The purpose and function of equipment detection is to reduce and avoid interference from the source. With the development of radio communication technology, the electromagnetic environment is becoming more complex and radio interference is becoming more and more complex. Under such circumstances, the importance of detection work is self-evident. To do a good job in testing, you need to start with four links, namely test instruments, equipment under test, test standards and test methods. We have more analysis and discussion on test instruments, test standards and test methods. However, the analysis and research of the equipment under test is also important for our inspection work. This article combines our practical work to explain the 800 MHz CDMA base station transmission performance from three aspects: CDMA communication principle overview, wireless transmission channel and transmission performance index. 1. Overview of CDMA communication principles CDMA is the abbreviation of English Code Division MulTIple Access, Chinese translates into code division multiple access, that is, code division multiple access, is one of the advanced technologies widely used in the field of communication. It is based on spread spectrum technology and uses pseudo-random sequences formed by spread spectrum technology to achieve multiple access. In other words, in CDMA communication systems, signals used by different users to transmit information are not distinguished by different frequencies or different time slots, but by Use different coding sequences to distinguish. The transmitting end generates the spread spectrum signal through two steps of modulation and spread spectrum. First, modulate the carrier with the information bits to be transmitted, and then use the pseudo-random sequence to spread the spectrum of the signal; or multiply the spreading sequence with the information bits (spread the spectrum of the information), and then modulate the carrier. The receiving end recovers the original information through two steps of despreading and demodulation. Despreading means that the receiver multiplies the received spread-spectrum signal with the same pseudo-random sequence, also known as correlated reception. After the despread signal undergoes regular demodulation, information can be recovered. In order for the receiver to correctly detect the information transmitted by the transmitter, the pseudo-random sequence generated locally by the receiver must be the same and synchronized with the transmitter, but the pseudo-random sequence itself is random, and it is impossible to achieve the same sequence at both ends of the transceiver. In order to reduce the detected error rate, the pseudo-random sequence must have good correlation characteristics. In CDMA systems, specific orthogonal sequences, such as Welsh sequences, are used for spectrum spreading. 2. Wireless transmission channel As mentioned in Part 1, pseudo-random sequences are very important and critical in CDMA communications. The Welsh sequence used in the CDMA system is of variable length, with 64 Welsh codes, 128 Welsh codes, and 256 Welsh codes. Below we will use 64 Welsh codes as an example to illustrate the CDMA wireless transmission channel. Figure 1 is a physical channel structure diagram of a CDMA cellular system with 64 Welsh codes as an example, including 1 pilot channel, 1 synchronization channel, 7 paging channels, and 55 traffic channels. 2.1 Pilot channel The pilot channel transmits continuous pilot signals sent by the base station. The pilot signal is a non-modulated direct sequence spread spectrum signal that allows the mobile station to quickly and accurately capture the timing information of the channel and extract coherent carriers to demodulate the signal. The mobile station can determine when it needs to perform handover by detecting and comparing the pilot signals of different base stations around it. 2.2 Synchronization channel The synchronization channel mainly transmits synchronization information. During synchronization, the mobile station uses this synchronization information to perform synchronization adjustment. Once the synchronization is completed, it is usually no longer used. When there are many communication services, it can be temporarily changed to a service channel. 2.3 Paging channel The paging channel transmits information for paging the mobile station during the call connection phase. The mobile station usually selects a paging channel (which can also be designated by the base station) after listening to the system to monitor the paging information and other commands sent by the system. When there are many communication services, it can also be changed to a service channel until all are used up. 2.4 Service channel The forward traffic channel has four transmission rates (9600 bit / s, 4800 bit / s, 2400 bit / s, and 1200 bit / s), which can be changed frame by frame to dynamically adapt to the communicator's voice characteristics. 3. Launch performance index Through the description of the communication principles and wireless channels in the first two parts, we have an overall understanding of the CDMA communication process and the physical layer structure of the wireless interface. Next, we will combine the actual work to explain the CDMA base station transmission performance from specific indicators. The test is based on YD / T 1047-2000 "800MHz CDMA Digital Cellular Mobile Communication Network Equipment General Test Specification: Base Station Part". The main test contents of CDMA base stations include: RF output total power, pilot power, code domain power, occupied bandwidth, frequency Tolerance, waveform quality, conducted spurious emission and other emission performance indicators. 3.1 Total RF output power (1) Definition: refers to the total transmit power of the base station under full load. (2) Significance: Power characteristics are the basic characteristics of base stations. (3) Impact on the network: If the total power is too low, the anti-interference ability of the adjacent base station will be reduced, and if the total power is too high, it will cause interference to other channels. (4) Requirements: between +2 dB and -4 dB of the rated power specified by the manufacturer. 3.2 Pilot power (1) Definition: ratio of pilot channel power to total power. (2) Significance: Determine the coverage of the base station to the mobile station. (3) Impact on the network: When the pilot power seriously deviates from the ideal value, it will affect the coverage characteristics of the network. (4) Requirements: within ± 0.5 dB of the configured value. 3.3 Code domain power (1) Definition: The power of each Welsh code, the base station detection mainly refers to the power of each inactive Welsh code. (2) Significance: The ability of the base station to accurately control the power of each Welsh code is a prerequisite for handling vulnerable RF multi-user links to ensure interference-free transmission. (3) Impact on the network: The deviation of the code domain power from the standard value will affect the communication quality. (4) Requirements: The power of each inactive channel should be 32 dB or lower than the total output power. 3.4 Occupied bandwidth (1) Definition: The average power transmitted below the lower frequency limit and above the upper frequency limit of this band is equal to the specified percentage of the total average power of a given transmission β / 2, generally the value of β / 2 should be taken as 0.05% . (2) Significance: Occupied bandwidth is a basic characteristic of a base station. (3) Impact on the network: If the occupied bandwidth is too narrow, the information will be distorted, and if the occupied bandwidth is too wide, adjacent channel interference will occur. (4) Requirements: meet the frequency template, currently less than 2 MHz. 3.5 Frequency tolerance (1) Definition: refers to the maximum allowable difference between the actual CDMA transmission carrier frequency and the specified CDMA transmission carrier frequency. In actual measurement, for ease of representation, it can also be expressed by the ratio of the maximum allowable difference to the specified CDMA transmission carrier frequency. The ratio is a dimensionless value, expressed as "ppm". If the actual frequency is represented by f, and the specified frequency is represented by f0, it can be represented by (︳f-f0︳ / f0) & TImes; 106ppm. The unit of frequency is Hertz. (2) Significance: To ensure that the transmission channel does not cause interference to adjacent channels. (3) Impact on the network: If the frequency tolerance exceeds the allowable value, it will cause interference to adjacent channels. (4) Requirements: The maximum allowable difference between the actual CDMA transmission carrier frequency and the specified CDMA transmission carrier frequency <± 5 & TImes; 108 of the specified frequency; or (︳f-f0︳ / f0) <± 0.05 ppm. 3.6 Waveform quality Ï (1) Definition: Ï is a correlation coefficient, which is the ratio of the actual transmitted signal power distribution to the ideal CDMA signal power distribution. (2) Significance: Ï indicates how close the actual transmitted signal is to the ideal CDMA signal power distribution, and the quality of the modulation can be judged. The closer the value of Ï to 1, the closer the actual signal to the ideal signal. (3) Impact on the network: Poor waveform quality will reduce system efficiency and communication quality. (4) Requirements: Ï> 0.912. 3.7 Conducted spurious emissions (1) Definition: transmission at the out-of-band frequency assigned to the CDMA frequency. (2) Significance: Examine the CDMA signal radiation outside the channel. (3) Impact on the network: Excessive spurs will cause interference to other CDMA channels or other communication frequency bands. (4) Requirements: â‘ Spurious emissions in the cellular band Within the bandwidth of 869 MHz to 894 MHz and the resolution bandwidth of 30 kHz, the ratio of the measured total spurious emission power level to the average output power level should meet the following requirements: (A) For frequency deviation from the center frequency of CDMA channel> 750 kHz, the power ratio ≤-45 dB (B) For a frequency deviation from the center frequency of the CDMA channel> 1.98 MHz, the power ratio ≤-60 dB â‘¡ Spurious emissions outside the cellular band For frequencies other than 869 MHz to 894 MHz, the spurious emission specifications are shown in Table 1.
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