reduce the rate, thereby increasing the symbol period and weakening the impact of multipath interference. At the same time, it uses the cyclic prefix (CP) as the guard interval, which greatly reduces or even eliminates the inter-symbol interference, and guarantees the orthogonality between the channels, thereby greatly reducing the inter-channel interference. Of course, this also pays the price of bandwidth and brings energy loss: the longer the CP, the greater the energy loss.
——Power control technology. In CDMA systems, power control technology is an important method to solve the near-far effect, and the effectiveness of power control determines the capacity of the network. Relatively speaking, power control is not a basic requirement for OFDM systems. The purpose of introducing power control in OFDM systems is to minimize inter-channel interference.
OFDM inter-cell interference-network planning. Due to the technical characteristics of CDMA itself, the frequency planning problem of the CDMA system is not very prominent, but it is facing the problem of code design and planning. The most basic purpose of OFDM system network planning is to reduce the interference between channels. Because this kind of planning is based on frequency allocation, the designer can solve the problem of cell division as long as some frequency bands are reserved.
-Balanced technology. Equalization technology can compensate for ISI caused by multipath effect in time division channels. In a CDMA system, the channel bandwidth is much larger than the flat fading bandwidth of the channel. Due to the good auto-correlation of the spreading code itself, the delay spread in wireless channel transmission can be regarded as just the retransmission of the transmitted signal. If the delay between these multipath signals exceeds the length of one chip, they can be regarded as uncorrelated noise by the RAKE receiving end, and no more equalization is needed.
For OFDM systems, under general fading environment, equalization is not an effective method to improve system performance, because the essence of equalization is to compensate for multipath channel characteristics. The OFDM technology itself has taken advantage of the diversity characteristics of multipath channels, so the system generally does not need to be balanced
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