Analysis of sdr technology principle
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**Software Radio (SDR) Concept**
Software Radio, also known as Software Defined Radio (SDR), is a communication technology that uses a flexible hardware platform to implement various communication functions through software. This approach allows for greater adaptability and reconfigurability compared to traditional radio systems.
The two main elements of SDR are the "Universal Hardware Platform" and "Software." A universal hardware platform means that the same physical device can support multiple communication protocols and functions, rather than being limited to a single application. The "software" aspect refers to the use of programming to define and control the communication system, which contrasts with traditional radios that rely on fixed hardware circuits. In conventional systems, each function requires dedicated hardware, leading to longer development cycles, higher costs, and limited flexibility. With SDR, developers can modify or add features quickly, reducing both time and cost.
To better understand the difference between traditional and SDR-based base stations, we can look at Figure 1. On the left, a traditional large base station is shown, filled with numerous custom-designed hardware components. On the right, an SDR-based miniaturized base station is presented, where most of the communication functions are implemented through software running on a general-purpose processor.
SDR is often considered the third major revolution in communications, following the transition from wired to wireless in 1G and from analog to digital in 2G. It represents the future of communication systems due to its flexibility, scalability, and cost-effectiveness.
**Principle of SDR System**
Now let’s explore how an SDR system works. A typical GPP-based SDR system includes a general-purpose processor (GPP) and peripheral hardware. For example, using a laptop connected to a USRP B200, we can see how the internal components operate.
**Transmitter**
In the transmitter section (see Figure 2), the SDR program running on the laptop handles the entire communication protocol stack, such as LTE or WiFi. These programs are usually written in C or C++ due to their real-time performance requirements. The SDR program processes baseband data, which is then sent to the USRP B200 via a USB 3.0 interface.
The USRP B200 contains an FPGA that manages the digital up-conversion (DUC) and transmit control modules. The DUC converts the baseband signal to an intermediate frequency (IF), which is then converted to an analog signal by the DAC. After filtering, the signal is modulated to the desired RF frequency and amplified before transmission.
**Receiver**
On the receiving side (see Figure 3), the signal first goes through a low-noise amplifier (LNA) to reduce noise. It is then down-converted to IF using a local oscillator, filtered, and digitized by the ADC. The digital signal is processed by the FPGA, which performs digital down-conversion (DDC) to bring the signal back to baseband. Finally, the data is sent to the laptop for further processing.
**Why Two Conversions?**
As shown in Figure 4, the signal undergoes two frequency conversions: first to IF, then to baseband. This is necessary because high-frequency RF signals cannot be directly sampled by ADCs due to Nyquist constraints. By converting to IF first, the system ensures that the sampling rate remains feasible. While zero-IF receivers exist, they are typically used in lower-frequency applications due to design complexity.
SDR is a powerful and evolving field, offering endless possibilities for innovation in wireless communication. Understanding its principles opens the door to advanced research and development in this exciting domain.