ECE 5233 Satellite Communications Prepared by Dr Ivica

ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 8: Satellite link design (Section 4. 1, 4. 2) Spring 2014

Outline ØObjectives of link design ØElements of satellite link ØFree space path loss equation ØSignal to noise ratio and link capacity ØExamples Important note: Slides present summary of the results. Detailed derivations are given in notes. Florida Institute of technologies Page 2

Objective of a link analysis Ø Link analysis determines properties of satellite equipment (antennas, amplifiers, data rate, etc. ) Ø Two links need to be planned o Uplink – from ground to satellite o Downlink – from satellite to ground Ø Two way communication – 4 links (two way maritime communications) Ø One way communication – 2 links (example – TV broadcast) One way communication Ø Two links are not at the same frequency Ø Two links may or may not be in the same band o Fixed / broadcast satellite services – usually same band o Mobile satellite services may use different bands Ø In some systems satellite links may be combined with terrestrial returns Florida Institute of technologies Two way communication Page 3

Elements of a satellite link Ø Transmit power Ø TX antenna gain Ø Path losses o Free space o TX/RX antenna losses o Environmental losses Ø RX antenna gain Ø RX properties o Noise temperature o Sensitivity (S/N and ROC) Ø Design margins required to guarantee certain reliability Note: satellite signals are usually very weak – requires careful link budget planning Florida Institute of technologies Page 4

Free space path loss – transmit side Ø Free Space Path Losses (FSPL) due to dispersion of EM wave energy Power flux in the direction of maximum radiation Ø Antenna used to focus the energy of the wave in the direction of the receiver Ø Note: antenna gain is usually quoted in the direction of radiation maximum. For other direction need to use the actual radiation pattern Florida Institute of technologies Page 5

Free space path loss – receive side Received power Using One obtains Effective antenna gain (effective aperture) h. A – aperture efficiency of the antenna (50 -90%) FSPL equation Florida Institute of technologies Page 6

Free Space Path Loss (FSPL) Equation for FSPL (linear) R = distance between TX and RX l = wavelength of the RF wave Equation for FSPL (logarithmic) – Friis’ equations Notes: FSPL grow 20 d. B/dec as a function of distance FSPL grows 20 d. B/dec as a function of frequency FSPL curves 1 -32 GHz range FSPL curves are straight lines in log-log coordinate system For Geo-Stationary satellites – loss may be above 200 d. B! Florida Institute of technologies Page 7

Additional losses Ø Additional losses o Misalignment of the antennas o Atmospheric losses o Radome losses o Component mismatch losses Ø The additional losses are taken into account through appropriate design margins Ø Typical design margin 5 -10 d. B o Component accuracy o Operating frequency o Required reliability Link equation AL – additional losses Florida Institute of technologies Page 8

Shannon capacity formula Ø Shannon capacity formula – establishes fundamental limits on communication Ø In the case of AWGN satellite channel Minimum energy per bit normalized to noise power density that is required for a given spectrum utilization C – capacity of the channel in bits/sec B – bandwidth of the channel in Hz S/N – signal to noise ratio (linear) Define g = R/B - bandwidth utilization in bps/Hz, where R is the information rate in bps. Note: g is the fundamental measure of spectrum utilization. Ultimate goal of every wireless communication system is to provide largest g for a given set of constraints. Florida Institute of technologies Page 9

Bandwidth utilization vs. power trade-off Ø Bandwidth utilization increases with an increase of available power Ø In power limited regions small increase of power produce significant increase in bandwidth utilization Ø In bandwidth limited region large power increase is required for increase in bandwidth utilization Ø For systems that are in bandwidth limited region – capacity is increased through frequency reuse Ø By combining power and reuse methods, contemporary systems reach spectrum utilization of 3 -7 bps/Hz Note: most of contemporary satellite systems are bandwidth limited – lot of efforts invested in means for spectrum reuse Florida Institute of technologies Page 10

Examples Example 4. 2. 1. A satellite at a distance of 40000 km from a point on Earth surface radiates power of 10 W into antenna gain of 17 d. B. Find the flux density on the Earth surface and the power received using antenna with effective aperture of 10 square meters. Example 4. 2. 2. The satellite in Example 4. 2. 1 operates at a frequency of 11 GHz (Ku band). The gain of the receiving antenna is 52. 3 d. B. Find the received power. Answer: Received power: -126 d. BW Answers: Flux density: 2. 49 e-14 W/m 2 Received power: -126 d. BW (-96 d. Bm) Florida Institute of technologies Page 11