DOWNLINK DESIGN
Two options were considered while designing our downlink communication link.
1. Direct Rover to Earth Communication without a landing station. 2. Rover to earth Communication via a Landing station
Option 1 is suitable if the target downlink data rate is in kilo bits/s. However, if the target data rate is in Mbps (like the X-prize); there are severe constraints on the rover to maintain an allowable SNR at the receiving Earth station. More data rate means increased bandwidth which in turn leads to more noise at the Earth station. Hence the rover needs to produce more power, have bulky antennas to produce more gain, etc. This significantly increases the dimensions and mass of the rover which means difficult maneuvering and navigation through the unknown lunar territory.
Option 2 means that the rover size and mass can be significantly reduced because the distance between the landing station and rover is never more than 5 km (requirement for X-prize bonus). The landing station being stationary can be packed with high gain dish antennas and more solar panels which can produce enough power to maintain a good SNR at the Earth station with HDTV data rates.
As the rover travels over the required 5 km distance, the probability of losing line-of-sight with the lander cannot be ignored. This risk can be minimized by using Infrared (IR) sensors and navigation cameras for intelligent route design to avoid craters, hills, and depressions. In addition, transmission at UHF (between rover and landing station) will diffract slightly about the curvature of the moon, further reducing the chance of losing line-of-sight. However, should the rover lose line-of-sight or experience equipment failure, a direct rover-to-earth transmission is also necessary. The direct rover to earth communication can transmit data rate in the order of kilo bps till until the control signals transmitted from Earth bring the rover back in the line of sight with the lander.
After weighing the factors described above; for our design, the primary downlink (for HDTV transmission) from the rover to the Earth is via the landing station. A secondary downlink (with rates in kilo bps) would be used for direct communication from the rover to the Earth if the rover loses line of sight communication with the landing station, till line of sight between landing station and the rover is restored by the control signals transmitted to the rover from the Earth.
PRIMARY DOWNLINK DESIGN (Rover → Landing Station → Earth)
ROVER TO LANDING STATIONA UHF frequency of 730 MHz is selected because a range of 5 km falls within the line of sight range of UHF.UHF can also diffract slightly from the surface of the moon. Another advantage of UHF transmission is the physically short wave that is produced by the high frequency. The size of transmission and reception equipment, (particularly antennas), is related to the size of the radio wave. Hence, smaller antennas size for rover to lander communication can be used.
The rover transmits HD video signals to the landing station. The following diagram shows the block schematic of the communication from the rover to the landing station.
The CMOS image sensor is a type of active image sensor which captures the visual data and converts it to a voltage. Additional circuitry on the chip converts the voltage to digital data.
The Texas Instruments’ MPEG-4 encoder compresses the data to 10 Mbps. This binary data is encoded with a rate 2/3 convolution turbo code (Forward error correction). The encoded data is then assigned raised cosine pulses. The RC pulses are QPSK modulated (carrier frequency 730 MHz) and transmitted to the landing station.
RC pulses minimize inter-symbol interference and spray less than 50 dB power outside the assigned frequency band (requirement of FCC), if designed correctly. For our design, a roll-off factor of 0.55 sprays less than 50 dB outside our assigned Bandwidth. The MATLAB simulation of the RC pulse is given below. For the MATLAB code of the designed pulses, click HERE.
CALCULATING THE BANDWIDTH REQUIREMENTOutput data rate of the MPEG-4 encoder = 10 Mbps Data rate with the redundant 2/3 turbo code included = 3*10/2 Mbps = 15 Mbps
Because QPSK is used, symbol rate (RS) = 0.5 * bit rate RS = 0.5 * 10 = 5 M sps (symbols per second)
BRF = RS * (1+ α), where α = roll off factor = 0.55
BRF = 5M * (1+ 0.55) = 7.75 MHz.
CALCULATION OF THE LINK BUDGET
Calculations for each link were done using excel. This provided an efficient way to switch parts to verify that link constraints were satisfied. The calculations are done for each stage and carried over to the next. The excel sheet can be downloaded HERE. This file includes a sheet for each link budget calculations (a total of 3). The basic equation for link design is:
With a transmit power of -0.024 dBW, a power of -89 dBW is received at the landing station
LANDING STATION TO THE EARTH STATIONFor the communication link between the landing station and the Earth station, a carrier frequency of 8.4 GHz is chosen (Space Research Band). At higher frequencies (> 30 GHz), rain attenuation starts to severely degrade the link. The websites of the Office of Space Communications (NASA) and FCC suggest that it is possible to get 7.75 MHz of bandwidth in the 8400 MHz to 8500 MHz frequency. Hence a carrier frequency of 8.4 GHz is selected.
The landing station has a Bent pipe transponder. It does not perform any signal processing but converts the 730 MHz signal to an 8.4 GHz signal. The block diagram of the communication system at the landing station is given below.
Signal Power CalculationThe transmit power Pt of the landing station is given by: PT = PR* GLNA * GBPF * GM1 * GBPF * GM2 * GBPF * GHPA
NOTE: During the design of the amplifiers, the amplifier output cannot exceed their maximum power ratings.
Noise CalculationsTotal Noise at the transmitter is can be approximated by: NT = k * B * GLNA * GBPF * GM1 * GBPF * GM2 * GBPF * GHPA * [TPHY + TLNA + TBPF/ GLNA + TM / (GLNA* G M1)]
[FOR DETAILED LINK BUDGET AND DEVICE DESIGN CALCULATION, CLICK HERE]
The minimum SNR at the Earth station with 2/3 turbo coding is 2 dB. An additional link margin of 4 dB is provided in the design.
The table below summarizes the different parameters and their values for the primary downlink design.
SECONDARY DOWNLINK DESIGN (Rover → Earth)
The secondary downlink serves as a backup incase the rover loses line of sight of the landing station. The secondary downlink can transmit up to bandwidth of 10 kHz to satisfy the minimum SNR required at the earth station. Again, the calculations can be found in our EXCEL SHEET HERE. This bandwidth can support the transmission of still images from the rover to the earth until the line of sight between landing station and rover is reestablished.
The frequency of 8.4 GHz (X band) is used for this link. The block schematic of this communication link is provided below.
CALCULATION OF MAXIMUM DATA RATE
BRF = RS * (1+ α), where α = roll off factor = 0.55 →RS = 10k/(1+0.55) = 6.45 sps = 12.9 kbps →Real data rate= 2/3 * 12.9 kbps = 8.6 kbps
The table below summarizes the different parameters and their values for the secondary downlink design. For the detailed design, CLICK HERE.
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