Design Review For Medium Range Radio Frequency

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					               Design Review
                    For


   Medium Range Radio Frequency
Communications Link Between Pilots and
  Ground Personnel at Airport Gates


            For ECE 445 – Senior Design




                 Developers

Daniel Fisher, Tom Stilwell, and Ryan Tennill


                      Date

             February 22, 2010
                         1
                                    Wireless Airport Radio

We chose this project because we see great value in the creation of a wireless communication
system for airport grounds crew. At this point in time the pilot can only talk to one crewmember
via a wired headset connection. It seems very beneficial to have a system where more than one
person can communicate to the pilot and each other without having to walk over and plug in.

Objectives:

The main goal is to create a wireless radio system that will transmit and receive to and from all
others. One set will contain three headset adapters and one plane adapter that communicate to
one another. The each set will work on a specific channel of the 2.4 GHz ISM band so that each
gate can communicate without interference from the others.

       Benefits:
        Complete freedom of movement while maintaining ability to communicate to others
        Eliminates wired connections and the safety risks that are associated with them
        Multiple users can servicing a single plane without interfering other gates
        No modification required to the aircraft hardware or structural design
        No modification of current headsets is required


       Features:
        Operates on the 2.4 GHz frequency band reserved world-wide by 802.15.4 radio
          communications reducing the presence of interference while also providing adequate
          range
        Interchangeability. The wireless circuit will be located in a pocketed sized container
          that plugs into the current headset allowing for quick replacement in case of breakage
        High quality audio communications provided by 250 kbps data rates
        Low power. Maximum power expected to be below 200mA. Additional power
          savings accomplished using low power modes to selectively power down the
          transceiver and controller
        Expandability. The 2.4 GHz band is divided into 16 channels 5 MHz wide. Each
          channel can be further subdivided for use by multiple nodes
        Simple channel changing. Easy to use method for changing channels via dip switches
          or digital keypad on container
        Compatible with ¼“ audio jack plugs
        Hands free communication


Schematics:

See next 4 pages for schematics.




                                                2
     System Overview

           Headsets                               Radio Component
         Speakers                           BP Filter            MSP430
                                                              Microcontroller

        Microphone          ¼”
                           Audio            Battery            Xbee RF Chip
                           Jack
                       Figure 1: Component Overview of a Single Unit

     BP Filter



In


                                                                                Out




                         Figure 2: Circuit Design for Band Pass Filter




                       Figure 3: Frequency Response Curve of BP Filter
                                              3
MSP430 1611 Microcontroller

                                                             Microphone Input




                                                          Analog Bandpass Filter
                                                             500Hz-4500Hz




                                                              MSP430F1611

                 MSP430F1611


                                                        Analog to Digital Converter


            Receive data to RF module
                    (UsART)

                                                               RAM (10kB)



                   RAM (10kB)


                                                       Energy Estimation Algorithm
                                                     E= |x(t)|^2, ~100ms blocks of data
                Decompress data
             (ADPCM or other scheme)

                                          no

                                                              E > threshold?


                                                                         Yes
            Digital to Analog Converter

                                                     Read previous 2 frames from ram




                                                            Compress data
                                                        (ADPCM or other scheme)



                De-emphasis filter                       Send data to RF module




                Headset Speakers




Figure 4: Microcontroller Data Flow (Left – Data to Audio / Right – Microphone to Data)




                                          4
             Figure 5: Block View of Microcontroller Functionality R1.


XBee RF Module




             Figure 6: Block View of Data Flow in XBee RF Module R2.




                                        5
Schematic Descriptions:

      System Interconnection:

          1. This is the overview of the interconnection of the components of a single unit
          2. Headset consists of both a microphone and mono-speakers with a ¼“ audio plug
          3. Box consists of the battery, microcontroller, radio chip, and filter circuit.

               The box is the housing unit for the circuit components, chips, and batteries. It
               must be a waterproofed enclosure that is easy to be carried around whether it be
               clipped to a belt or stuck in a pocket. Our first choice would be to use the left ear
               as the enclosure for the components. This will be done if the components and
               PCB are small enough to fit and the ear can be safely waterproofed. Otherwise,
               we will use the black box (or a container similar to it) to house the components.

      Band-Pass Filter:

          1.   Analog filter design that filter out noise
          2.   Reduces load and requirements of Microcontroller
          3.   Keeps speech relevant frequencies (500 – 4500 Hz)
          4.   Provides little latency to keep real-time communications
          5.   5th Order Butterworth Low Pass Filter R3 with 2nd Order High Pass Filter

      Microcontroller:

          1. Controls RF Module.
                Communicates over UART.
          2. Converts microphone voice to digital (ADC)
                This will be a 12-bit linear conversion scheme.
          3. Level detection
                This will determine when the user is speaking into
                the microphone, so that we don’t send empty packets

                  In the micro we will use a rotating buffer of length fsample/Tdetection (aprox. 2000
                  address.) When each new sample comes in we will subtract the old sample
                  and add the new one. (see picture below)




                                                 7
   4. Compression (ADPCM)

               The ADPCM compression will convert the 12-bit output from the ADC
               down to an 8-bit logarithmic gain output. To avoid large processing times
               we will use a step-wise log scale for the 8 most significant bits of the input
               and the remaining 4 will be linearly added to the output.




                                                                    R4


        This algorithm is for a 14 bit input, so we will modify it for our 12-bit system.

   5. Converts digital to audio for speaker (DAC)

        The DAC can output up to 3.3V but we have measured that the headsets only
        need a 1.1V supply. So we will divide the digital output signal by 3 and then run
        the analog signal to a buffer connected to the speakers.


Radio Frequency Module:

   1.   Enables the transmission and reception of signals on the same antenna
   2.   Packets and sends input data in bursts to minimize power
   3.   Buffers and decodes received data
   4.   Accepts and responds to API instructions from Microcontroller that will modify
        transmission power and throughput during operation

                                          8
   5. Transmission frequency set by microcontroller to allow multiple networks of
      radios to work simultaneously in the same cell. This frequency will be set by 4
      dip switches connected to the digital inputs of the micro.


Power:

   1. Two rechargeable 3.7 V batteries
   2. 2050 mAh
   3. 300 mA step down switching voltage regulator with 90% efficiency


Optional Feature: Dynamic Control of Transmit Power

         One of the primary goals of this project is to minimize power consumption for
enhanced battery life. XBee-Pro International version module has the ability to change
maximum transmit power via the command mode interface. The microcontroller can be
used to put the device into command mode and specify the new parameter value using
available API commands. The ability to change this parameter is limited to four different
settings: 10, 8, 2, and -3 dBm. It is desired to use this capability to periodically evaluate
received signal strength and adjust transmit power to the lowest power level necessary.
Signal strength can be evaluated directly from received packets and the power adjusted as
necessary. Power consumption can be greatly reduced by only broadcasting the data as
far as is necessary. The RF module has a maximum transmit power of 10dBm and a
corresponding current draw of 150mA where as the module limited to 0dBm only draws
45mA while transmitting.




                                          9
Performance Requirement:

The unit should be able to transmit and receive within a 1000 ft (305 m) line of sight if
necessary. If it can transmit the 1000 ft, the unit should be very efficient in the 100/300 ft range.
However, the FCC limits the transmission of signals to less than 10 dBm while inside an airport.
This will constrain the total distance we can get. Using the Friis Transmission formula and the
specifications of the XBee chip, we estimated the distances achievable for the lower power
levels.

       PR = PT*GT*GR*λ2 / (4πd)2

       Power Transmit levels: 10/8/2 dBm                     10.00/6.30/1.58 mW
       Receiving Sensitivity: -96 dBm                        250E-12 mW
       Frequency: 2.4 GHz                                    0.125 m
       Estimated distance at max power: 750 m

       250E-12 = 10*G2*0.1252 / (4π*750)2                    G = 0.38

       250E-12 = 6.3*0.382*0.1252 / (4π*d)2                  d = 590 m
       250E-12 = 6.3*0.382*0.1252 / (4π*d)2                  d = 296 m


Having researched different number of chips, this XBee chip is the only RF module that both
transmits the required distance and satisfies the FCC’s regulation criteria. With a distance of 590
m in ideal conditions, this easily satisfies the range requirement while leaving ample room for
lowered results in the field.

The unit must also be able to last an entire work day for airport personnel. We assume the
average usage of this headset will be 18 hours per day. We also assume that they will be used at
50% duty cycle (whether it be transmitting or receiving) so the product will need to function for
9 hours per day (the rest of the time it will be sleeping). Digital I/O lines on microcontroller is
connected to the sleep line on the X-Bee so it can remotely be put to sleep if no microphone
activity is detected in a set period of time. We also assume the user is only talking for 25% of
that time.

       Microcontroller Power:          2.5 mA
       X-Bee Power Sleep:              0.05 mA
       X-Bee Power Idle:               55 mA
       X-Bee Power Transmit:           150 mA

This will result in a usage of 755 mAh per day of use.




                                                 10
End Product Testing:

    Distance:

          The effective range of the radios is a large requirement of the project, because they
    must be extremely reliable at 100 feet but must not be too strong to avoid interference at
    adjacent gates. To verify this we will collect data on the signal to noise ratio (SNR), and
    analyze the graph over distance. In practice these radios will usually be transmitting about
    30 feet, and maybe through the roof the cab of a car. To be conservative we are aiming to
    have reliable digital communication for up to 300 feet line of sight.
          The X-Bee transceiver we are using has an API function (called DB) that can report
    the signal strength back to the MCU over the serial line called.

    Delay:

          Because this is a real time system, and real people will be using this system the total
    delay of the system must be minimized. The International Telecommunication Union says
    that for voice communication a one-way delay of less than 150ms is acceptable for almost
    all user applications. We will try to reach this goal, and we will measure this in the lab.
    Because propagation delay is so small for these distances we can ignore it for testing delay
    and send an impulse into one microphone and measure the time till it is reproduced by the
    headset speakers that are in the same room. Because the radios are only for voice
    communication, their frequency response is unimportant as long as speech is easily
    recognized. Our delay requirement makes sure that this recognizable speech arrives in a
    timely matter.


    Multi-access:

          Multiple cells of radio users in the same area is another design requirement for our
    radios, and we will hopefully be able to make four radios in order to test their functionality
    in the real world. But in order to get data on multiple systems interacting and the systems
    channel rejection, we will set one device to constant transmit, and monitor the SNR of the
    link between two communicating radios nearby. These radios will either be on a different
    channel or if throughput is fast enough they might be just We will then graph this SNR
    against the distance from the constant transmitter. In use the radio transceivers will usually
    be at least 100 feet away from each other because of airplane gate spacing, so this short
    distance test is conservative.

    Voice Quality:

          In order to maximize our battery life and allow many radios to work simultaneously
    we are aggressively trying to minimize the data size of the recorded audio. To do this we
    are band-filtering, sampling, and using a non-linear gain compression to reduce the amount
    of bits being sent. In addition because this is a multicast system there will be no re-
    transmissions, so end user voice quality is an important metric to measure while tweaking

                                               11
    the design. To measure this we will periodically have a small group of fellow EE’s listen
    to a few pre-recorded messages and rate the voice quality on the standardized MOS scale of
    1-5. The International Telecommunication Union (ITU) even has a set of predefined set of
    phrases to test.


             5 - Perfect. Like face-to-face conversation or radio reception.
             4 - Fair. Imperfections can be perceived, but sound still clear. This is (supposedly)
              the range for cell phones.
             3 - Annoying.
             2 - Very annoying. Nearly impossible to communicate.
             1 - Impossible to communicate

          The scores do not need to be integers, and the arithmetic mean of all participants is
    computed and compared. Because the test is so quick and the sample size (~10) so small it
    will be a quick and efficient way to find the perfect codec for our system.


Modular Testing:

   Analog Filter:

           Before we connect the microphone to the ADC, we will verify that our active analog
    filter is functioning with the correct response by measuring the output with the oscilloscope
    on FFT.

   Level Detection:

          In order to find the right threshold for the level detection, it will take some testing.
    By connecting an LED to an I/O pin on the MCU that turns on when the system is
    transmitting, we will have a visual feedback for the system. Then by varying the speaking
    level and background noise level we will try to find an optimum threshold.

   ADPCM:

          The ADPCM will compress the 12-bit signal to 8-bits and modify it with a non-linear
    gain. In order to make sure that it is working correctly we will have a loopback testing
    mode that by passes the X-Bee radios and sends the signal straight to the talker’s
    headphones.

   X-Bee Radios:

          We have already begun testing the X-Bee communications with a chip in the lab. We
    will verify that the radios are working independently from our project by connection both
    to serial ports on lab computers. They we will program each with the API and send test



                                               12
     data, from one computers terminal to the others.    This will be facilitated with the XCTU
     software provided by the manufacturer (Digi).



Tolerance Analysis:

         One of the most important facets of this product will be the length of time that the set
  will transmit and receive communications. We will be using a rechargeable battery, but it
  must be able to last an entire day of use.

        In order to test the power tolerance of the system, we will connect the entire circuit to a
  constant voltage supply. Then we will measure how much current is being used while we
  change the operational characteristics of the system. Both radios will be set to the same
  duty cycle so half of the active time will be sending and half receiving.

  Duty cycle                       Average Current                   Battery life
  5%
  10%
  20%
  30%

       After calculating average current usage we can use the equations used earlier to make an
  accurate estimation of the battery life, based on its available mW/hrs.




                                                13
Cost Analysis:

        This cost is for a full gate set: 3 personnel headsets and 3 communication boxes as well
as the 1 communication box that plugs into the airplane.

                                         Parts Received

        Part #            Mft                Desc              For       Price    Qty      Total
                       David
Headsets                             Headphone w/ Mic       Comm       $250.00      2     $500.00
                       Clark
Battery Box                          Plastic container      Unit          $5.00     2      $10.00
Total                                                                                     $510.00

                                          Parts Needed

        Part #             Mft               Desc             For       Price     Qty      Total
                        David
Headsets                             Headphone w/ Mic       Comm       $250.00      1     $250.00
                        Clark
Battery Box                       Plastic container         Unit         $5.00      2      $10.00
296-18102-1-ND          TI        MSP430F1611               MC          $19.09      4      $76.36
XBP24-AWI-001J          Digi      Xbee-Pro RF Int’l         Radio       $32.00      4     $128.00
LM2904N                 Intersil  Dual Op Amp               Filter       $0.32      8       $2.56
                        Yageo     1 kΩ Resistor             Filter       $0.32     20       $6.40
                        Yageo     175 Ω Resistor            Filter       $0.32      4       $1.28
                        Yageo     375 Ω Resistor            Filter       $0.32      4       $1.28
                        Kemet     1000 nF Capacitor         Filter       $0.32      4       $1.28
                        Kemet     1450 nF Capacitor         Filter       $0.32      4       $1.28
                        Kemet     7 nF Capacitor            Filter       $0.32      4       $1.28
                        Kemet     14 nF Capacitor           Filter       $0.32      4       $1.28
                        Kemet     18 nF Capacitor           Filter       $0.32      4       $1.28
                        Kemet     24 nF Capacitor           Filter       $0.32      4       $1.28
                        Kemet     83 nF Capacitor           Filter       $0.32      4       $1.28
                                  PCB Board                 Unit         $5.00      4      $20.00
                        Kemet     100 nF Capacitor          Battery      $0.32      8       $2.56
                        Kemet     10 nF Capacitor           Battery      $0.32      4       $1.28
                        Kemet     100 uH Inductor           Battery      $0.32      4       $1.28
CGR 18650               Panasonic Li Battery 2050mAh        Battery      $3.29      8      $26.32
                        National  3.3 500mA
LM2674                                                      Battery      $3.47      4      $13.88
                        Semicond. Regulator
13-06238                Spruce    “Remove” Streamer         Safety       $4.35      1        $4.35
GH7184-ND               Grayhill  Dip switch x4             MC           $0.96      4        $3.84
Total                                                                                     $558.32

GRAND TOTAL FOR ONE SET: $1068.35


                                                14
Schedule:

     Week Of                                          Description
                       Find necessary chips and circuit components – Tom
Feb. 14th – Feb 20th   Draw circuit layout sheets and pin-outs – Ryan
                       Complete Design Review documentation and layout – Dan
                       Design Reviews – ALL
                       Order remaining parts & Call FCC – Tom
Feb 21st – Feb 27th
                       Build power supply – Ryan
                       Design PCB layout – Dan
                       Microcontroller Coding – Tom
 Feb 28th – Mar 6th    Submit PCB for printing – Ryan
                       Test Analog Filter – Dan
                       Microcontroller Coding – Tom
Mar 7th – Mar 13th     Microcontroller/RF interface coding – Ryan
                       Solder PCB Board – Dan
                       Progress Reports – ALL
                       Microcontroller Coding – Tom
Mar 14th – Mar 20th
                       Complete PCB / Resubmit if Problematic – Ryan
                       Test packet reception – Dan
                       Spring Break – ALL
                       Prepare Mock-up Demo Notes – Dan
Mar 21st – Mar 27th
                       Debugging – Ryan
                       Test – Tom
                       Test – Tom
Mar 28th – Apr 3rd     Debugging/Improvements - Dan
                       Multi Channel test – Ryan
                       Test – Dan
 Apr 4th – Apr 10th    Debugging/Improvements – Tom
                       Real time delay functioning – Ryan
                       Finalize Assembly Box – Dan
Apr 11th – Apr 17th    Debugging/Improvements -Ryan
                       Test Interference – Tom
                       Finalize Product for Demonstration – Tom
Apr 18th – Apr 24th    Finish Paper – Dan
                       Edit Paper – Ryan
                       Rehearse Presentation – ALL
Apr 25th – May 1st     Prepare PPT for Presentation – Dan & Tom
                       Prepare for Demo – Ryan




                                             15
References:

R1.     “MSP430F1611.” Texas Instruments. Feb 16, 2010
<http://focus.ti.com/lit/ds/symlink/msp430f1611.pdf>.

R2.    “XBee & XBee-PRO 802.15.4 OEM RF Modules.” Digi. Feb 17, 2010
<http://ftp1.digi.com/support/documentation/90000982_B.pdf>.

R3.  “Design of a 5th Order Butterworht Low-Pass Filter Using Sallen & Key Circuit.” UIC
ECE Dept. Feb 19, 2010 <http://www.ece.uic.edu/~jmorisak/blpf.html>.

R4.    “μ-law algorithm” Wikipedia. Feb 17, 2010 <http://en.wikipedia.org/wiki/Mu-
law_algorithm>.




                                            16

				
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