1. Satellite-Earth

Satellite-Earth

Design Specifications

(1) ISM Band: 5.725-5.875 GHz (B = 150 MHz)

(2) DSSS to obtain a PSD 3dB below that of a receiver with Tsys = 150 K in the downlink.

(3) DSSS with a PN sequence duration greater than 1 minute

(4) Bit Error Rate (BER) (Uncoded)

(5) Data Rate is to be maximized within these limits

Assumptions

Typical values are taken for receiver temperatures. For the earth station, , while for the satellite, . The temperature for the satellite is much higher, as the whole field of view of the dish sees Earth (~290 K) which adds up to the total system temperature.

As the locations for the communications earth stations have not been chosen yet, we assume a worst case distance from earth station to satellite of 40 000 km.

The antenna for the satellite is chosen to have a diameter o 1.0 m such that the whole communications system can be shipped assembled and ready to operate. The earth station dish will have a diameter of 10 m. For both antennas, an aperture efficiency of 0.3 is an adequate estimate.

This yields gains of 30 and 50 dB respectively, a path loss of 200 dB, and an overall loss of 120 dB.

Link Design

Pulse shape and symbol rate

Root-raise cosine pulses with parameter will be used to represent each digital symbol. As we are using DSSS, each symbol is a "chip". This allows a maximum chip rate of:

Modulation

QPSK is used for modulation which has a BER that can be approximated by:

The maximum BER specified yields a minimum SNR (C/N) of 11.3 dB. Using

yields minimum received carrier powers of -113 and -109 dBm by the earth station and the satellite respectively, while the minimum transmitted powers need to be 7 dBm and 11 dBm.

DSSS and Data Rate

Our data stream is encoded with a pseudo-random sequence of length bits, or 85.9 seconds. This is a non-maximal length sequence, based on a shift register longer than 32 bits, and can be changed by pre-loading the register adequately. If an intruder does not know both the sequence (which we will change over time) and the starting time of the sequence, it would require a prohibitively large amount of computing power to decode our signal. By spreading the signal in the spectrum, interference and potential jamming is also mitigated.

To achieve the specified PSD reaching the ground, the SNR for the spread signal has to satisfy

is the system temperature of the hypothetical receiver in specification (2), while is the system temperature of our earth station receiver. This yields a processing rate of

which we round up to M=32 for modulo-2 convenience. This in turn yields our maximum data symbol rate and data bit rate for our modulation scheme

Forward Error Correction

We'll incorporate a Reed-Solomon code with rate 7/8, which will result on a BER several decades below the specified uncoded rate. This will result in a practically error-free link with a data rate of 2.73 Mbps.

Encryption

Using frequency spreading as described previously, the communications signal will be buried below the noise floor. To further ensure system security, 256-bit Advanced Encryption Standard (AES) will be employed. A 256-bit key is essentially impossible to crack, and this is the same encryption used by the U.S. Government to protect Top Secret information. SunWire customers' investments in our reliable space solar power system are secure and guaranteed.

REFERENCES

[1] T. Pratt, Satellite Communications, Second Edition, Wiley, 2003.

[2] B. P. Lathi, Modern Digital and Analog Communication Systems, Third Edition, Oxford, 1998.