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i i i generate, create ean-13 supplement 2 none for software projects Microsoft Office Excel Website i i TranterBoo Software EAN-13 k 2003/11/18 16:12 page 20 #38. The Role of Simulation 1 . and more detail ed models and simulations of subsystems and components. As the implementation begins, the measured characteristics of components and subsystems are included in the simulation model. We now describe the various phases of the design process and how simulations are used during various phases of the design process.

. Link Budget and System-Level Speci cation Process The design proc ess for a communications system begins with the statement and analysis of user requirements and performance expectations including throughput, error rate, outage probability, and constraints on bandwidth, power, weight, complexity/cost, channel over which the system is expected to operate, and the life expectancy of the system. Based on the user requirements, the systems engineer arrives at an initial concept for the system such as the modulation schemes to be used, the coding and equalization techniques if necessary, and so on. A set of parameter values called A-level speci cations such as power levels, bandwidths, and modulation index are also established during this initial stage of the design.

The overall goal at this point in the design process is to determine a system topology and the parameter values that will meet performance objectives and also meet the design constraints. As stated earlier, the system performance will be a function of the signal-to-noise ratio (SNR, or equivalently the value of Eb /N0 ) and the total distortion introduced by all the components in the communication link. The signal-to-noise ratio is established though a process called link budgeting, which for the most part is a power calculation that takes into account such factors as the transmitted power, antenna gains, path losses, power gains, and noise gures of ampli ers and lters.

1 While the link budget is not the primary quantity of interest in simulations, it does establish a range of values of S/N or Eb /N0 over which simulations for performance estimations have to be carried out. Since it is impossible to build ideal components, practical implementation of components like ampli ers and lters will produce nonideal behavior. As a result, signal distortion will be induced, which will impact system performance.

This is taken into account in the link budget by calculating the performance of the system with ideal components and then including implementation losses that account for performance degradation due to the signal distortion induced by nonideal components. The implementation loss is a measure (often an estimate based on prior experience) of how much the Eb /N0 must be increased in order to overcome the e ect of the distortion induced by nonideal components. Sometimes the implementation losses are also referred to as communication or distortion parameters.

Note that some parameters, such as lter bandwidths, might a ect the noise power at. 1 A link budget European Article Number 13 for None typically takes the form of a spreadsheet in which all of the system gains and losses (both signal and noise) including propagation loss, antenna gains, ampli er noise, cabling losses and other e ects are identi ed and numerically de ned. These are usually expressed in dB. After the spreadsheet is complete, the SNR necessary at the receiver input for the required level of performance is determinied.

Using the spreadsheet, one can then work backward and determine the transmitter power required to achieve the required performance.. i i i i i TranterBoo Software EAN 13 k 2003/11/18 16:12 page 21 #39. Section 1.5. The Role of Simulation various points in the system and this in turn will impact the link budget as well as the distortion. The system designer starts with an initial con guration for the system, the A-level specs and the link budget. The link budget is expressed in a spreadsheetlike format, and the bottom line in the link budget is the net Eb /N0 at a critical point in the system after all the implementation losses have been taken into account.

This critical point is often the receiver input. The link budget is said to be closed or balanced if the link has su cient Eb /N0 , with a safe margin, to produce acceptable system performance. There are many di erent measures of system performance.

As examples, analog systems often use mean-square error as a performance measure while a typical performance measure for digital systems is the bit error rate. At this point in the design process, the performance metric is computed from approximate formulas and not simulated. Since all of the implementation losses have been accounted in the net Eb /N0 , the BER, for example, can be computed using the formula for an ideal system.

If the link budget does not close or balance, then the A-level speci cations, the implementation losses, and even the system con guration are changed and the link budget is recomputed. For example, the bandwidth of one or more lters may be changed, the antenna size (gain) may be increased, and the speci cation of the noise gure of an ampli er might be lowered. This process is continued until the budget is balanced or closed with an adequate margin.

Based on the initial system con guration, the A-level speci cations and the link budget, which is now assumed to be closed, it should be possible to construct a simulation model that can be used to verify the link budget and re ne the design. Performance measures can be estimated accurately and performance degradations due to nonideal implementations can be veri ed through detailed simulations. If the allocations in the link budget are veri ed through simulation and the link budget is still closed, the design process then proceeds to the next stage, which involves the detailed design and implementation of subsystems and components.

If the link budget does not close, then some of the distortion allocations are changed and the system topology and A-level specs might have to be changed. For example, the coding gain might have to be increased and the speci cations on the linearity requirements of an ampli er might be changed. Also if the simulation indicates that the distortion due to a component is less than what was allocated to that component in the link budget, the resulting savings can be applied to relax the requirements for some other component (i.

e., more distortion can be tolerated elsewhere in the system). This iterative process continues until the link budget is balanced.

A balanced link budget provides the initial speci cations for hardware (and software) development. This initial phase of the design involves a considerable amount of art, and it is usually done by someone who has a considerable level of experience in designing communication systems. In most cases the initial design will be based on previous designs for similar systems with minor modi cations.

In other words, new designs are often evolutionary or incremental in nature..
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