With the rapid development of the communication industry, the volume of information being transmitted has significantly increased. Whether in public or private communication networks, traffic is growing at an unprecedented rate. Traditional infrared and optical systems have reached their limits, while the microwave spectrum is already heavily congested. As a result, millimeter-wave communication has emerged as a promising solution due to its unique advantages.
Millimeter waves have a short wavelength, which allows for smaller, lighter, and more power-efficient devices. These characteristics make them highly maneuverable and ideal for various applications. With the same antenna aperture, millimeter waves can produce narrow beams with low side lobes, enabling precise target tracking and recognition. Their high resolution and narrow beam width also enhance system stealth and resistance to interference. To achieve maximum flexibility, a software-defined radio (SDR) platform can be used to create a universal millimeter-wave hardware system. This requires all components to be programmable, flexible, and miniaturized, ensuring openness, digitization, standardization, and programmability. Digital up-conversion and down-conversion are key technologies in this process. This paper introduces a two-step frequency conversion method for the millimeter-wave transmitter, using Altera's Cyclone series EP1C12F324 to handle baseband digital signal processing, control the AD9857, and implement functions such as interpolation filtering, quadrature modulation, and D/A conversion, ultimately achieving QDPSK modulation on a 70 MHz IF carrier.
The transmitter is a critical component in millimeter-wave communication systems. It processes the modulated signal through frequency conversion, amplification, and filtering before sending it to the antenna feed system, either directly to the communication partner or through a relay station. The frequency conversion schemes for transmitters can be divided into direct conversion and two-step conversion methods. While direct conversion integrates modulation and up-conversion into a single circuit, it suffers from limitations in output power, dynamic range, and harmonic suppression. In contrast, the two-step conversion method separates modulation and up-conversion, first modulating at a lower intermediate frequency and then converting it to a higher millimeter-wave frequency. This approach improves system adaptability, frequency flexibility, and anti-jamming performance by effectively suppressing harmonics and intermodulation distortion.
This design uses a two-step frequency conversion method to upconvert a 70 MHz signal to 31 GHz. Due to the challenges of extracting the output signal after power amplification, filters are not practical in this case. Therefore, the intermediate frequency signal is first modulated and then upconverted to the millimeter-wave band. The design is illustrated in Figure 1.
In Figure 1, the baseband signal is first IF-modulated to produce a 70 MHz intermediate frequency signal. This signal is then mixed with a 2.93 GHz local oscillator, filtered, and converted to 3 GHz. Next, it is mixed with a 29 GHz signal to reach the final 31 GHz output. A bandpass filter suppresses sideband noise and interference from the mixer, while a radio frequency amplifier compensates for signal loss during mixing. For the first local oscillator, a phase-locked loop method ensures high frequency stability, low phase noise, and fine frequency resolution. For the second local oscillator, operating near the millimeter-wave band, a microwave phase-locked loop followed by frequency multiplication is used.
When selecting a suitable modulation scheme for millimeter-wave communication, several factors must be considered. Millimeter-wave channels are typically nonlinear, and non-constant envelope or multi-carrier signals can cause spectral spreading or crosstalk. This leads to in-band distortion that affects modulation accuracy and increases bit error rates, while out-of-band distortion interferes with adjacent channels. Additionally, due to the high cost and limited power of millimeter-wave power amplifiers, the channel is power-limited, requiring coherent demodulation at the receiver end.
Common modulation techniques include ASK, FSK, and PSK. Among these, PSK offers the best noise immunity, followed by DPSK, with FSK performing worse. However, PSK is prone to "phase ambiguity." In terms of bandwidth efficiency, PSK and ASK are more efficient than FSK. Based on these considerations, QDPSK is chosen for this design because it offers higher spectral efficiency than BPSK, better noise immunity than 8PSK and 16QAM, and is relatively simple to implement with lower costs.
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