Development and Challenges of Software-Defined Radio (SDR)
With the evolution of mobile communications, the concept of Software Radio began gaining widespread attention in the early 1990s. Due to the coexistence of multiple digital wireless communication standards - such as GSM and CDMA-IS95 - each standard imposed different requirements on mobile devices, making interoperability between different systems impossible. To address this issue, the Software Radio approach proposed sampling, quantizing, and digitizing all radio signals across the 2 MHz to 2000 MHz spectrum, then processing them via software. In other words, it aimed to receive and digitally process all possible wireless communication signals, enabling communication with base stations of any wireless standard. Theoretically, a Software Radio-based mobile device could be compatible with any wireless communication system.
Although Software Radio held great theoretical promise, its practical implementation required extremely high-speed hardware and software processing capabilities. Due to limitations in hardware technology at the time, a pure Software Radio system has not been widely adopted in commercial products to this day. However, Software-Defined Radio (SDR), a technology inspired by the Software Radio concept, has attracted increasing attention. At the ITU-8F conference in October 2001, SDR was recommended as a highly promising direction for the future of wireless communications.
SDR is a system and architecture that must possess reprogrammable and reconfigurable capabilities, enabling devices to support multiple standards, frequency bands, and functions. It not only employs programmable components for baseband digital signal processing but also allows programming and reconfiguration of RF and intermediate frequency (IF) analog circuits. Desired features of SDR include:
- Reprogrammability and reconfigurability
- Ability to provide and modify services
- Support for multiple standards
- Intelligent spectrum utilization
It is important to note that SDR is not an isolated technology but rather a common platform that can be utilized by various technologies. A key distinction between SDR and pure Software Radio is that SDR does not require receiving the entire frequency band (2 MHz–2000 MHz). Instead, it manually or automatically scans frequency bands one by one to identify the most suitable frequency and standard for communication.
As is well known, IMT-2000 (3G) did not achieve its original goal of a unified global standard. Instead, it fragmented into multiple standards, including Europe’s WCDMA, North America’s cdma2000, and China’s TD-SCDMA. This diversity raised challenges related to roaming and compatibility across different standards. Moreover, to ensure a smooth transition from 2G to 3G, ideal 3G devices should also support GSM and CDMA-IS95. By adopting SDR technology, a universal software platform could manually or automatically search through possible frequency bands and modes, process received digital signals with tailored software solutions, and select and switch to the most suitable frequency and standard for communication. This approach would enable full compatibility across all modes, offering significant advantages.
However, achieving the goals of SDR presents considerable challenges, including:
- Architectural design
- Broadband programmable and configurable RF/IF technologies
When implementing different wireless communication standards using SDR, TD-SCDMA is particularly well-suited for integration with SDR due to its inherent characteristics. It is the only standard that explicitly incorporates smart antennas and high-speed digital modulation techniques into its design and intends to be implemented using Software Radio technology. Moreover, implementing TD-SCDMA with SDR is relatively straightforward:
1. Narrow Bandwidth per Frequency Band: The signal processing requirements are manageable, making it easier to implement on a software platform without needing extremely high-speed hardware. This simplifies migration to an SDR-based solution.
2. Synchronous Transmission: Both uplink and downlink signals in TD-SCDMA use synchronous transmission, enabling the use of simpler coherent demodulation schemes rather than complex non-coherent demodulation. This reduces software processing demands and facilitates implementation.
