Astronaut to RFID Link

Propagation Model

The propagation model used for the astronaut to RFID link is considered to be idealized and uses the Friis transmission equation, equation 4 from Pratt [1].

P_R	= P_T + G_T + G_R – 20log10(4πR/λ)	(4)
Where,
P_T	= Transmit power (dBW)
G_T	= Gain of the transmitter (dBi)
G_R	= Gain of the receiver (dBi)
R	= Link distance
λ	= Wavelength

Noise Power

The noise power is dependent on the bandwidth and the system temperature. The same RF chain which was designed for the astronaut to Lander link was used; therefore, the system temperature is identical, 345 K. Bandwidth requirements for the astronaut to RFID link are low, since the amount of information contained on the RFID is small. A final bandwidth of 20 kHz was chosen. Table 5 shows the noise power given our final specifications.

Table 5: Noise power

P_N	k	T_sys	B
-160.18 dBW	1.39 x 10-23 JK^-1	345 K	20 kHz

Transmit Power

For the astronaut to RFID link, increasing the transmit power allows a greater distance between tag drops as well as an increased coverage area. However, increasing the transmit power has the drawback of draining the system battery at a faster rate. Additionally, safe effective isotropically radiated power levels were considered. Taking all of these factors in to consideration, a value of 10 W transmission power was used.

First, given the minimum received power at the RFID to turn on, -50 dBW, the maximum distance was derived, shown in Table 6.

Table 6: Maximum distance for RFID to turn on

P_T,astro	P_R,RFID	G_T	G_R	λ	R
10 dBW	-50 dBW	0 dBi	0 dBi	0.123 m	9.79 m

Using the received power of -50 dBW as the reflected transmit power for the RFID, the received power at the astronaut given a 9.79 m distance can be derived. The minimum possible received power for proper demodulation depends on the noise power and the target CNR. In order to achieve a low BER of 10^-6, a target value of 7dB CNR could be used, assuming the addition of block error correction codes (127, 64) discussed in the Error Correction section below and BPSK modulation. Using equation 4, the received power at the astronaut (-110 dBW) is above the minimum value needed for 7dB CNR (-153.2 dBW); therefore, a maximum distance of 9.79 m is achieved. In other words, the maximum distance is not limited by the receiver characteristics at the astronaut. It is limited by the transmit power and the minimum received power required by the RFID to turn on.

Modulation Scheme

Given the small amount of data to be retrieved from the RFIDs, BPSK, a simple modulation scheme is chosen, since it requires the smallest CNR for a target BER and throughput is not a concern. Using equation 3 with a bandwidth of 20 kHz and a roll-off factor of 0.5, the symbol rate is 13.3 ksps. With BPSK, the symbol rate is equivalent to the bit rate; therefore the bit rate is 13.3 kbps. Likely, a smaller bandwidth could be chosen, which would reduce the noise power and consequently the required receive power at the astronaut. However, as noted in the Transmit Power section, the astronaut to RFID link is not limited by the receiver characteristics at the astronaut; rather, it is limited by the transmit power and the minimum receiver power required for the RFID to activate.

Error Correction

It was important to keep the hardware at the RFID simple; therefore, for error correction, a simple 127, 64 block code was used. From Pratt [1], using BPSK modulation, this block code allows for approximately 7dB CNR, giving a BER under 10^-6, and a coding gain of approximately 3.6 dB.

References:

[1] T. Pratt, C. Bostian, T. Allnutt, Satellite Communications, 2nd edition, Wiley, 2002.
[2] Larry Foore and Nathan Ida, Path Loss Prediction Over the Lunar Surface Utilizing a Modified Longley-Rice Irregular Terrain Model. NASA Technical