利用階層式調變訊號偵測正交分頻多工系統之載波頻率飄移

Abstract

中文摘要 正交分頻多工技術 ( Orthogonal Frequency-Division Multiplexing, OFDM )可以在有限的頻寬內增加資料的傳輸量，其利用多個相互正交的子載波在有限的頻寬內傳送資料，且具備抵抗多重路徑、符號之間干擾 ( Inter-Symbol Interference, ISI )的優點，但是其對子載波之間的干擾 ( Inter-Carrier Interference, ICI )非常敏感 ，都卜勒效應會使子載波之間失去正交性，本篇論文使用階層式調變 ( Hierarchical modulation )技術估計都卜勒效應造成的載波頻率飄移 ( Carrier Frequency Offset, CFO )。 階層式調變最初的目的是為了在傳輸資料時，針對不同重要性的資料在作調變時給予不同的錯誤率保護程度。本論文將正交分頻多工技術中的調變改成階層式調變，利用階層式調變每一個階級的位元錯誤率在不同的載波頻率飄移下各不相同的特性來估計都卜勒效應的程度。 本篇論文提出兩種估計載波頻率飄移的設計，第一種是將領航訊號( pilot signal)作階層式調變，接收端利用各階層位元錯誤率在不同載波頻率飄移程度下有不同錯誤率的特性估計載波頻率飄移，模擬結果發現，實際的位元錯誤率會高於數學分析的各階層位元錯誤率，導致系統高估載波頻率飄移程度。第二種設計是將資料訊號作階層式調變，為了盡可能同時滿足資料訊號的服務品質以及估計載波頻率飄移，針對通道品質好壞不同的情況下，設計不同參數的階層式調變以及相對的載波頻率飄移判斷機制，目標是能夠更即時的估計都卜勒效應，並且在滿足資料訊號服務品質的前提下，能夠對都卜勒效應程度作粗略的估計。

ABSTRACT As a result of its high spectral efficiency and multipath immunity, orthogonal frequency-division multiplexing(OFDM ) has considered one of the major modulation schemes for high-speed communication systems. OFDM systems are expected to operate at high carrier frequencies, high speed, and high throughput for mobile users. However, it is well known that OFDM systems are very sensitive to local oscillator carrier-frequency offset, local oscillator phase noise, time synchronization error, and Doppler frequency spread, since these factors can result in the loss of the orthogonality between sub-carriers and create inter-carrier interference (ICI). In this essay, we exploit hierarchical 64-QAM modulations for acquiring the information of carrier frequency offset. Hierarchical constellations offer a different degree of protection to the transmitted messages according to their relative importance. In this thesis, it is observed that the bit error rate and error floor of each level caused by additive white Gaussian noise (AWGN) and inter carrier interference (ICI) are different. Therefore, we estimate the degree of Doppler effect by applying the hierarchical modulations. In this thesis, we proposed two novel designs for the carrier frequency offset (CFO) estimation. In the first design, we exploit the hierarchical 64-QAM to modulate the pilot signals at transmitter in the OFDM system. Based on the factor of different error floor level at different carrier frequency offset among each level bit, the carrier frequency offset resulting from the Doppler effect can be estimated at the receiver. The simulation results show that the BER of each level is higher than the BER of theoretical analysis. This result makes the estimation of CFO is higher than accurate CFO. In the second design, we use the hierarchical 64-QAM modulation to modulate data signals at the transmitter. It is because data signals can estimate CFO more promptly than pilot signals. Considering the tradeoff between quality of service(QOS) of data signals and information of carrier frequency offset, we design four hierarchical 64-QAM modulations in accordance with the different channel qualities. We expect that this design not only can estimate carrier frequency offset (CFO) when the channel is bad, but satisfies the quality of service.