带通采样数字相干接收机：自由运行本地振荡器在相位调制光纤传输中的应用

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"这篇研究论文探讨了一种新颖的基于光纤的无线电（RoF）数字接收机架构，适用于相干异频检测。该架构采用带通采样技术，旨在解决传统接收机高采样率需求和前端硬件依赖的问题。特别是，研究人员针对独立自由运行的光学本地振荡器产生的频率偏移，修改了均匀带通采样定理。实验中，他们通过相位调制和数字信号处理辅助的RoF链路，展示了40Mbit/s的QPSK调制数据信号在2.4GHz射频载波频率下的实际演示。本文的核心知识点包括： 1. 数字相干接收机：这种接收机利用数字信号处理技术进行相干解调，能够有效提高光通信系统的灵敏度和误码率性能，尤其是在长距离传输中。 2. 带通采样：带通采样是信号处理的一种方法，它只对信号的特定频带进行采样，而非全带宽。这种方法可以降低对采样率的要求，节省硬件资源，但需要正确处理由于频率偏移导致的问题。 3. 自由运行的本地振荡器激光器：在光纤通信系统中，本地振荡器（LO）用于与接收到的信号进行相干检测。自由运行意味着LO没有外同步，可能会产生频率漂移，这需要特别的补偿技术来保证系统性能。 4. 频率偏移：由于自由运行的LO产生的频率不稳定性，会导致接收信号与LO之间出现频率差，这会影响相干检测的效果。文中提出的方法修改了带通采样定理，以适应这种频率偏移。 5. 相位调制：这是一种光或无线电信号调制方式，通过改变信号的相位来携带信息。在RoF系统中，相位调制可以高效地利用频谱资源。 6. QPSK（四相相移键控）：QPSK是一种常见的数字调制方式，通过改变信号的相位在四个可能的状态之间切换，每个状态代表两个比特的信息，从而实现高数据传输速率。 7. RoF（Radio-over-Fiber）技术：RoF是将无线电信号通过光纤传输的技术，能实现长距离、大容量的无线信号传输，并结合了光纤通信的高带宽和无线通信的灵活性。 8. 数字信号处理（DSP）：在RoF链路中，DSP用于信号的解调、均衡、噪声抑制等，以改善信号质量和恢复原始数据。这篇论文提出了一种创新的数字相干接收机设计，通过带通采样和对自由运行LO频率偏移的处理，降低了系统复杂性，同时在相位调制的RoF系统中实现了高效的数据传输。这一成果对于优化光纤通信系统和未来无线通信网络的性能具有重要意义。"

Bandpass Sampling in Digital Coherent Receiver with

Free-Running Local Oscillator Laser for Phase

Modulated Radio-over-Fiber Systems

Minghua Cao, Jianqiang Li, Kun Xu*, Yitang Dai, Feifei Yin and Jintong Lin

State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and

Telecommunications, Beijing 100876, P. R. China

Email: xukun@bupt.edu.cn

Abstract—We have experimentally presented a novel

radio-over-fiber (RoF) based digital receiver architecture for

coherent heterodyne detection. It demonstrates the application of

using bandpass sampling in a RoF communications system to

overcome the high sampling rate requirement of conventional

receivers and front-end hardware dependency. In particular, we

modified the uniform bandpass sampling theorem to cope with

frequency offset produced by independent free running optical

local oscillator. Thereafter, a 40Mbit/s QPSK modulated data

signal at 2.4GHz RF carrier frequency is experimentally

demonstrated by using phase modulated and digital signal

processing aided RoF links.

Keywords—Radio-over-Fiber; Bandpass Sampling; Coherent

Communications

I. INTRODUCTION

Resent large-scale deployment of broadband wireless

access networks such as WiFi, WiMAX, 3G and LTE/4G

demand high-bit-rate, power-efficient and cost-effective

solutions for high performance fronthaul links over sufficiently

long distances. The use of RoF for the linear transport of radio

frequency (RF) is considered as a flexible approach for

reducing system complexity by using a centralized architecture

that incorporates a simplified antenna located closer to the

customer[1-3].

Most approaches utilized in RoF links employed intensity

modulation with direct detection (IM-DD), which are

inherently non-linear and result in limited dynamic range for

uplink. An alternative approach to IM-DD link is to use optical

phase modulation at the antenna and linear coherent detection

(PM-Coh) at the central unit. Such a phase modulator allows an

electrical signal to vary the phase angle of an optical carrier

with inherent linearity and does not require bias control. In

addition, coherent receiver techniques offer improved receiver

sensitivity by using high-power optical local oscillator (LO)

and higher modulation depths [4, 5].

Fig.1 Direct sampling and digital coherent detection

The general architecture of DSP aided PM-Coh receiver of

interest here represented in Fig.1. This design eliminates the

requirement of using analog mixers for frequency

down-conversion. In addition, by incorporating PM-Coh

technology there remains enough room for enhancing the

performance of RoF transmission systems [6].

In digital signal processing (DSP) aided coherent RoF

receiver, the signal is first down-converted from the optical

carrier to the modulated electrical carrier. The electrical RF

signal is then sampled by ADCs for further down-conversion to

the baseband and subsequent demodulation operations carried

out by DSPs. In practice, when RF carrier working at GHz

frequency band, the required sampling rate for ADCs can be

too high to be achieved if the Nyquist sampling theorem is to

be satisfied [7, 8]. Moreover, it would be a waste to process at

this rate for information only appeared within a relatively

narrow band. Therefore, we propose to employ bandpass

sampling [9] which is a technique that samples high data rate

signals with smaller sampling rate to relax the demand for

ADCs and DSPs. As a result, the amount of memory needed to

capture a given time interval of a continuous signal can be

reduced, thereby decreasing overall computational complexity.

Moreover, bandpass sampling can realize direct frequency

downconversion which can simplify the architecture of the

base stations by eliminating analog mixing devices required.

Unfortunately, in an optical coherent receiver with free

running optical LO, it is well known that frequency offset is

present [10], which complicates bandpass sampling design. In

this paper, we propose and experimentally demonstrate a

complete bandpass-sampling-based digital coherent receiver

with free-running LO laser for cost-efficient PM-Coh RoF

links.

II. P

RACTICAL CONSIDERATION OF APPROPRIATE

SAMPLING RATES

Fig.2 Spectral replications in frequency domain from bandpass sampling

of (a)a bandpass signal, (b)aliasing-free sampling, (c)local oscillator

frequency offset caused aliasing

Due to the current lack of inexpensive high speed ADCs

and DSPs, bandpass sampling is an attractive option for

TuEG-2

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