Simulating the Performance 0f Communication Links with Satellite Transponders Bruce Elbert Application Technology Strategy, Inc. [email protected] Maurice Schiff Elanix, Inc. [email protected] Introduction Some time ago, the Defense Department adopted a procurement policy “fly before you buy”. In the modern world of telecommunications the equivalent concept might be termed “simulate before you build”. The economics associated with the link performance are quite severe. Even a small degradation will affect the system data rate or coverage, both of which are related to capital and operating expenses. It is crucial to have all of the system design parameters optimized before a heavy commitment to implementation. Furthermore, when things go wrong in the actual article during construction or initial operation, a simulation model can be used to track down the offending element. The simulation will also be useful for pre-testing any corrective action before attempting it either in space or on the ground. In this article we use SystemView by Elanix to explore the end-to-end simulation of communications links involving satellite transponders. We will describe the various impairments to the system and computer models used to simulate their effects. Satellite Transponder Communications Link A transponder is a broadband RF channel used to amplify one or more carriers on the downlink side of a geostationary communications satellite. It is part of the microwave repeater and antenna system that is housed onboard the operating satellite. Examples of these satellites include AMC 4 and Telstar 5, located at 101 and 97 degrees west longitude, respectively. These satellites and most of their cohorts in the geostationary orbit have bent-pipe repeaters using C and Ku bands; a bent pipe repeater is simply one that receives all signals in the uplink beam, block translates them to the downlink band, and separates them into individual transponders of a fixed bandwidth. Figure 1 shows the basic concept. Each transponder is amplified by either a traveling wave tube amplifier (TWTA) or a solid state power amplifier (SSPA). Satellites of this type are very popular for transmitting TV channels to broadcast stations, cable TV systems, and directly to the home. Other applications include very small aperture terminal (VSAT) data communications networks, international high bit rate pipes, and rural telephony. Integration of these information types is becoming popular as satellite transponders can deliver data rates in the range of 50 to 150 Mbps. Achieving these high data rates requires careful consideration of the design and performance of the repeater. The most significant impairments to digital transmission come about in the filtering, which constrains bandwidth and introduces delay distortion, and the power amplification, which produces AM/AM and AM/PM conversion. These effects will be discussed in detail later in this article. For maximum power output with the highest efficiency (e.g., to minimize solar panel DC supply), this amplifier should be operated at its saturation point. However, many services are sensitive and susceptible to AM/AM and AM/PM conversion, for which backoff is necessary. With such an operating point, intermodulation distortion can be held to an acceptable level, however, backoff also reduces downlink power. The transponder itself is simply a repeater. It takes in the signal from the uplink at a frequency f1, amplifies it and sends it back on a second frequency f2. Figure 2 shows a typical frequency plan with 24-channel transponder. The uplink frequency is at 6 GHz, and the downlink frequency is at 4 GHz. The 24 channels are separated by 40 MHz and have a 36 MHz useful bandwidth. The guard band of 4 MHz assures that the transponders do not interact with each other. System Impairments The transponder is a central element in the end-to-end communications link, illustrated in Figure 1. This drawing provides a simplified system block diagram that shows the impairments that affect the system performance. Thus, the transmitting earth station on the uplink side will cause its share of distortion as will the receiving earth station on the downlink side. Some of this distortion is uncorrelated, which means that its contribution can be added more or less algebraically. However, for this to be correct, one must know the individual contributions. Other types of distortion, notably group delay, AM/AM and AM/PM, interact with one another and independence is no longer assured. Simple link budgeting techniques are available for evaluating links with additive noise, however, a communications simulation tool like SystemView by Elanix is necessary for analyzing related impairments and their interaction. Another benefit of this approach is that both theoretical and measured data can be included in the simulation models. Filter gain and phase distortion: These are common elements in a communication link. Most filters are designed in the frequency domain in terms of their type (Elliptic function, Chebechev, Bessel, etc.) and order. This information is linked to the filter poles and hence the frequency response. In a time domain simulator such as SystemView by Elanix the time impulse response is derived from the frequency response, and the filtering action is a convolutional operation. In a frequency domain type implementation, the data is processed in blocks, which leads to signal discontinuities at the block transitions. Thermal Noise Thermal noise is the most common impairment in a wireless communication system. There are three general sources, 1) The noise that enters the antenna with the signal, aptly called antenna noise, 2) the noise generated due to ohmic absorption in the various passive hardware components, and 3) noise produced in amplifiers through thermal action within semiconductors. The noise is simulated as a Gaussian random variable with noise power spectral density No = kT = 1.381E-23T w/Hz. The system temperature T is computed by adding the contributions of the three system noise sources. This is easily simulated with SystemView by Elanix because each noise source is generated from a different key (seed) to insure that they are not correlated. Antennas and low noise amplifiers are typically rated in Kelvin (degrees above absolute zero), which allows the simple translation to noise power spectral density. If the bandwidth of the RF carrier is known, then the total noise power is simply the product NoB. TWT AM/AM and AM/PM conversion The TWTA is a common element in earth station s and communication satellites. For an input sine wave of frequency f and amplitude r, the TWTA is characterized by the relationship y(t)=A[r(t)]sin(2πft+φ[r(t)]) The empirical relations 2 A(r)=arr r/[1+b r ] 22 ϕ(r)=aϕϕ r /[1+b r ] describe A(r) and φ(r). The first term is called AM/AM conversion, and the second is AM/PM conversion. The four contestants [arr , b ,aϕ ,bϕ ] can be determined from the actual TWTA tube measurements via a least square fit. Another and simpler approach is to enter the measured TWTA data into a text file and use simple table look up for the required values. The term A(r)/r is the nominal gain. A plot of A(r) shows the output power increasing with the input, and then leveling off and actually decreasing as the input power continues to increase into the overdrive region. This is the saturation phenomena mentioned above. As discussed later for DVB-S, the TWTA must be operated a little bit below saturation to control sideband regeneration. Figure 3 shows the typical TWTA AM/AM curve indicating the definition of the important parameter, back off (BO). The operating point should be optimized, as described later, for the specific transmission system. Pre-Amplifier and mixer non linearties Amplifier types other than the TWTA as well as the mixers used to translate the signal frequencies have nonlinear aspects as well. Generally the transfer function of such devices isdescribed in terms of a polynomial 2 yt()=+ a bxt () + cxt () + .... where the coefficients are chosen to satisfy common figures of merit such as the two tone third order intercept point , IP3. The coefficient, a, is the linear gain term. The problem arises when there are two or more input signals in the input x(t) of the form, xt( )=+ A sin(2π ft12 ) B sin(2π ft ) This is common in shared transponder operation where several carriers occupy the usable bandwidth. Substituting into the above and using standard trigonometric identities show that the output y (t) can have frequency intermodulation (IM) products with frequency values ±±mf12 nf , where m and n are integers. Generally the power in the IM product decreases with increasing m and n. The worst case generally occurs in the so-called third IM product when m = 2 and n = 1 and vice versa. Using figure 2, consider two wideband signals in the uplink to the satellite, one at f1 = 6105 MHz (channel 9 uplink), and one at f2 = 6065 MHz (channel 7 uplink). The typical satellite employs a wideband frequency-translating receiver that provides about half of the 100 dB total repeater gain. Each pair of carriers creates two third order IM products: f3 = 2*6105 – 6065 MHz = 6145 MHz (uplink channel 11), and f3 = 2*6065 – 6105 MHz = 6025 MHz (uplink channel 5). Figure 4 shows the signal spectra just described. Careful modeling of these third order and higher IM products is therefore essential. Another requirement is accurate accounting of all such products that can be produced within the transponder bandwidths. Even weak, high order IM products in the wrong place can be disastrous. We want to insure that the satellite does not jam itself. This mechanism can affect multiple signals within one channel as commonly employed by single channel per carrier (SCPC) systems.