Constraints and parameters for the system design » History » Version 6

GAY, Adrien, 03/23/2015 10:25 AM

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h1. Constraints and parameters for the system design
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* Required bit rate
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In order to define the bit rate required for the transmission of video streams we need some input parameters, provided in the mission statement:
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> # aspect ratio of the video : AR=1,78 
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> # quality of the encoded video L=720p
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> # frame cadence FPS=12 fps
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We used the following formula
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p=. Bit rate = FPS*5,0692 * L ^1,391^ /(1000*AR) 
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So 
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p=. *Bit rate=322 Kbps*
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As the link will allow video transmission, we will need an adapted encapsulation protocol, so we must take into account extra bits: we decide to define a bit rate of 500 kbps. 
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   # Network constraints
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> * Frame format
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The application foreseen aims at transmitting multimedia content (video on the downlink), so we choose to use the MPEG-4 coding standard. The associated frame format MP4 is particularly adapted for the encapsulation of multimedia data type.
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> * Protocol stack
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   # Antenna choice
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> * On board antenna
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The antenna placed on the aircraft must be compliant with the flight constraints listed in the introduction. Indeed, we should maintain the communication link while the aircraft is performing a turn ( +/- 15° attitude), landing or taking off ( +/- 45° inclination). Therefore, he position of the antenna must be carefully determined in order to be at LOS. 
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Another constraint is the embedded 
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> * Ground station antenna
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h2. Constraints for the physical layer and RF equipment:
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(calcul Rb)
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From the given allocated frequency band, the following parameters are defined:
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* f : Central frequency of the emitted signal
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* B: Larger of the allocated bandwidth
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* EIRP: Maximum power that can be emitted in a given direction 
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From the specifications, the following parameters are defined:
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* Rb: Useful bit rate of the transmission
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* R : Minimal distance for the transmission
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The value of these parameters constrain the parameters of the physical layer and the RF equipment for the design of the system.
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h3. Physical layer:
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The study of the physical layer will be limited to the choice of the modulation, the coding and the shaping filter. We will consider a SRRC filter (Square Root Raised Cosine) for the shaping filter as it is commonly used in telecommunication systems for its good performances. 
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Then, the parameters of the physical layer are:
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* M : Modulation (M=4 : QPSK, M=8 : 8PSK etc)
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* rho : Coding rate (rho <1)
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* alpha : roll-off of the SRRC filter
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In fact all these parameters are linked through the spectral efficiency T of the system, which is fixed by B and Rb:
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T= cst  et  T=
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Then, the parameters of the physical have to comply with the following relation:
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T>cst
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h3. Link budget:
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Here is the expression of the link budget:
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(link budget)
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We can notice that all the parameters are already known, except:
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* (G/T): Figure of merit of the receiver (ISAE antenna)
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* Lmarg: Margin on the link budget to take into account all the perturbations (antenna   
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depointing, atmosphere attenuation, interferences, non-ideal demodulator …)
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Lmarg being only linked to physical parameters, we don’t have any influence on it. Then, it has to be evaluated but it is not really a parameter of the design.
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Power amplifier
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h2. Conclusion:
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From these considerations, our aim will be to:
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* Choose the modulation and the coding (according to the shaping filter)
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* Compute the gain of the receiving antenna
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* Propose some technical solution for the receiving antenna
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We will also develop tools to visualize the influence of the bandwidth, EIRP, useful bit rate and distance on the system design.
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The aircraft antenna will be considered able to fulfil the required antenna pattern, but we will not discuss about technical solutions for this antenna, as it can be really tricky.