With the development of communication industry, the amount of information transmission is increasing. Regardless of the public communication network or the dedicated communication network, the communication traffic is growing rapidly. The infrared and optical systems have been limited, and the microwave spectrum is already very crowded. Millimeter wave communication has been widely used in various fields due to its unique advantages.
The millimeter wave has a short wavelength, and its device is small in size, light in weight, low in power consumption, and good in maneuverability. Under the same aperture antenna, the short-wavelength millimeter wave can realize narrow beam and low side lobes, thus providing target tracking and recognition. Extremely high accuracy and good resolution, while narrow beams also improve the system's concealment and anti-interference ability. It can be systemized by building a millimeter-wave universal hardware platform based on software radio principle, and the millimeter-wave hardware platform based on software radio principle requires the components of the system to be programmable, flexible and miniaturized. To maximize the openness, digitization, standardization and programmability of the hardware platform. Digital up-conversion and down-conversion technology are key technologies for building a millimeter-wave universal hardware platform. Based on this, this paper presents a two-frequency conversion method for the millimeter wave transmitting end up-conversion scheme, and uses Altera's Cyclone series EP1Cl2F324 to complete the baseband digital signal processing, realize the control of AD9-857, and complete the baseband digital signal in the digital domain. The functions of interpolation filtering, quadrature modulation, D/A conversion, etc., enable QDPSK modulation on a 70 MHz IF carrier.
1 mm wave transmitterThe transmitter is an important component of the millimeter wave communication equipment. Its function is to transmit the modulated wave to the antenna feeder system after being processed by frequency conversion, amplification, filtering, etc., and send it to the communication partner or the relay relay station. The frequency conversion scheme of the transmitter can be divided into two types: direct conversion method and two-step conversion method. The direct conversion method combines modulation and up-conversion into one circuit and completes it in one circuit. The two-step conversion method separates the modulation from the up-conversion, first modulates at a lower intermediate frequency, and then up-converts the modulated signal to Higher carrier frequency (millimeter wave frequency).
Although the direct conversion method is simple, due to its power limitation, the circuit cannot effectively provide sufficient output power and a large dynamic range, and other harmonic levels will be much higher than the required signal, for the filter and amplifier. The requirements are also very high. The two-step transform method can attenuate the shortcomings of the direct conversion method, and has strong adaptability to the carrier, good frequency flexibility, and reasonable frequency configuration can effectively suppress harmonics and intermodulation components generated in various spurious and frequency conversion processes, and improve The anti-jamming performance of the system. This scheme adopts a two-step transformation method, and
The system works in the millimeter wave band, and its operating frequency is relatively high, using two or more frequency conversion schemes.
This design is to upconvert the 70 MHz signal to the 31 GHz output, considering that the strong transmit signal leakage after power amplification will affect the transmitter performance index, and it is very difficult and expensive to use the filter to extract the output signal. Therefore, the two-frequency conversion method is adopted, and the intermediate frequency signal is modulated and then upconverted to the millimeter wave frequency band. The design scheme is shown in Figure 1.
In Figure 1, the baseband signal is IF-modulated to obtain an intermediate frequency signal of 70 MHz. The intermediate frequency signal is mixed with 2.93 GHz by intermediate frequency amplification and low-pass filtering to obtain 3 GHz, and then mixed with 3 GHz and 29 GHz to obtain 31 GHz. That is, the frequency is upconverted to the millimeter wave band. The bandpass filter is used to suppress the sideband noise and the interference caused by the multiplier, and the radio frequency amplifier is used to compensate the multiplication loss. For the first local oscillator to obtain higher frequency stability, phase noise index and frequency resolution, the mixing phase lock method can be used. For the second local oscillator, since its frequency reaches 29 GHz, close to the millimeter wave band, it can be realized by microwave phase locking and then multiplying.
2 IF modulation mode selectionThe millimeter wave channel is generally a non-linear channel. Mainly based on digital constant envelope modulation, when the non-constant envelope modulation signal or multi-carrier signal passes through the millimeter-wave nonlinear channel, it will cause spectrum spread or generate cross-talk signal. The in-band distortion component interferes with the modulated signal, causing vector deviation, affecting the modulation accuracy, and increasing the bit error rate during reception demodulation; the out-of-band distortion component will interfere with adjacent channels. At the same time, due to the high cost of the millimeter wave power amplification technology and limited power output, the millimeter wave channel is a power limited type, and a coherent demodulation technique should be adopted at the receiving end. Therefore, when selecting a modulation method suitable for a millimeter wave communication channel, pay attention to the following points:
(1) Pay attention to the matching degree between the system and the system in terms of signal-to-noise ratio. Try to use the modulation method with lower error rate under the same signal-to-noise ratio, and consider the utilization of the frequency band.
(2) To consider the performance degradation of the nonlinear channel, try to use the constant envelope modulation method;
(3) Analyze its anti-fading performance and consider using appropriate measures to compensate.
In the digital communication system, there are three basic modulation methods: ASK, FSK and PSK. Comparing the performance of the commonly used modulation and demodulation methods, it can be concluded that the 2PSK/2DPSK equipment is simple and anti-interference ability in terms of modulation mode implementation. Strong, adaptability to fading channels and non-linear channels, but spectrum utilization is not high. 2FSK equipment is simple, adaptable to fading channels and non-linear channels, but its spectrum utilization and anti-interference ability are worse than 2PSK/2DPSK. The spectrum utilization of 4PSK/4DPSK is twice that of 2PSK/2DPSK, and the anti-interference ability is the same as the latter. The complexity of the equipment is only slightly increased, the adaptability to the fading channel is moderate, and the linearity requirement of the channel is not too high. Compared with 4PSK/4DPSK, 8PSK has higher frequency utilization, but the complexity of the equipment has increased, and the fading and distortion characteristics of the channel are more sensitive than the latter. Some measures are needed to improve the performance.
In terms of noise immunity, PSK has the best performance, followed by DPSK, and the third is FSK, while ASK has the worst performance. However, although the performance of the PSK system is superior to the DPSK system, it is prone to "phase ambiguity". From the perspective of system bandwidth utilization, PSK and ASK occupy a narrower channel bandwidth than FSK, that is, PSK and ASK are more effective, so PSK is all binary keying methods from the perspective of anti-noise performance and improved channel bandwidth utilization. The best one.
Through the above analysis, since the spectrum utilization rate of QDPSK is higher than that of BPSK, and the anti-noise performance is higher than 8PSK, 16QAM, etc., and the engineering implementation is simple and the cost is low, the design adopts QDPSK modulation mode.
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