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Part II Front End Technology Front End design – including RF Architecture, Data Conversion and Digital Front Ends – has emerged as a key issue as SDR techniques are finding themselves increasingly embodied by stealth into today’s new products. The radical solution – ‘Pure’ Software Radio, with A/D conversion at the antenna – is not yet feasible at GHz carrier frequencies. However, recent technology advances suggest it may be nearer than had been thought. Software Defined Radio Edited by Walter Tuttlebee Copyright q 2002 John Wiley & Sons, Ltd ISBNs: 0-470-84318-7 (Hardback); 0-470-84600-3 (Electronic) 2 Radio Frequency Translation for Software Defined Radios Mark Beach, John MacLeod, Paul Warr University of Bristol In an ideal world, a software defined radio (SDR) would be able to transmit and receive signals of any frequency, power level, bandwidth, and modulation technique. Current analog receiver and transmitter hardware sections are still a long way from being able to achieve this ideal behavior. It is the aim of this chapter to explain why this is the case, to present some design techniques for synthesis of SDR RF translation architectures, and to consider where the breakthroughs in technology are required if the RF hardware component of the ideal SDR 1 is to become a reality. This chapter is structured in four parts. Initially, we gather data to define the requirements for the illustrative design of SDR hardware for commercial wireless applications. In the second part, we attempt to define the problems that are associated with the design of SDR hardware, both the receiver and the transmitter aspects. In the third, we consider techniques which may be of value in solving these problems before finally drawing some conclusions. In considering these requirements, the chapter is based around the proposition that our SDR must be able to process the major European air interface standards. The performance requirements that these standards demand are then surveyed. Basic receiver design is considered by looking first at the architectural issues. The pros and cons of single and multiple conversion architectures are then discussed. The important issue of circuit linearity, and its relationship with blocker specifications, is examined. Alternative specifications for radio linearity are defined, and the reason for their use is briefly explained. ‘Signal budget,’ as a basis for the receiver design, is discussed. An example is given to show how performance specifications relate to this budget, and the consequent performance speci- fication of individual circuit elements. Image signals and the problems they cause in the 1 The term ‘ideal SDR’ used in this chapter should not be confused with the concept of the ‘pure’ software radio. The latter refers to a software radio with A/D conversion at the carrier frequency; the former refers to a fully flexible and dynamically reconfigurable ‘pragmatic’ software radio, still incorporating radio frequency translation stages prior to the A/D converter and digital processing subsystem. Software Defined Radio Edited by Walter Tuttlebee Copyright q 2002 John Wiley & Sons, Ltd ISBNs: 0-470-84318-7 (Hardback); 0-470-84600-3 (Electronic) design of a superheterodyne SDR receiver are then covered. The relationship of the image signal to the IF frequency, as well as the relationship of the required image rejection to the receiver blocker specifications, is also explained. The design of an SDR transmitter is introduced by explaining that the filter problems within a receiver mirror their analogs within the SDR transmitter. Transmitter architectures are examined, and similar conclusions are drawn about their architectures as were drawn about the receiver. Transmitter linearity and the related efficiency issues are highlighted and techniques of PA linearization briefly summarized. The zero IF stage is examined compre- hensively, including a detailed description of the specific issues which arise from this archi- tecture. Flexible filtering is then discussed and a brief summary of the ways in which it could be realized, or has been realized, is presented. Emphasis is placed on ensuring that the tuning does not introduce nonlinearities into the circuit performance and an introduction is presented into the potential role that micro-electro-mechanical system (MEMS) technology could play in future designs. The body of the chapter concludes with an examination of the ‘Low IF’ design as a possible compromise between the superheterodyne and zero IF architectures. 2.1 Requirements and Specifications There are three driving forces for the development of SDR. The first impetus derives from the requirement that a mobile phone can provide ‘world roaming.’ This means that the phone, as well as being able to operate in Europe to the GSM radio standard, should be able to operate in the United States to their IS54 and IS95 systems, and in Asia and Japan with their PDC and PHS systems. The second stimulus revolves around trying to combine the performance features of a radiotelephone (GSM, DECT, and UMTS), with the functionality of a personal area network (PAN), (e.g. Bluetooth), and that of a local area Network (LAN) (e.g. HIPER- LAN). The third motivation is that SDR could drive down the production costs through the scale economies of a single radio platform for multiple standards and hence markets. Improvements could be made via ‘software upgrades’ and the radio could be ‘future proofed’ to some degree. In this section we review the radio specifications of some of the major European commu- nications standards to define the performance that will be required of an SDR capable of encompassing all these air interface standards. It would be possible to spread the net wider and also embrace North American and Asian standards; however, the European standards are sufficiently representative to highlight all the major issues involved. [1–8] 2 2.1.1 Transmitter Specifications The most important design parameters when dealing with SDR transmitter design are: X output power level X power control range X spurious emissions Software Defined Radio: Enabling Technologies26 2 These standards embrace standard voice telephony (GSM and DECT), 3G standards (UMTS), personal area networks (Bluetooth), and local area networks (HIPERLAN/2). Frequency of operation is also the other obvious parameter – this is discussed following the separate transmitter and receiver specification discussions, with requirements summarized later in Figure 2.2. 2.1.1.1 Transmitter Output Power Levels The power levels to be provided by a mobile station depend on the standard and on its class. In all cases, the transmitter should provide output power control over a significant range to comparatively fine tolerances. This will impose critical challenges on the architecture used. Table 2.1 summarizes this information. 2.1.1.2 Spurious Emission Specifications All air interface standards specify spurious emission with a mask specification. They are best summarized graphically. A graphical summary of all ‘in-band’ spurious emission specifications is included in Appendix A1 to this chapter. Out-of-band emissions are also specified in the relevant standards. For information on these the reader is referred to references [1–8]. 2.1.2 Receiver Specifications The most important design parameters when dealing with SDR receiver design are: X input sensitivity X maximum expected input signal X blocker specifications 2.1.2.1 Input Sensitivity and Maximum Input Level Table 2.2 summarizes the sensitivity requirements of our target group of air interface stan- dards. 2.1.2.2 Blocker Specifications All air interface standards specify blocker signal levels with a mask type of specification. Again, this is best summarized graphically and is included as Appendix A2 to this chapter. 2.1.3 Operating Frequency Bands Table 2.3 lists the frequency bands for the air interface standards considered in this chapter; this information is also shown graphically in Figure 2.1. Radio Frequency Translation for Software Defined Radios 27 Software Defined Radio: Enabling Technologies28 Table 2.1 Transmitter power output and power control specifications Air-interface standard Nominal maximum output power Nominal minimum output power (dBm) Power control Terminal class Maximum power (dBm) Levels Power range (dBm) Step GSM 900 2 39 5 0–239 337 3–15 37–13 2 dB 433 16–18 11–72dB 529 19–31 5 DCS 1800 1 30 0 29 36 224 30–31 34–32 2 dB 336 0–830–14 2 dB 9–13 12 – 42dB 14 2 15–28 0 Nominal output power Level Power (dBm) DECT 1 4 224 UMTS-FDD 1 33 2 44 Steps 227 1 324 2 421 3 UMTS-TDD 2 24 2 44 321 Bluetooth 1 20 1 4 P min ,24 P min to P max 24 2 6 P min ,230 P min to P max 30 – P min ,230 P min to P max Band (MHz) Power (dBm) HIPERLAN/2 5150–5350 23 EIRP 5470–5725 30 EIRP 2.2 Receiver Design Considerations 2.2.1 Basic Considerations The basic receiver function is to take a real, low power, RF signal and down-convert it to a complex (in-phase and quadrature, I/Q) baseband signal. During this process, the signal power level is increased. The following list describes the characteristics of the input signal to a hypothetical SDR receiver and the output signal from that receiver. The characteristics of the input signal are: X signal type real X low power down to –107 dBm X high dynamic range up to 2 15 dBm X spectrum band pass, with center frequencies varying from 876 MHz to 5725 MHz Radio Frequency Translation for Software Defined Radios 29 Table 2.2 Input signal level specifications Air interface standard Reference sensitivity level (dBm) Maximum input level (dBm) GSM 900 Small MS 2 102 2 15 Other MS 2 104 DCS 1800 Class 1 or Class 2 2 100/ 2 102 2 23 Class 3 2 102 PCS 1900 Normal 2 102 2 23 Other 2 104 DECT 2 86 2 33 UMTS (FDD) 12.2 kbps 2 92 64 kbps 2 99.2 144 kbps 2 102.7 384 kbps 2 107 UMTS (TDD) 2 105 Bluetooth 2 70 2 20 HIPERLAN/2 6 Mbps 2 85 Receiver class 1 ¼ 2 20 9 Mbps 2 83 Receiver class 2 ¼ 2 30 12 Mbps 2 81 18 Mbps 2 79 27 Mbps 2 75 36 Mbps 2 73 54 Mbps 2 68 Worst case parameter 2 107 2 15 The characteristics of the output signal (to a digital subsystem) are: X signal type complex (I/Q) X spectrum baseband, with bandwidth up to 20 MHz X dynamic range reduced by AGC to meet requirements of the ADC In doing this, the receiver must X keep the signal power sufficiently greater than the noise power, to ensure the output signal-to-noise ratio is sufficiently high to allow appropriate BER performance of the modulation scheme used; X ensure that high power input signals do not overload components of the receiver; X ensure that high power nearby signals (blockers) do not effect detection of the wanted signal; X ensure that signals of the wanted frequency can be separated from signals at the image 3 frequency. The first two points on this list are generally accommodated by careful design. The latter two points are problems that can be tackled by selection of an appropriate architecture and Software Defined Radio: Enabling Technologies30 3 Image signals are discussed further under ‘Image Rejection’ within this section on receiver design. Table 2.3 Frequency of operation of major European air interface standards Air interface standard Uplink (MHz) Downlink (MHz) Duplex spacing (MHz) GSM 900 890–915 935–960 45 E-GSM 900 880–915 925–960 45 R-GSM 900 876–915 921–960 45 DCS 1800 1710–1785 1805–1880 95 PCS 1900 1850–1910 1930–1990 80 DECT 1881.792–1897.344 1881.792–1897.344 Not applicable – a TDD system UMTS FDD (Europe) 1920–1980 2110–2170 190 UMTS FDD (CDMA 2000) 1850–1910 1930–1990 80 UMTS TDD 1900–1920 1900–1920 – (Europe) 2010–2025 2010–2025 UMTS TDD (CDMA 2000) 1850–1910 1850–1910 – 1930–1990 1930–1990 1910–1930 1910–1930 Bluetooth USA, Europe, & most other countries 2400–2483.5 2400–2483.5 – Spain 2455–2475 2455–2475 – France 2446.5–2435 2446.5–2435 – HIPERLAN\2 5150–5350 5150–5350 5470–5725 5470–5725 – application of appropriate technological ‘fixes’ such as image reject mixing, linearization, and variable preselect filters. Important commercial requirements, which place constraints on this, are: X ability to be manufactured as an integrated circuit, with a minimum of external compo- nents; X low power consumption to allow portable operation with long battery life. The next section discusses the comparative advantage of various receiver architectures; subsequent sections describe important considerations in practical receiver design. Radio Frequency Translation for Software Defined Radios 31 Figure 2.1 Diagrammatic representation of the operating frequencies of the major European air interface standards (excluding HIPERLAN/2) 2.2.2 Receiver Architectures The primary distinction between receivers is the number of stages taken to down-convert a signal to baseband. Direct conversion takes one down-conversion; superheterodyne receivers employ two or more. In general, complexity increases with the number of down-conversions. As we explore alternative architectures it will be shown that the simplicity of direct conver- sion brings with it several technical problems which would appear to make direct conversion architecture inappropriate for an SDR receiver. These issues are treated in more detail later in the chapter. 2.2.2.1 Direct Conversion Architecture A basic direct conversion receiver architecture is shown in Figure 2.2. This receiver consists of a low noise amplifier (LNA) which provides modest RF gain at a low noise figure. The output signal from the LNA is filtered in a preselect filter, and down-converted in a complex (I,Q) mixer. The majority of the gain and automatic gain control (AGC) is provided in a high gain baseband amplifier. Its advantages are: X low complexity X suitable for integrated circuit realization X simple filtering requirements X image signal suppression is easier (compared to multiple conversion architecture) Its disadvantages are: X A local oscillator is required, in which the two output signals are accurately in phase quadrature and amplitude balance, over a frequency range equal to the frequency range of the input signal. Software Defined Radio: Enabling Technologies32 Figure 2.2 Direct conversion receiver architecture X The mixers needs to be balanced and to be able to operate over a correspondingly wide frequency band. X Local oscillator leakage through the mixer and LNA will be radiated from the antenna and reflected back into the receiver from that antenna. The reflected signal will vary with the physical environment in which the antenna is placed. This ‘time varying’ DC offset caused by ‘self-mixing’ is a problem. X Most of the signal gain occurs in one frequency band creating the potential for instabil- ity. X 1/f noise is a major problem. X Second order distortion product mix down ‘in-band’. All of these points are explained in more detail later in the chapter. 2.2.2.2 Multiple Conversion Architecture A multiple conversion receiver is shown in Figure 2.3. Its advantages are: X good selectivity (due to the presence of preselect and channel filters; X gain is distributed over several amplifiers operating in different frequency bands; X conversion from a real to a complex signal is done at one fixed frequency; therefore a phase quadrature, amplitude balanced, local oscillator is only required at a single frequency. Its disadvantages are: X the complexity is high; X several local oscillator signals may be required; X specialized IF filters are required; this makes it impossible to achieve single chip reali- zation of a superheterodyne receiver. Radio Frequency Translation for Software Defined Radios 33 Figure 2.3 Multiple conversion superheterodyne architecture [...]... certainly exceed the noise floor of the ADC 2.2.6 Image Rejection A problem that arises uniquely with software radio RF design is how to accommodate image signals An image signal is a signal of such a frequency that, along with the wanted signal, it Radio Frequency Translation for Software Defined Radios 45 Figure 2.13 Received signal levels for W-CDMA will be mixed down to the set IF frequency Image signals... useful notation when dealing with intercept points of other orders (second-order intercept point IP2) 6 The ‘two tones’ refers to the two input signals, at frequencies f1 and f2 7 Other components such as 2f1 1 f2 will appear a long way out-of-band, and will thus not pass through the IF filtering Software Defined Radio: Enabling Technologies 36 Figure 2.5 Typical spectrum analyzer display used to calculate... Equation (2) is extremely well known and is quoted in a number of references; see for example [11], pp 169–171 Equation (3) is less well known, and is a worst case description Radio Frequency Translation for Software Defined Radios Figure 2.6 37 Cascade connection of amplifiers of the overall distortion performance of an amplifier chain; see [12], pp 219–232 and pp 367–371 Although Equations (2) and (3)... dB This figure can be now substituted into Equation (1) to derive the required input TOI of the receiver as Figure 2.8 Scenario of blockers producing in-band third-order products Radio Frequency Translation for Software Defined Radios TOIin ¼ 223 1 87 ¼ 120:5 dBm 2 39 ð4Þ The output TOI is calculated by adding the receiver gain (in dB) to the input TOI specifications (in dBm) The further we progress down... adjacent channel powers over the channel bandwidth 9 In other words, even if the TOI of the LNA were 20 dBm, the overall TOIOUT would still be about 50 dBm (actually 49.6 dBm) Radio Frequency Translation for Software Defined Radios 41 Figure 2.10 Different ways of quantifying the IMD distortion for wideband modulated or multichannel signals Noise power ratio (NPR) is an alternative way of characterizing... approximates the analog signal that it is converting It can be shown that the signal-to-noise ratio of an ADC is given by    FS SNRQF ðdBÞ ¼ 6:02b 1 1:76 1 10log dB ð7Þ 2BC Radio Frequency Translation for Software Defined Radios 43 where b is the resolution of the ADC in bits, FS is the sampling frequency and BC is the bandwidth of the channel being sampled We will now redraw Figure 2.11 to concentrate... the ADC is set by the resolution of the ADC, combined with the maximum input signal of the ADC † The required AGC range is set by the difference between maximum input signal to the radio and the minimum input signal to the radio (Pin(max) 2 Pin(min)) dB, less the difference between the maximum input power to the ADC and the noise floor of the ADC (PADCmax 2 nADC) dB, plus the Eb/N0 for the modulation... fundamental component increases This is because the power in the third-order component is proportional to the cube of the input power 4 Local oscillator balance and DC offset Radio Frequency Translation for Software Defined Radios 35 Figure 2.4 Illustration of the concept of third-order intercept Second, were it not for saturation at the output, the third-order distortion product would eventually reach... the second IF will require a bigger down-conversion This will cause the image signal to ‘close in’ on the wanted Figure 2.14 Image problems arising from a large second conversion Radio Frequency Translation for Software Defined Radios 47 signal For example Figure 2.14 shows an E-GSM signal being down-converted to a 500 MHz first IF The local oscillator is set to perform a high side conversion and its frequency... having the equivalent of a low IF receiver In the transmitter this will cause the wanted sideband to be closer to the unwanted sideband, making it difficult to remove by filtering Radio Frequency Translation for Software Defined Radios 49 2.3.2.1 Direct Conversion Transmitter A direct conversion transmitter is shown in Figure 2.16 Its advantages are X X X X low complexity suitable for integrated circuit . software radio, still incorporating radio frequency translation stages prior to the A/D converter and digital processing subsystem. Software Defined Radio. also shown graphically in Figure 2.1. Radio Frequency Translation for Software Defined Radios 27 Software Defined Radio: Enabling Technologies28 Table 2.1