Advanced Optical Modulation Techniques - Literature review Example

Name: Course: Instructor: Date: Advanced Optical Modulation Techniques – A Literature Review There has been a significant development in the field of optical modulation techniques in the recent years [12]. Prior to the current shift towards the advanced optical modulation technologies, the Wavelength-Division Multiplexed (WDM) optical transmission techniques were widely used from as early as the 1990s [3]. Later on, the technological advancements in the modulation techniques necessitated improvement in spectral efficiencies, which is what set the journey on the advancement in the optical modulation techniques leading to such high-speed fiber-optic techniques as the Quaternary Phase Shift Keying (QPSK), Binary Phase Shift Keying (BPSK), Differential Phase Shift Keying (DPSK), optical Quadrature Amplitude Modulation (m-QAM), and Differential Quadrature Phase Shift Keying (DQPSK) in which both phase and amplitude could be modulated [19]. This paper, therefore, performs a literature review of the advanced optical modulation techniques. Differential Phase Shift Keying is a technique that is well known for its lack of absolute carrier phase reference [5]. Typically, DPSK is characterized by the fact that the phase reference as the transmitted signal. For that reason, DPSK may be said to be a phase modulation technique in which the conveyance of data is carried out through the inverted phase of the carrier wave [19]. The very first step in DPSK entails encoding the data in question differentially prior to employing a Phase Modulator (PM) (also used is the Mach-Zehnder Modulator, MZM) to modulate the encoded data onto the optical carrier [11]. Behind the scenes, the MZM acts by converting the initial optical phase into a 180-degree phase shift. Most instances involving the encoding of the encoded data onto the optical carrier are characterized by the use of MZM typically because of its superb tolerance of chromatic dispersion [2]. Practically, it is not possible to perform demodulation of DPSK, at least directly [3]. In order to counter this property, therefore, a Delay Interferometer (DI) is usually placed along the path of light at the receiver. This helps in the conversion of the differential phase modulation into intensity modulation [1]. Alternatively, the MZM produces intensity modulation by retaining the constructive or destructive interference courtesy of the input electrical voltage. Most advanced optical modulation techniques are known for their excellent performance in modulation [13]. Additionally, they can independently modulate the optical path’s phase as well as intensity. Consequently, therefore, almost all advanced optical modulation techniques are based on MSMs [7]. According to [3], the process of conversion of phase modulation into intensity modulation is achieved through the destructive interference of the two optical fields (usually at the destructive end), and constructive interference of the two optical fields in the event that there is a variation of phase between subsequent bits. [5] points out that the delay interferometer along the optical route conserves energy within it, and for that reason, the constructive end generates a data pattern that is logically upside down [5]. The greatest disadvantage with DPSK is the fact that it performs poorly beyond certain levels of data rates [14]. It is practically impossible, for example, for DPSK to function effectively at a data rate of, say, 50 GB. That is where the use of Differential Quadrature Phase Shift Keying (DQPSK) comes in handy. In optical systems, DQPSK accomplishes this by employing a multi-level technology, usually characterized by several bits per symbol [16]. Due to this characteristic, the symbol rate for DQPSK is generally quite low [1]. The transmitter of a DQPSK takes advantage of the bi-phase modulators with some phase shift to ensure a 900 shift of the output especially when the two outputs come together. In comparison to the DPSK, the DQPSK’s receiver sensitivity is far much lower. On the flip side, though, the DQPSK is known for its excellent tolerance to chromatic dispersion as well as outstanding spectral efficiency [18]. In practice, the DQPSK operates under four distinct phase shifts i.e. π, +π/2, -π/2, and 0 [1]. The π may be represented by either of the following: sin(ωt), sin(ωt + π/2), sin(ωt – π/2), and sin(ωt + π) [3]. It is these four phase shifts that enhance DQPSK’s data modulation. For the DQPSK, the aggregate bit rate is twice the symbol rate operation of the DQPSK [21]. The receiver end through which the DQPSK signal passes divides that signal equally into two sections which are detected by a bi-balanced receiver system. The arrangement of the duo is usually in parallel which has the advantage of simultaneously demodulating the DQPSK signal and its two binary data streams [6]. In DQPSK demodulation it is mandatory that the DI generates a delay that equals to the bit duration multiplied by two [9]. Essentially, this implies a delay that equates to the symbol duration. M-ary Quadrature Amplitude Modulation (m-QAM) is an optical modulation technique for systems whose transmission rate and efficiency of bandwidth is quite high [10]. What distinguishes QMA from other optical modulation techniques is the fact that it integrates both phase and amplitude modulation [1]. The equation for M-ary QAM can be represented as follows: The M-QAM signal is composed of two phase quadrature carriers each of which undergoes modulation by a set of discrete amplitudes [20]. As a matter of fact, that is the genesis of its QAM name. These signals can also be represented in form of a signal space, and can be illustrated in values of powers of 2 as follows: M = 2k, Where k = 2, 3, 4, 5,……n Based on the illustration shown above, the even values of k yields constellations that are literally square in nature i.e. 4-QAM, 16-QAM, 64-QAM, etc. On the other hand, odd values of k yield constellations whose shape is cross-like – the reason for which they are referred to as cross constellations. Such constellations are 32-QAM, 128-QAM, etc. square constellations are usually characterized by QAM that is in line with the independent modulation of amplitude of an in-phase carrier and a quadrature carrier [5]. The quadrature carrier and an in-phase carrier are also referred to as the sine carrier and cosine carrier respectively [11]. An even number of bits per symbol for an M-QAM square constellation may be adequately represented by: and it can be said to be a Cartesian square of an L-ary PAM constellation that is one-dimensional. The symbol error possibility of a square-constellated M-QAM can be represented as follows: In order to produce an M-QAM constellation that is square in nature by use of a single I/Q modulator that is under the control of two quadrature signals whose amplitude levels are represented by , it is imperative to employ the use of two digital to analog converters [12]. It is also imperative that each of the digital to analog converters exhibits a minimum resolution of bits with a sampling rate that equals the symbol rate [1]. With the recent need for signals and data to be transmitted over relatively longer distances, a better modulation technique that serves this purpose had to be developed. In addition to accomplishing longer transmission distances, the RZ-DPSK is also fundamental in ensuring excellent system tolerance to distortions that are not linear [4]. In the RZ-DPSK, it is worth noting that the encoding of binary data is done through a phase shift of “zero” or “pi” in the midst of bits that are next to each other [21]. Just as the name suggests, the RZ-DPSK is characterized by the fact that the signal optical power is always returning to zero at the very end of every slot of bit. The unique characteristic of this modulation technique is that it has an extra bit synchronized intensity modulation for which its signal optical power is always varying [7]. The sister modulation technique to the RZ-DPSK is also referred to as the Non-Return-to-Zero Differential Phase Shift Keying (NRZ-DPSK) whose optical signal pulse width is far much wider than that of the RZ-DPSK. As a result, therefore, in comparison to the RZ-DPSK, the NRZ-DPSK happens to have a narrower optical spectrum [9]. In order to ensure the highest possible efficiency in the optical systems, it is prudent that such a system does the transmission of the highest amount of information within not only the lowest possible bandwidth, but also at the very lowest cost [4]. The various advanced optical modulation techniques discussed above i.e. the DQPSK, DPSK, M-QAM, NRZ-DPSK, and RZ-DQPSK are generally aimed at accomplishing this supposed goal. One way through which these optical modulation techniques may achieve this function is by enhancing the WDM channel capacity [10]. While the DPSK and DQPSK are known for taking advantage of the multiple phases to transmit more than one bit in every signal element, the M-QAM is synonymous with making the channel capacity more efficient through its transmission of more than one bit in every signal element. In this case, the M-QAM uses different amplitudes and different phases altogether. The RZ-DPSK/NRZ-DPSK, on the other hand, encodes binary data through a defined phase shift. In conclusion, therefore, the field of optical modulation is definitely set to experience even more advancements in the years to come with infinitively small bandwidths being used to transmit huge amounts of information. Works Cited 1 P. J. Winzer and R-J. Essiambre, “Advanced Modulation Formats for High-Capacity Optical Transport Networks”, J. Lightwave Technology, vol. 24, no. 12, pp. 4711-4728, 2006. 2 P. J. Winzer, “Modulation and Multiplexing in Optical Communication Systems”, IEEE LEOS Newsletter, February 2009, pp. 4-10. 3 P. J. 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New York: Academic, pp. 95–130, 2008. Read More