數位調變之向量誤差改善

Abstract

本論文中主要在探討不同的數位調變下訊號由PA放大發送後，信號失真的程度有何不同。分析其中代表信號品質的參數:向量誤差量(Error Vector Magnitude)，發現在此實驗中:振幅誤差(Magnitude Error)是主要影響EVM的成因，另外，在IEEE 802.11b for WLAN的standard包含1M/2M/5.5M/11M bps (BPSK/DQPSK/OQPSK/QPSK)等四種不同的調變方式，不同的調變下所得到的EVM performance有所不同，於是我們將從編碼方式來找出造成EVM在不同調變下有所差別的原因。

為了改善信號隨著Power愈大，信號品質劣化的情形，本論文設計了一個補償平台，將向量信號分析儀(Vector Signal Analyzer)解調得到的參數紀錄之後，建立成一LUT(Lookup Table)，透過自動儀器控制，將需要LUT中補償的值，反向送回原來發送端，藉以改善原來較差的信號，發現改善程度可達15~29%，且不需要耗費複雜的closed-loop 運算。

本論文使用的PA是一般巿面上可取得的IC，型號SST12LP14A，一般增益為29dB，操作頻率為 2.4G~2.5GHz，在802.11b模式下，Output Power=23dBm時，耗電流240mA，效率25%。

The paper treat the result of signal distortion by different modulated signal after Power Amplifier. We analyze the parameters, EVM (Error Vector Magnitude) and find Magnitude error is the dominant factor. By the way, IEEE 802.11b for WLAN includes 4 kinds of digital modulation such as 1M/2M/5.5M/11M bps , BPSK/DQPSK/OQPSK/QPSK. Each modulation has different EVM performance. Then we compare the difference by different encoding of 802.11b modulation.

To improve the worse signal performance with larger Output Power, we design a compensated system which can de-modulate parameter and record data to LUT(Look-up table).We transmit compensated value in LUT to the data source by automatically controlling instruments and improve signal quality. The EVM performance can be improved 15~29% and doesn’t need any closed-loop algorithm.

The Power Amplifier Sample is normal IC , which can be get easily, Part No.SST12LP14A,typical gain=29dB,operating frequency= 2.4G~2.5GHz Current consumption=240mA, efficiency=25%.