Basic knowledge of CDMA technology

1. What is CDMA technology?

CDMA literally means code division multiple access, which is a technology developed on the basis of spread spectrum communication, a branch of digital communication technology. Spread spectrum, in simple terms, is to expand the spectrum. CDMA technology uses direct sequence spread spectrum, which is to use a carrier with noise characteristics and a much wider frequency band than the bandwidth required for simple point-to-point communication to transmit the same data.

Like frequency modulation and amplitude modulation technology, direct sequence spread spectrum is a modulation technology that uses a code sequence (high speed) to modulate the original data information (low speed) so that the modulated information can be transmitted at high speed.

CDMA technology is a direct sequence spread spectrum method. Like frequency modulation and amplitude modulation technology, direct sequence spread spectrum is a modulation technology that uses a code sequence (high speed) to modulate the original data information (low speed), so that the modulated information can be transmitted at high speed.

CDMA literally means code division multiple access, which is a technology developed on the basis of spread spectrum communication, a branch of digital communication technology. Spread spectrum, in simple terms, is to expand the spectrum. CDMA technology uses direct sequence spread spectrum, which is to use a carrier with noise characteristics and a much wider frequency band than the bandwidth required for simple point-to-point communication to transmit the same data.

2. The Origin of CDMA Technology

The origin of spread spectrum technology can be traced back to World War II. The original intention of this idea was to prevent the enemy from interfering with one's own communications. As we know, since the bandwidth used in narrowband communication is only tens of kHz, it is very easy to interfere with the opponent's communication by using a transmitter with the same transmission frequency and sufficient power. This is because it is difficult to recover the original information from a poor signal-to-noise ratio environment regardless of the amplitude modulation or frequency modulation technology.

The idea of ​​CDMA is to spread the signal energy to a wide frequency band through special code processing, so that it is buried in the noise. Only the same code can be used to recover the signal at the receiving end (the whole process is like encryption and decryption). We call it direct sequence spread spectrum. Since the signal is buried in the noise, it is difficult for the enemy to detect it. Therefore, this technology has a wide range of applications in the military field.

3. What is CDMA's soft capacity?

According to the analogy of the CDMA system above, new talkers may constantly enter the room. Of course, the total number of talkers has a certain limit, which is closely related to the size of the room, the volume of people, and the distance between talkers. Here we introduce several new analogies: the size of the room is the capacity of a single carrier for the CDMA system; the volume between talkers is equivalent to the transmission power of the mobile phone in the CDMA system; volume control corresponds to a very important technology in CDMA-power control; the distance between talkers corresponds to the distance between the mobile phone and the base station. Through this example, we can summarize some characteristics of the CDMA system: the CDMA system is a self-interference system; the capacity of a single carrier frequency in the CDMA system is not fixed like FDMA and TDMA, which is what we often call "soft capacity"; therefore, power control plays an important role in the CDMA system, which directly affects the system capacity.

4. What is a CDMA short code? How is it different from a CDMA long code? What are their uses?

The CDMA system uses two pseudo-random code sequences, namely short codes and long codes.

Short code: A short code is a periodic sequence of length 215-1.

In the forward channel of the CDMA system (from the base station to the mobile phone), the short code is used to modulate the forward channel so that the forward channel carries the mark of the base station. Different base stations use short codes with different phases to distinguish themselves from each other.

In the reverse channel (from the mobile phone to the base station), the short code is used to modulate the reverse traffic channel, which has the same function as the short code in the forward channel.

Long code: The long code is a periodic sequence of length 242-1.

In the forward channel of the CDMA series (from the base station to the mobile phone), the long code is used to scramble the traffic channel (similar to encryption).

In the reverse channel (from the mobile phone to the base station), the long code is used to directly spread the spectrum, so as to distinguish different accessing mobile phones.

Walsh code: In addition to long codes and short codes, the CDMA system also uses 64-bit long Walsh codes. Walsh codes have good orthogonality in mathematics. The so-called orthogonality means that people who speak different languages ​​and do not understand each other's language cannot communicate with each other using language. Walsh codes can be used to distinguish different forward channels.

5. Why is power control so important in CDMA systems?

As mentioned above, power control of CDMA system is particularly important, and power control is considered to be the core of all CDMA key technologies. To explain the importance of power control, we must first understand the concept of "near-far effect". We can imagine that if all users in a cell transmit signals with the same power, the signal from a mobile phone close to the base station will be strong, while the signal from a mobile phone far away from the base station will be weak, which will cause the strong signal to cover the weak signal. This is the problem of "near-far effect" in mobile communications.

Because all users share the same frequency (carrier), the "near-far effect" is more prominent. The purpose of CDMA power control is to overcome the "near-far effect" so that the system can maintain high-quality communications while not causing unnecessary interference to other users occupying the same channel.

6. Why can CDMA mobile phones maintain low transmission power?

This is because the CDMA system has a set of precise power control methods. The power control in the CDMA system is divided into forward power control and reverse power control. Reverse power control is further divided into open-loop control in which only the mobile phone participates and closed-loop power control in which both the mobile phone and the base station participate. The reverse open-loop power control is completed independently by the mobile phone. The mobile phone quickly adjusts the mobile phone's transmission power according to the changes in the power it receives in the cell. It is precisely because of these precise power controls that CDMA mobile phones can maintain appropriate transmission power.

7. What is CDMA soft switching? How is it different from hard switching?

Mobile communication is built on the move. With frequency reuse, there will inevitably be the problem of frequency switching on the move. The quality of a network is mainly reflected in indicators such as dropped calls and frequency loss in the wireless aspect. The switching method will affect these indicators. Through the comparison of the soft switching and hard switching methods below, we can draw a conclusion on which is better.

Hard Switch

In FDMA and TDMA systems, all handovers are hard handovers. When a handover occurs, the mobile phone always releases the channel of the original base station before obtaining the channel assigned by the new base station. It is a "release-establishment" process. The handover process occurs between two base station transition areas or sectors. The two base stations or sectors are in a competitive relationship.

If the signal strength of two base stations changes dramatically in a certain area, the mobile phone will switch back and forth between the two base stations, resulting in the so-called "ping-pong effect". This will increase the burden on the switching system and increase the possibility of dropped calls.

Soft Switch

In CDMA systems, the handover situation is different. When a mobile phone is in handover, two or more base stations will monitor it at the same time. The base station controller in the system will compare the signal quality reports about the mobile phone from each base station frame by frame and select the best one.

It can be seen that the handover of CDMA is a process of "establishment-comparison-release". We call this kind of handover soft handover to distinguish it from the handover in FDMA and TDMA. Soft handover can be the handover between different base stations under the same base station controller or between different base stations under different base station controllers.

8. What is CDMA's "softer switching"?

There is another type of handover in the CDMA system called "softer handover". It refers to the handover between different sectors of the same base station with the same frequency. In addition, the CDMA system can also provide handover between different carriers guided by pilot beacons, as well as some software-controlled handovers. All these handover measures bring more reliable wireless channels to the CDMA system.

What is the difference between the existing cellular system hierarchical reception and CDMA hierarchical reception?

In the CDMA system, the Rake receiver is used to overcome the multipath effect, turning the disadvantage into an advantage. This is a unique path classification technology in CDMA.

The path-level reception works like this: at the base station, each reverse channel has four digital demodulators, and each digital demodulator contains a search unit and a demodulation unit. The search unit is responsible for searching for different multipath signals and handing them over to each demodulation unit for demodulation. In this way, each base station can demodulate four multipath signals at the same time and perform vector merging. The recovered signal is better than any other signal. In the mobile phone, there are three digital demodulation units and one search unit, so the mobile phone can also demodulate three multipath signals at the same time and perform vector merging. Due to the use of multipath receivers, both base stations and mobile phones can receive stable signals.

9. Why does CDMA need to synchronize the entire network?

If the code sequence has a transmission delay during transmission, the original data cannot be demodulated and restored at the receiving end. It is necessary to compensate for the delay caused by transmission and digital signal processing through artificial delay at the receiving end. To achieve this compensation, we must establish a synchronization system, that is, the code sequences generated by the receiving and transmitting ends must be synchronized. This is the synchronization problem of the CDMA system. Since the code rate in the CDMA system is very high, it is necessary to have a set of high-precision synchronous clocks as a reference to coordinate the work of all base stations in the entire network.

At present, the Global Positioning System (GPS) is the best choice for this clock reference. GPS is an antenna navigation system composed of 24 satellites orbiting the earth. Its advantages are global coverage, high system clock accuracy, and not easily affected by electromagnetic storms and low-frequency interference sources. As a backup, the Long Range Navigation (LORAN-C) system is also a good choice. This system uses ground wave propagation technology and also has the characteristics of accurate clock, no influence from ionospheric changes, low attenuation, stable phase and amplitude, etc.