摘要
2010年全球移动用户数量突破50亿,同时,移动用户对业务的需求日趋多样化。移动通信系统需要具备更大的系统容量和更高的数据速率以满足众多移动用户的需求。第三代移动通信系统采用非恒包络调制方式及多载波技术,增大了系统容量,提高了数据速率,但也增大了信号的峰均比。
为了线性放大具有高PAPR的信号,功率放大器必须工作在效率低下的功率回退区域,这不仅造成能源浪费,而且耗散在功率放大器上的大量热能,严重影响移动通信系统的稳定性及功率放大器的寿命。因此,提高功率放大器的效率不仅可以节能减排、保护环境,而且能提高运营商的竞争优势。
在众多提高功率放大器效率的技术中,多合体(Doherty)放大器具有电路结构简单、成本低、效率提升明显等优点,结合数字预失真技术,其线性度可得到较大改善,因而成为业界的首选方案。
本文首先讨论了高效率功率放大器的关键技术,详细分析了Doherty功率放大器的工作过程,概述了Doherty功率放大器的技术现状。针对经典Doherty功率放大器上下两路不能同时饱和的缺陷,找到解决方法,即采用不等功率输入的非对称Doherty电路,然后在ADS软件平台上进行电路仿真,在Altium Designer软件平台上完成印制电路板设计。最后对不等功率输入的非对称Doherty功率放大器进行调试,并完成了测试。
TD-SCDMA 9载波测试信号下,平均输出功率43dBm时,功率放大器功率附加效率达到41.1%,数字预失真纠正后,邻道ACLR为–44.3dBc,EVM为11.5%。测试结果表明,非对称Doherty功率放大器具有较高的效率优势,与数字预失真技术结合后,能满足TD-SCDMA系统对信号线性度的要求,可应用于TD-SCDMA系统。
The amount of global mobile subscribers exceeded 5 billion in 2010; meanwhile, the business needs of mobile users are increasingly diversified. Greater capacity and higher data rates are required for the mobile communication systems to meet the requirements of subscribers. Non-constant envelope modulation and multi-carrier technique are adopted in third generation mobile communication systems (3G), enlarging system capacity, increasing data rates, and also resulting in higher peak to average power ratio (PAPR).
The power amplifier should operate in power back-off region to linearly amplify signals with high PAPR, resulting in low efficiency, which not only cause energy waste, and large numbers of thermal energy dissipation on the power amplifier, seriously affected the life of the stability and power amplifiers for mobile communication systems. Therefore, improvement of the efficiency of power amplifier is helpful to energy-saving, emission reduction and protection of the environment, and can improve the operator's competitive advantage.
Among numerous techniques of efficiency enhancement, Doherty is widely accepted and adopted owing to its simple circuit structure, low cost, and excellent performance. Besides, linearity of Doherty power amplifier can be greatly improved with combination of digital pre-distortion (DPD).
In this article, key techniques of high efficiency PA are first illustrated, then detailed analysis on operation of Doherty amplifier is given, and current research achievements in Doherty PA are reviewed. Uneven structure is employed to overcome drawbacks of classical Doherty PA. Circuit simulation and design of PCB are fulfilled by means of ADS and Altium Designer respectively. Finally, the uneven Doherty PA is debugged and tested.
The power added efficiency is 41.1% at 43dBm average output power, tested under TD-SCDMA 9 carriers signal, ACLR and EVM are improved to–44.3dBc and 11.5% respectively after linearization of DPD. It’s shown that, uneven Doherty PA is highly efficient, and applicable to TD-SCDMA system with the combination of digital pre-distortion technique.
引文
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