(Aktu Btech) Optical Communication Important Unit-5 Digital Receiver Performance

B.Tech AKTU Quantum Book delves into the field of Optical Communication. Learn how to grasp this cutting-edge technology by accessing essential notes, repeated questions, and important insights. Unit-5 Digital Receiver Performance

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Important Questions For Optical Communication:
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Q1. Write a short note on probability of error sensitivity. 

Ans. The chance of an inaccurate bit identification by the receiver’s judgement circuit, or bit-error rate, determines the performance standard for receivers.

BER: 

1. Bit error rate (BER) is defined as the ratio of number of errors occurring over a time interval to the number of pulses transmitted during the interval. 

where, 

N_e is the number of errors occurring during the interval,  

N_t is the number of pulses transmitted during the interval, and 

B is the bit rate (1/T_b) or pulse transmission rate. 

2. BER for optical fiber communication system is ranging between 10^-9 and 10^-12. BER of receiver depends on S/N ratio. 

3. To compute the BER at receiver probability distribution of output signal is considered. 

where, V is the pulse amplitude and 𝜎 is standard deviation.  

Quantum limit:

• 1. For an ideal photo detector quantum efficiency 𝜂 = l and has zero dark current then the minimum received. 
• 2. Power for a specific bit-error rate is known as quantum limit. 
• 3. Let an optical pulse of energy E is incident on photo detector in time interval 𝜏.
• 4. Then the probability of emitting zero electrons during the interval is 𝜏

where, N is average number of electrons hole pair. 

Q2. How to measure the BER and Q factor in digital transmission ?

Ans. A. BER: 

• 1. BER is a statistical metric whose value is influenced by the measurement period and the sources of mistakes.
• 2. To provide a statistically accurate BER assessment, a measurement time in which roughly 100 errors occur may be required if the mistakes are caused by Gaussian noise in a somewhat steady transmission channel.
• 3. A bit error happens once every 100 seconds (BER). Even lower bit error rates may not be acceptable at this level.
• 4. For example, to detect 100 errors for measuring a 10^-12 BER in a 10 Gb/s link will require 2.8 hours. Thus test times on installed links could run anywhere from 8 to 72 hours. To reduce such costly and time consuming test periods, a Q-factor technique can be used. 

B. Q-factor measurement:

• 1. This strategy reduces the receiver threshold, which raises the likelihood of errors and shortens test duration.
• 2. Q-factor based assessments are more complicated and use a degraded signal to quantify performance, which is more comparable to what is seen on fielded links.
• 3. The IEEE 802.3 specification for testing 10-Gigabit Ethernet (10-GbE) equipment outlines this technique.

Q3. Explain the term receiver sensitivity in reference to digital receiver performance.

Ans.

• 1. A particular minimum average optical power level must reach the photodetector in order to achieve the necessary BER at the specified data rate. The receiver sensitivity refers to the value of this minimal power level.
• 2. A common method of defining the receiver sensitivity is as an average optical power (P_avg) in dBm incident on the photodetector.   
• 3. It can also be described as an optical modulation amplitude (OMA) expressed as a peak to peak current at the output of the photodetector. The receiver sensitivity provides a measurement of the minimum average OMA power required to maintain the highest possible (worst case) BER at a particular data rate. 
• 4. The receiver sensitivity factor Q in terms of signal currents from 1 and 0 pulses (I₁ and I₀, respectively) and their corresponding noise current variations (𝜎₁ and 𝜎₀, respectively), and assuming there is no optical power in a zero pulse, yields.  

• 5. The receiver sensitivity P_sensitivity is found from the average power contained in a bit period for the specified data rate as:

where R is the unity gain responsivity and Mis the gain of the photodiode. 

• 6. Thermal noise and shot noise are the two types of noise that are most prevalent in the receiver in fiber transmission links without optical amplifiers.
• 7. The shot noise depends on the received power, whereas the thermal noise is independent of the incoming optical signal power. 
• 8. Assuming there no optical power in a received zero pulse, the noise variances for O and 1 pulses, respectively, are 

• 9. The shot noise variance for a 1 pulse is  

• 10. The thermal noise current variance is:

• 11. Substituting