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Communications Systems Design

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Communications Systems Design White Paper Communications Systems Design This white paper presents an over- view of system level design for wireless communications equip- ment. It will be useful not just to those involved in the development of new communications systems, but also to designers of new prod- ucts using existing systems, or anybody seeking to develop new architectures and/or components to reduce product cost and/or size. It also describes the benefits and implications of developing highly Figure 1: Wireless street lighting control system (courtesy of Telensa) integrated System on a Chip (SoC) Introduction limitations, legislative restrictions, solutions, which dominate in today's Radio (wireless) communications development budgets, unit cost targets high volume communications prod- systems have moved a long way in the and timescales. The design process is ucts. last 100 years. From radio broadcast- complex, requiring an in-depth under- ing, the first mass market application, standing of technology, radio systems, to today's ubiquitous Smartphones digital processing and the ability to offering an array of applications and accurately simulate the effects of a communication at ever increasing data practical realisation at a system level. rates. This march of increasing func- tionality and complexity was enabled Applications and Requirements by the advent of the integrated circuit Although consumer demand for wire- and the microprocessor, which together less data is relentless, it would be have facilitated miniaturisation and wrong to think that all current develop- low-cost high volume manufacture. ments in communications are aimed at maximising traffic throughput over Modern communications systems pro- wide channel bandwidths. This type of liferate throughout the available spec- application certainly occupies an enor- trum and use a wide range of channel mous amount of global development bandwidths, modulation schemes and effort, but there are also applications data rates. The main traffic for wireless where only a tiny amount of informa- communications systems is now data tion needs to be communicated between rather than voice and the required data sites and on an infrequent basis. rates are ever rising. When designing a communications system the imple- A good example of a modern low mentation choices depend on a host of data-rate application is in the monitor- requirements, some of which can be ing and control of street lighting, such conflicting. as that depicted in Figure 1 (courtesy of Telensa). These include the required functional- ity and performance, the available Here each street lamp is fitted with a technology, operational conditions and small two way radio link allowing Sheet•Code••RFi0611 Plextek RF Integration Plextek Ltd., London Road, Great Chesterford, Essex CB10 1NY UK T: +44 (0) 1799 533200 E: [email protected] W: www.plextekrfi.com monitoring and control by the local communications beyond the range of (excessive re-charging of mobile council (City authority). The council the normal cellular services. devices) but for other applications can control when the street lights are (vehicle security devices, remote meter turned on, and to save energy late into Satellite Satellite receivers are very reading devices) a battery life of many the night, when they can be dimmed to sensitive. The signal levels are very years is essential for effective commer- a lower level or switched off. The street low by the time they arrive on the cial deployment. Such a requirement light can report back the state of its bulb ground and special attention is paid to must be considered from the outset and and ballast, or more recently LEDs, so minimising their noise figure. The will be a key parameter in the system that replacement can be undertaken down side is that they are more prone design process. when near end-of-life. The amount of to interference from high power data that needs to be communicated systems on nearby frequencies. Special Modulation Schemes and Data to/from each light is small and infre- protection is given to the satellite Rates quent. There is currently strong and frequency bands by restrictions on The one resource that all radio applica- growing interest in low data rate M2M other users nearby to ensure their tions have in common is the radio (machine-to-machine) communications radiated power was not excessive. spectrum and this has to be shared by such as this. However, the demand for growth of all users. As the number of applications cellular services has recently put this continue to increase, it becomes more Another growth area in communica- protection under pressure with the and more important that the use of this tions is high data rate links for personal allocation of cellular band frequencies resource is undertaken efficiently. consumer applications. High data-rate close to some satellite bands. WLAN and cellular communications is In all cases, the modulation must be now the norm and is expected by users. Battery life is an important design appropriate for the application, as effi- As higher and higher wireless data rates parameter in many communications cient as possible and constrained to the have become available, new applica- products. bounds of the allocated channel to tions evolve to make use of the addi- avoid spectral leakage into adjacent tional capacity. The volume of data that In some cases this simply needs to be must be transferred is greatly amplified long enough to avoid inconvenience in the back-haul network where the live data from thousands of users needs to be transferred between the local cellular tower and the cellular operator regional control centre. Microwave line of sight links are often used for this purpose, operating at high frequencies where wide channel band- widths are available. In addition to using wide channel bandwidths, micro- wave links make use of higher order modulation schemes to squeeze more data through each channel. Satellite systems allow truly global coverage. Global Navigation Satellite Systems (GNSS) provide one-way communications of navigational infor- mation worldwide. Two way satellite services are provided by other organi- sations and are used by emergency services in times of disasters as well as Figure 2: Combined I-Q constellation diagram by news reporters and those who need Plextek RF Integration Plextek Ltd., London Road, Great Chesterford, Essex CB10 1NY UK T: +44 (0) 1799 533200 E: [email protected] W: www.plextekrfi.com channels which could potentially inter- fere with users there. There are many different modulation schemes operating in systems around the world today. For microwave point- to-point links, so called "high order" modulation schemes such as QAM64, QAM256 & QAM512 are commonly used. These offer high data transfer rates as each symbol of the modulation represents 6, 8 and 9 bits of data respec- tively. Their high data density comes at a price as it places challenging require- ments on the error vector magnitude (EVM), the recovered eye diagram and the control of the symbol detection points. Figure 3: BER versus Eb/No for different modulation schemes,M=4: To illustrate this, Figure 2 overlays four QPSK, M=16: QAM16 etc. separate modulation schemes on a single constellation diagram. The four distortion required to displace one par- The higher the order of modulation schemes: QPSK, QAM16, QAM32 & ticular constellation point from its used the higher the BER for a given QAM64 all have the same mean power region into that of another is smaller noise level. Figure 3 shows a plot of level, but it is clear that the individual than that required for a low order BER versus Eb/No for a range of constellation points of the highest scheme. modulation schemes, where Eb is the scheme (QAM64) are much closer to energy per bit and No the noise spectral each other than are those of the lowest The presence of noise in the receive density. It is clear that squeezing more scheme (QPSK). channel sets the fundamental limit on data through a given bandwidth by the the Bit Error Rate (BER) that can be use of higher order modulation schemes This makes the higher order scheme achieved for a given modulation has an associated penalty in terms of much more susceptible to noise and scheme. the power that must be transmitted to distortion since the amount of noise or achieve the same BER (assuming a Figure 4: Constellation and eye diagram for QAM256 modulation Plextek RF Integration Plextek Ltd., London Road, Great Chesterford, Essex CB10 1NY UK T: +44 (0) 1799 533200 E: [email protected] W: www.plextekrfi.com Figure 5: QAM 256 modulation spectrum (Blue) and Figure 6: QAM 256 modulation suffering transmit spectral mask (Green) for point-to-point links excessive spectral regrowth due to PA distortions receiver with the same noise perform- In addition to corrupting the constella- It is possible to simulate the entire ance). There is also additional com- tion and causing eye closure, distortion communications system from bits in plexity in the radio design itself. in the transmitter can cause the signal to bits out. This is a time consuming to self-intermodulate and produce task but can be worthwhile particularly Figure 4 depicts the constellation and spectral regrowth (leakage) into adja- in cases where custom System on Chip eye diagram for QAM256 modulation cent channels. Figure 6 shows the (SoC) ICs are under development. without any real-world corruption. spectral regrowth that PA non-lineari- Figure 9 compares the simulated BER ties can cause. Here the regrowth to measured BER for a low-IF demod- In a practical system imperfections in exceeds the statutory limits imposed ulator designed by Plextek RFI and the transmitter, such as amplitude and by the regulatory authorities. implemented as part of a custom ASIC phase distortion in the RF amplifiers now in volume production.
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