Comparison of CDMA and OFDM techniques

Spectrum utilization, support high-speed multimedia service, system capacity, anti-multipath channel interference and other factors is the most fixed broadband wireless access equipment supplier selected CDMA (Code Division Multiple Access) or OFDM (Orthogonal Frequency Division Multiplexing) the main starting point for the technology as a key point to multipoint (PMP) of. Both technologies have their own merits and in these areas, so equipment manufacturers need the actual situation on balance, a comprehensive analysis to make the best choice.
　　CDMA technology is based on the modulation and multiple access spread spectrum communication theory connection technology. OFDM technology is a multi-carrier modulation technique, the basic idea is that channel is divided into a number of orthogonal subchannels using a subcarrier modulated on each sub-channel, and each sub-carrier transmitted in parallel. OFDM and CDMA technologies have advantages and disadvantages. CDMA has known advantages, the new technology using a variety of OFDM also exhibit good extensibility network structure, higher spectrum efficiency, more flexible modulation scheme and multipath interference. The following mainly from the modulation, the peak to average power ratio equal angular Narrowband interference analysis of specific differences between the two technologies in the performance.
　　- modulation technique. In general, the spectral efficiency of a wireless system by using the 16QAM (Quadrature Amplitude Modulation), 64QAM or even higher order modulation manner is improved, and a good communication between the system should get the best spectral efficiency and error rate balance.
　　In the CDMA system, the downlink can support a variety of modulation, but the modulation symbols for each link must be the same, and the uplink does not support multiple modulation, the CDMA system which makes the loss of some flexibility. Further, in such a non-orthogonal link, using higher order modulation of the user will be using low-order modulation users have a great noise.
　　In an OFDM system, each link can be independently modulated, so that the system both in the uplink or on the downlink can easily accommodate a variety of mixed modulation scheme. This can introduce the concept of "adaptive modulation". It increases the flexibility of the system, for example, under good channel conditions may be employed as the terminal 64QAM modulation order to obtain a higher maximum spectral efficiency, while the poor channel conditions may be selected QPSK (Quadrature Phase Shift Keying) modulation and other low-level modulation to ensure that the signal to noise ratio. In this way, the system can achieve the best balance between spectral efficiency and bit error rate. Further, although the inter-channel interference limits the modulation scheme for a particular link bar, but this can be solved by the network frequency planning and radio resource management means.
　　- peak to average power ratio (PAPR). This is also an important factor device makers should consider. Because too high PAPR causes the transmitting side high power amplifier linearity requirements, which means to provide additional power, a battery backup and enlarging the size of the device, thereby increasing the cost of the base station and user equipment.
　　PAPR CDMA system typically 5 ~ 11dB, and will increase the data rate and the number of used codes increases. There are a lot of technology can reduce PAPR CDMA.
　　In an OFDM system, since the non-constant envelope of the signal, making the system very sensitive to non-linear. If the nonlinear susceptibility of the measures did not improve, OFDM technology will not be used for battery-transmission systems and mobile phones. There are many OFDM technology can reduce the PAPR.
　　- narrowband interference. CDMA's biggest advantage is its performance in anti-narrowband interference. Since the interference only affects a small part of the spread spectrum signal; and narrowband interference in OFDM also affects only a small part of its band, and the system may not use parts of the bands subject to interference, or with the use of a lower order before error correction and modulation and other means to resolve.
　　- multipath interference. In the radio channel, the multipath receiving signal propagation effects caused by overlapping, mutual interference between the signal waveforms at the receiver to determine the error. This will seriously affect the quality of signal transmission.
　　To counteract this self-interference signal, CDMA RAKE receiver uses a diversity reception technique to distinguish between bound and multiplexed signal energy. In order to reduce sources of interference, RAKE receiver to provide some diversity gain. However, due to multipath signal energy are not equal, the test proved that, if the path exceeds 7 or 8, the dispersion of this signal energy that would reduce the accuracy of channel estimation, RAKE reception performance will decline rapidly.
　　OFDM technology and different ideas RAKE reception, which is the information symbol to be transmitted by the serial-parallel conversion, the rate of reduction, thereby increasing the symbol period, in order to weaken the influence of multipath interference. It simultaneously using a cyclic prefix (CP) as a guard interval, greatly reduce or even eliminate the inter-symbol interference, and to ensure orthogonality among the channels, thereby greatly reducing the inter-channel interference. Of course, this also paid the price of bandwidth, and brought energy losses: the longer the CP, the greater the energy loss.
　　- Power control technology. In the CDMA system, 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. The power control relatively basic needs are not OFDM system. OFDM system introduces object of power control is to minimize inter-channel interference.
　　--network layout. Due to the technical characteristics of its own CDMA, CDMA frequency planning system is not very prominent, but faced with the problem of design and planning codes. The most basic purpose of network planning is to reduce interference between OFDM system channel. Because of this plan is based on the frequency distribution, as long as the designer set aside some bands can solve the problem of cell splitting.
　　- equalization. ISI equalization techniques may compensate for time-division channels due to multipath effects produced. In the CDMA system, the channel bandwidth much greater than the bandwidth of the channel is flat fading. Since the spreading code is transmitted again from the good correlation of its own, so that delay spread in the wireless channel may be viewed as simply transmitted signal. If the delay between the multipath signals from each other than the length of one chip, can be considered uncorrelated RAKE receiving end noise, without the need for equalization.
　　OFDM system, in general fading environment, equalization is not an effective method to improve system performance, because the substance is balanced to compensate the multipath channel characteristics. OFDM technology itself has the advantage of diversity characteristics of the multipath channel, so that the system generally do not have the balanced
　　process generated OFDM