光学曼彻斯特编码FSK/IM正交调制系统性能研究

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"该研究探讨了一种采用频率移键控(FSK)标签和光学曼彻斯特编码(MC)载荷的正交调制系统的性能。通过引入光学MC载荷，FSK和强度调制(IM)正交调制系统的可用消光比(ER)值可从5dB提高到9dB，这相比于传统的非归零(NRZ)载荷显示出了显著的优势。此外，标签和载荷的误比特率(BER)特性也优于NRZ编码，证明了其作为通信系统理想候选方案的潜力。" 本文主要关注的是在光通信领域中，如何通过优化调制技术来提升系统性能。首先，FSK是一种常见的数字调制技术，它通过改变载波频率来表示二进制数据，具有抗噪声性能强、功耗低的优点。在本文中，FSK被用作标签，用于区分不同的信号。其次，光学曼彻斯特编码(MC)是一种特殊的编码方式，它在每个比特时间内交替改变光的强度，以实现数据的传输。与传统的非归零编码(NRZ)相比，曼彻斯特编码具有自同步的特点，即使在没有时钟的情况下也能从信号中恢复数据，因此在实际应用中更为稳健。在本研究中，将光学曼彻斯特编码应用于载荷部分，显著提高了系统的消光比，消光比是衡量光调制器性能的重要指标，更高的消光比意味着更好的信号质量和系统稳定性。系统性能的提升体现在两个方面：一是消光比的增强，从5dB提升到9dB，这有助于改善信号质量，减少误码率；二是误比特率(BER)的改善，表明无论是标签还是载荷部分，采用MC编码都比NRZ编码更能抵抗传输过程中的干扰和噪声，从而提高数据传输的可靠性。这项研究为光通信系统的设计提供了一个新的视角，即结合FSK和光学曼彻斯特编码，可以构建出一个性能更优的正交调制系统。这种系统不仅提高了信号的消光比，降低了误码率，还具有良好的自同步能力，对于未来的高速、高可靠性的光通信网络具有重要的理论和实践意义。

464 CHINESE OPTICS LETTERS / Vol. 8, No. 5 / May 10, 2010

Study of FSK/IM orthogonal modulation system with

optical Manchester-coded payload

Rui Zhou (±±± bbb)

∗

, Xiangjun Xin ($$$), Qi Zhang (ÜÜÜ lll),

Kun Zhao (ëëë ÃÃÃ), Tonggang Zhao (ëëëÓÓÓfff), and Chongxiu Yu ({{{DDD)

Institute of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications,

Beijing 100876, China

∗

E-mail: zhourui85@gmail.com

Received September 18, 2009

We study the performance of an orthogonal modulation system with frequency-shift keying (FSK) label

and optical Manchester-coded (MC) payload. Simulation result shows that by introducing an optical MC

payload, the available extinction ratio (ER) value of a FSK and intensity modulation (IM) orthogonal

mo dulation system can be improved from 5 to 9 dB for system optimization, exhibiting great advantages

over the traditional non-return-to-zero (NRZ) payload. Besides, the bit error rate (BER) characteristics

of both label and payload show a more remarkable advantage than that of NRZ coding, verifying itself as

a p erfect candidate for the payload coding method in orthogonal modulation systems.

OCIS co des: 060.2330, 060.4080.

doi: 10.3788/COL20100805.0464.

In recent years, optical burst switching (OBS)

[1]

and

optical label switching (OLS)

[2]

technologies have been

studied as promising solutions for optical packet switch-

ing (OPS). In OLS networks, the low-bit-rate optical

label contains routing information and propagates to-

gether with data payload. At each intermediate node,

the OLS router can transparently forward the payload in-

formation directly in the optical layer based on the label

information, which avoids the costly optical-electrical-

optical (OEO) conversion of the high-speed data.

Several labeling methods have been proposed and

demonstrated as possible solutions

[3]

. Among them, an

orthogonal modulation method named optical frequency-

shift keying/intensity modulation (FSK/IM) has been

regarded as a feasible scheme due to its compact spec-

trum, simple label swapping, and remarkable scalability

to high bit rates

[4−8]

. However, in FSK/IM systems FSK

label introduces crosstalk to the intensity modulated pay-

load, thus limiting the scalability of the labeling scheme

and reducing the system modulation performance

[5−7]

Manchester coding, also referred to as bi-phase coding,

has been experimentally demonstrated to reduce such

crosstalk

[9,10]

, for which the data signal is electrically

encoded by an encoder before adding it to the optical

carrier by intensity modulator.

Recently, a novel method has been proposed for the

direct generation of Manchester encoded optical signal,

through a dual-drive Mach-Zehnder modulator (DD-

MZM)

[11]

. In this case no extra encoder is needed, which

simpliﬁes the system and reduces the total cost. How-

ever, the optical Manchester-coded (MC) signal has not

yet been used as high-speed payload in OLS systems. In

this letter, we discuss the feasibility of introducing opti-

cal Manchester coding in a FSK/IM orthogonal modula-

tion system, achieving FSK label with optically encoded

Manchester payload. Simulation results prove that by

adjusting the modulation voltage of DD-MZM, a good

trade-oﬀ between lab el and payload performances can be

achieved at high extinction ratio (ER), without much bit

error rate (BER) penalty of payload or label.

Optical Manchester coding can be obtained by driv-

ing the two arms of a DD-MZM with electrical non-

return-to-zero (NRZ) data and an electrical clock signal,

respectively, and adjusting the modulation voltage ap-

propriately to make an exclusive OR (XOR) operation

between the NRZ and clock signals

[11]

. The principle

of Manchester coding is to use either rise or fall of the

signal in the middle of each bit perio d ‘T ’ to represent

‘0’ or ‘1’, as shown in Fig. 1.

Manchester (split phase) coding has been widely stud-

ied in optical and electrical communication systems due

to some of its intrinsic characteristics.

1) Compared with the usual NRZ code, its main ad-

vantages are easy timing extraction, zero direct current

(DC) content, and no laser pattern dependency. The

laser pattern dependency is caused by the possible long

strings of ‘1’s or ‘0’s and degrades the receiver sensitivity

through variations in laser power of ‘1’ level

[12]

2) Manchester coding greatly suppresses the crosstalk

in subcarrier multiplexing (SCM) signaling

[13]

and the

spectrum overlapping between label and payload in the

FSK/IM orthogonal modulation scheme

[10]

through spec-

trum shaping.

Also, proper decoding methods of MC signal have been

proposed to improve the system performance.

1) A weighted pro cessing of the two bit halves of

Manchester data brings a large BER improvement for the

data corrupted by multiplicative noise (signal-dependent

Fig. 1. (a) Manchester coding; (b) Manchester and NRZ.

1671-7694/2010/050464-04

° 2010 Chinese Optics Letters

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