Wcdma Uygulamaları İçin Yüksek Doğrusallıklı Güç Kuvvetlendiricisi Tasarımı

Abstract

Sayısal önceden bozma yöntemi kullanılarak 3. nesil iletişim sistemleri baz istasyonlarında kullanılmak üzere yüksek güçlü güç kuvvetlendiricisi tasarlanmıştır. Doherty tekniği ile çalışan çıkış katı kullanılarak yüksek verim, sayısal predistorsiyon yöntemi ile de yüksek doğrusallık elde edilmiştir. Sistem güç kazancını yükseltmek amacıyla sisteme ön iki kazanç katı daha eklenmiştir. Sistemin kazancı 36 dB, P1dB değeri 45 dBm olarak ölçülmüştür. Sistem WCDMA uygulamalarında kullanılan 2.11 – 2.17 GHz sıklık bandında çalışmaktadır. Sayısal predistorsiyon yöntemi ile sistemin doğrusallığı artırılarak ACLR (Komşu Kanal Güç Oranı) değerleri 1. ve 2. Komşu kanal için sırasıyla -50 dBm ve -55 dBm olarak elde edilmiştir. Sistem bu derece yüksek doğrusallığı sağlamasının yanında güç verimliliği de sunmaktadır. 40 dBm etkin güç değeri için hesaplanan verim değeri %37 olmuştur. Elde edilen verim ve ACLR değerleri modern haberleşme sistemlerinde kullanılabilecek standartlara ulaştığının bir göstergesi niteliğindedir. Tez çalışması kapsamında anlatılan sistem, 3. Nesil iletişim altyapısının ihtiyaç duyduğu özellikleri karşılamak amacıyla tasarlanmış ve gerçeklenmiştir. Sistem temel olarak arka arkaya bağlanmış üç kazanç katı ve doğrusallaştırma işlevini yerine getiren DSP tabanlı yongadan oluşmaktadır. Doğrusallaştırıcı yonga üzerinde sayısal önceden bozma yöntemi uygulanarak tasarlanan sistemin ACLR ve ara-kiplenim bozulması (IMD) performansı artırılmış ve WCDMA uygulamalarında kullanılan 2.11 – 2.17 GHz sıklık bandı ve 5 MHz bant-genişliği için optimize edilmişitir. Sistemin ACLR performansı doğrusallaştırma öncesine göre 20 – 25 dB düzeyinde artırılmıştır. Kullanılan kaskat bağlı ön kat kuvvetlendiriciler için empedans uyumlama devreleri tasarlanmış ve AWR yazılım ortamında benzetimleri yapılmıştır. Sistemin çıkışına da izolatör konularak sistem çıkışında karakteristik empedans olan 50 Ohm sonlandırma olmadığı durumlarda sistemin zarar görmesi önlenmiştir. Bu yönüyle, tasarlanan güç kuvvetlendiricisi sistemi pek çok özelliği bünyesinde barındıran bir uygulama özelliği taşımaktadır. Doğrusallaştırma işleminde kullanılan yonga için giriş işaretinden ve sistem çıkış işaretinden kuplörler aracılığıyla alınan örnekler, empedans uyumlama devreleri ile çalışma sıklığında en yüksek performans gösterecek şekilde tasarlanmıştır. Giriş ve çıkış işaretinden alınan örnekler sayısallaştırılarak çıkışta meydana gelen distorsiyonun giderilmesi için uygun genlik ve evrede düzeltme işareti oluşturularak sistem girişine yeniden uygulanmış ve çıkış işareti doğrusallaştırılmıştır.

A high power amplifier is designed for the 3rd generation base station transmitters by using digital predistortion technique. High power added efficiency has been obtained thanks to Doherty GaN output stage amplifier, and the linearity of the system has been increased using digital predistortion technique. Three-stage amplifier was used to obtain high power gain. The power gain of the system is 36 dB and 1 dB compression point measured is 45 dBm. Operating frequency band of the system is 2.11 – 2.17 GHz that is used for WCDMA (Wideband Code-Division Multiple Access) applications. Designed power amplifier with digital predistortion technique exhibits an ACLR (Adjacent Channel Leakage Ratio) performance of -50 dBc and -55 dBc for the first and the second channel, respectively. In addition to the high linearity feature, designed power amplifier system offers high power added efficiency, which is another critical design goal. Power added efficiency of the power amplifier system is 37% for the 40 dBm output power. Power amplifier linearity and efficiency have always been counter opposing goals in wireless communication systems. At this point, linearization techniques, that do not sacrifice efficiency, have become critical in order to improve linearity. Digital predistortion has recently been employed in wireless base stations and shown to improve linearity of power amplifiers. Advances in digital predistortion techniques have benefited by the improvements in digital signal processing algorithms. Predistortion is simply the process of modifying an input signal such that the distortion imposed by a nonlinear gain function. When the nonlinear gain function is only a function of the instantaneous input amplitude, the system is said to be memoryless. In this case, predistortion function must be the polynomial inverse of the nonlinear gain function. Unfortunately, RF power amplifiers rarely exhibit memoryless system behavior. The next level complexity must be considered when the phase distortion is available. As the signal bandwidth gets wider, such as that in WCDMA, power amplifiers begin to show memory effects. This is especially true for the high power amplifiers used in base stations. The causes of the memory effects of the memory effects can be attributed to thermal constants of the active devices in biasing network that have frequency dependent behavior. As a result, the current output of the power amplifier depends on not only the current inputs, but also the past input values. The power amplifier becomes a nonlinear system with memory effect. Amplitude response (AM/AM) of the amplifier is one of the key characteristics to determine the correctibility of a power amplifier. A highly linear power amplifier, in example operated in backoff, exhibits a straight line characteristic. This means that every 1 dB increase in input power results in 1 dB increase in output power. In addition, the power amplifier presents very little nonlinear distortion as long as the power amplifier is not operating in compression. However, this type of power amplifiers is not commonly used in wireless infrastructure equipment due to its very poor efficiency. Class AB amplifiers typically have a monotonic gain response that means the gain slowly decreases as the output power approaches to the saturation point. Advanced power amplifier architectures such as Doherty create much more complicated AM/AM responses. On the other hand, Doherty power amplifiers exhibit efficiency advantages. In this application, a Doherty GaN power amplifier was used at the output stage to achieve high efficiency. To reduce the nonlinearity, the power amplifier can be backed off to operate within the linear portion of its curve. However, newer transmission formats, such as WCDMA and orthogonal frequency division multiplexing (OFDM), have high peak to average power ratios meaning large fluctuations in their signal envelopes. This means that the power amplifier needs to be backed off far away from its saturation point, resulting in very low efficiency. Considering the number of base stations in a mobile system, improved power amplifier efficiency can reduce the power consumption significantly. To improve the power amplifier efficiency without compromising its linearity, power amplifier linearization is essential. One of the most important measurements on radio frequency signals for digital communication systems is the leakage power in adjacent channels. Leakage power influences the system capacity as it interferes with the transmission in adjacent channels. Therefore, it must be rigorously controlled to guarantee communication for all subscribers in a network. This leads to implement ACLR measurements strictly for the power amplifiers. ACLR is the logical extension of the distortion measurement except that the two tones replaced by the modulated signal. The 3rd order IMD product is usually defined as the ratio of the power in one tone of third-order tones to that in one of the main tones. ACLR is defined as the ratio of power in a bandwidth away from the main signal to the power in a bandwidth within the main signal. The IMD technique tests system’s nonlinear distortion by stimulating with multiple sinusoidal tones in the main channel and measuring the peak power ratio of the main channel to the adjacent channel. ACLR is an improvement on the IMD technique. ACLR technique stimulates with a signal specific to the usage application, for example WCDMA. The system described in this thesis was designed and implemented in order to meet the requirements of 3rd generation communication systems. Basically, the power amplifier system consists of three stage gain block and DSP-based linearizer chip. Adjacent Channel Leakage Ratio (ACLR) and Intermodulation Distortion (IMD) performance of the system has been increased by using digital predistortion technique that is applied on the linearizer chip. The system has been optimized for WCDMA frequency band and bandwidth, which are 2.11 – 2.17 GHz and 5 MHz bandwidth. ACLR performance of the system was increased by 20 -25 dB with respect to that without digital predistortion. Impedance matching circuits was designed for the first two stage cascaded amplifiers and simulated AWR software environment. Isolator placed at the output of the power amplifier prevents the system in the absence of proper termination. Samples are taken from input and output of the system through the couplers in order to be processed in digital predistortion construction. Linearizer produces correction signal and injects it to the input of the amplifiers to eliminate the distortions. Impedance matching circuits are also designed for these inputs and the correction output of the linearizer. Among all linearization techniques, digital predistortion is one of the most cost effective techniques. It adds a digital predistorter in the baseband to create a nonlinearity that is complementary to the compressing characteristics of the power amplifier. Ideally, the cascade of the predistorter and the power amplifier becomes linear and the original input is amplified by a constant gain. With the predistorter, the power amplifier can be utilized up to its saturation point while still maintaining a good linearity and high efficiency that are tough design parameters in order to achieve.