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Softwar e Radio Arc hitecture: Object-Oriented Approac hes to Wireless Systems Engineering Joseph Mitola III Copyright c !2000 John Wiley & Sons, Inc. ISBNs: 0-471-38492-5 (Hardback); 0-471-21664-X (Electronic) 3 The Radio Spectrum a nd RF Environment Radio is the penultimate medium for mobile communications, but it has also been used for many fixed-site applications such as AM/FM broadcast, satellite trunking, point-to-point microwave telephony, and digital TV. Although there are radio applications in very low frequencies (VLF) and extremely low fre- quencies (ELF), these bands require extensive fixed-site infrastructure w hose size and cost is dominated by the mile-long antennas and megawatt-power handling requirements. SDR insertion opportunities in these bands are lim- ited. Therefore, this text is concerned w ith the bands in which there are major economic opportunities for software-radio technology insertion: HF through extremely high frequencies (EHF). I. RF SIGNAL SPACE Figure 3-1 shows how terrestrial radio uses cluster in RF signal space. This figure shows the notional clusters of the significant band/mode combinations addressed by software radio technology. This space is the two-dimensional cross product of radio frequency and duty cycle. 17 Since coherent bandwidth is proportional to carrier frequency, the figure is also labeled in terms of nom- inal instantaneous bandwidth. A QPSK encoded T- or E-carrier signal is on continuously for a duty cycle of 1.0. Low-duty-cycle modes such as burst com- munications and ultra-wideband (UWB) have high peak power as suggested by the additional label on the axis. This is not an exact correspondence, but it shows a trend related to the thermal properties of power-handling devices. The PTT modes have the duty cycle of voice, which is about 25% dur- ing speech epochs. Given conversational pauses, a voice channel is typically occupied less than 10% of the time. The busiest military voice channels are occupied not more than 40% in a full duplex channel such as the typical LVHF military bands. On the other hand, troposcatter radios have high peak power and unity duty cycle. The tropo cluster was positioned to show the high peak power. HF communications may also have high power, but the duty cycle is typically that of voice or low-speed data. As the label on the right side of the figure suggests, the greater the ratio of peak power to minimum power, 17 Duty cycle is the ratio of signal on-time to the elapsed time of an epoch. 73 74 THE RADIO SPECTRUM AND RF ENVIRONMENT Figure 3-1 Communications modes cluster in RF signal space. the greater the dynamic range requirements on the ADC in the receiver. Since there is no wideband R F or ADC that can encompass all RF with the full dynamic range, designs historically have addressed a single mode. The SDR addresses a few clusters, while the software-radio architecture embraces mi- gration toward the entire signal space. It is therefore essential to consider each of these clusters in detail. A. Overview of Radi o Bands and Modes This section p rovides an overview of radio bands and modes. HF commu- nications consist primarily of voice, narrowband data, and Morse code, some of which is generated by machine and some of which is generated manually. The literature also presents successful research in the use of wideband spread spectrum at HF, including thousands-of-chips-per-bit and millions-of-chips-per-second (MHz) [119]. In addition, HF radar uses direct-sequence spread spectrum in a frequency-hopped pulsed signal struc- ture. Neither of these relatively exotic waveforms are s hown in order to focus the figure on the waveforms likely to be encountered in software radios. LVHF includes spectrum allocated to military users who traditionally have employed half-duplex PTT analog frequency modulated (FM) single-frequen- cy voice modes. Military LVHF also includes many FH spread-spectrum ra- dios. In addition, the literature describes burst signal structures such as meteor burst. These radios transmit data at relatively high data rates for tens to hun- dreds of milliseconds with high instantaneous data rates, a low duty cycle, and therefore relatively low average data rate. RF SIGNAL SPACE 75 The LVHF and VHF frequency bands also support frequency division multiplexed (FDM) multichannel radios with typically 4 to 12 radio relay telephony channels for military users. These may also employ pulse code modulation (PCM) for digital telephony as alternate modes of an FDM/PCM dual-mode radio. The FDM mode provides compatibility with older equip- ment, but the improved quality of PCM makes it the mode of choice for most applications. For long-haul telephony relay, the FDM or PCM signals may use very high-power propagation modes like troposcatter. Thus, the figure shows a high-power cluster for “relay and tropo.” These high power modes use constant-duty-cycle FDM and PCM waveforms, an excep- tion to the pattern that higher peak power typically implies lower duty cycle. Mobile cellular radio (MCR) operates in frequency allocations between 400 MHz and 2.5 GHz, with clusters at 900 and 1800 MHz. There are similar radio services such as special mobile radio (SMR) as low as 40 M Hz. The Instrumentation Scientific and Medical (ISM) bands at 2 and 5 GHz support personal communications systems (PCS) and RF LANs. MCR has become popular worldwide for rapid deployment of business and residential telephony in developing economies. MCR avoids the burial of fiber or cable for rapid build-out. Wireless local loop (WLL) has most features of MCR with reduced handset mobility [75]. The military employs specialized radar transponders for the Identification of Friends or Foes (IFF) and for other Integrated Communications, Navigation, and Identification Architecture (ICNIA) functions including tactical data links (e.g., remote radar plan position indicator displays) [120]. Distance measure- ment equipment (DME) and tactical air navigation (TACAN) also fall into this category of typically moderate duty cycle and moderate to high instantaneous data rate m odes. Software r adios for the military often must monitor multiple bands and modes for flight safety reasons. They typically require multiple navigation, IFF, and command-and-control communications for redundancy. These modes fall in a cluster of pulsed and lower-duty-cycle/high-peak-power signals. The Synchronous Optical Network (SONET) [5] carries most backbone telephony in developed n ations. Such fibers may be disrupted as much as six times per year per hundred miles of fiber (this rate was an industry rule of thumb in the United States in the early to mid-1990s). Consequently, SONET- compatible high-capacity microwave radios were developed with interoperable data rates of 155 (OC-3) and 622 Mbps (OC-12). Deployments in some in- frastructures protect fiber paths, while others cross obstacles where it may be difficult, expensive or impossible to lay fiber, such as extreme terrain and bodies of water. Interoperation with SONET networks connects SDR nodes to the larger PSTN. Finally, Figure 3-1 shows how radar signals typically emit the highest ra- diated power and employ the lowest duty cycles of any cluster in RF signal space. Impulse radar can create high-resolution maps of hidden objects (e.g., 76 THE RADIO SPECTRUM AND RF ENVIRONMENT by penetrating walls). UWB communications use the same subnanosecond pulse technology operating at baseband. Time Domain Corporation’s UWB system, for example, encodes data into an impulse train with an average of 40 million pulses per second (PPS). Since UWB communications employ subnanosecond pulses not readily synthesized with current-generation SDR hardware (e.g., FPGAs and DSP chips), UWB is not a focus of SDR stan- dardization. On the other hand, as the underlying digital technology continues to evolve into clock rates over 1 GHz, UWB will ultimately migrate into the domain of the SDR. At today’s rate of technology development, UWB will be accessible with SDR technology within 10 years. With the near-term excep- tion of UWB, any of the bands and modes of Figure 3-1 may be implemented using the SDR techniques described in this text. When used together a mix of modes across multiple radio bands provides a new dimension in QoS, reliability, and efficiency in the employment of the radio spectrum. After considering the top-level characteristics of these bands that are relevant to software-radio architecture, each band is considered in detail. B. Dynamic Range-Bandwidth Product As mentioned earlier, the right side of Figure 3-1 is labeled “ADC Dynamic Range.” This highlights the fact that the ratio of lowest to highest power signal in the receiver (total dynamic range) drives the requirements the ADC. As one accesses successively larger chunks of bandwidth, the sampling rate of the ADC must increase to at least 2.0 times the maximum frequency component ( f max ) to satisfy the Nyquist criterion. Sound engineering principles require sampling at 2.5 f max . In addition, the larger bandwidths are needed to service multiple subscribers with a single ADC. Narrowband analog recei vers employ AGC to accommodate m any decades of difference in recei ved signal strength from a high-power nearby subscriber to the weakest, most distant subscriber. Analog receivers also filter high-power interference out of the analog signal- processing band. The near–far ratio (NFR) is the ratio of the highest-power (presumably nearby) signal to the weakest (presumably most distant) signal. This ratio is 90 dB in GSM. Given a requirement for a 15 dB SNR for BER appropriate to the required QoS, the total dynamic range is at least 105 dB. Any in- band interference can raise this total dynamic range further. As the service bandwidth increases, the p robability increases that subscribers and interferers with much higher power will be present in the receiver’s RF band. In an HF band from 3 to 30 MHz, for example, the dynamic range of received signals is typically between 120 and 130 dBc (dB relative to full scale). Since ADCs nominally provide 6 dB of dynamic range per bit, one would need an ADC with 130 = 6= " 22 bits (at least) to service all potential HF subscribers. Contemporary ADCs with the necessary 70 M samples per second (Msps) sampling rates have only 14 (84 dB) of dynamic range. Thus, it is impossible to HF BAND COMMUNICATIONS MODES 77 access the entire HF band with today’s ADC (and DAC) technology. N ear-term implementations therefore m ust tailor the architecture by s tructuring access to each band so that the communications objectives of SDR applications are met within the numerous constraints of available technology, including the ADC. This tailoring process requires an understanding of the HF and other modes presented below. To extend this reasoning further, a multiband multimode radio such as SPEAKeasy was intended to service HF, VHF, and UHF military bands (from 2 MHz to 2 GHz). This means both sustaining the high dynamic range of HF and sampling the 2 GHz bandwidth, requiring a 5 GHz sample rate which is 96.9 dB-Hz. A useful figure of merit , F , for uniform digital sampling using ADCs and DACs is: F = Dynamic Range (dBc) + Sampling Rate (dB/Hz) SPEAKeasy would require F = 226 : 9dB/Hz(96 : 9 dB/Hz + 130 dBc), well beyond the state of the art of 140 to 160 dBc/Hz. Although we are mak- ing progress in ADC technology, practical engineering implementations of software radios avoid the frontal assault of a single ADC. Instead, the art and science of software radio systems engineering includes the partitioning of the total service bandwidth (e.g., from 2 MHz to 2 GHz) into multiple parallel RF bands. These are partitioned further into multiple parallel service bands (ADC/DAC channels). Each subband would have filtering, AGC, and digital signal processing that match the available ADC technology. The RF signal-space suggests regions within which a single ADC may provide effec- tive sampling. The subbands and modes developed subsequently further refine these regions. II. HF BAND COMMUNICATIONS MODES HF extends from 3 to 30 MHz according to international agreement. The def- inition of ITU frequency bands is taken from [5]. The length o f a full- cycle radio wave in these bands is 100 meters at 3 MHz and 10 meters at 30 MHz, with linear variation between these extremes according to c = f # ¸ , where c is the speed of light, ¸ is the wavelength, and f is the radio frequen- cy. Wavelengths determine the physical sizes of resonant antennas. Anten- nas resonate well across bandwidths that are less than 10% of the carrier frequency. To cover a full HF band using such a resonant structure would require about ten such antennas. The alternatives are to physically tune the narrowband antennas to operate on a specific subband, or to use a wide- band antenna to access more of the band at once. A multiband radio there- fore could employ a mix of wideband and tunable narrowband antennas drawn from the conventional antennas described in this and subsequent sec- tions. 78 THE RADIO SPECTRUM AND RF ENVIRONMENT Figure 3-2 The HF communications band. A. HF Propagation As Figure 3-2 suggests, HF radio waves are usually reflected from the iono- sphere, resulting in communications beyond line of sight (LOS). The iono- sphere has several layers from w hich the waves may reflect. These are identi- fied as the D, E, and F layers in order of increasing altitude. Two or more such skywaves may be received in what is called multimode propagation .These waves will add (as complex vectors) at the receiver resulting in phase and amplitude variability. The time differences between two reflected waves (HF propagation modes) will be about 1 ns per foot of altitude separation. Since the reflecting layers may be from 1 to 10,000 miles apart, this equates to 1 to 10 ms of delay-spread. In addition, the ionosphere and fixed transm itters on the earth are typically approaching or receding, imparting Doppler shift onto the RF carrier. Since the layers of the ionosphere may be moving in different directions, the Doppler spread at HF is large, typically 5 Hz. If the RF carrier is too low or too high, it will pass through the ionosphere. Beyond LOS, reflections from the ionosphere are only possible on radio fre- quencies between the least usable frequency (LUF) and the maximum usable frequency (MUF). Specific combinations of RF and antenna configuration can result in near vertically incident (NVI) propagation in which the waves reflect- ing from the ionosphere propagate only a few tens of miles. NVI is useful in mountainous areas for communications between subscribers in adjacent val- leys, for example. In addition, HF will reflect from water and from some land- masses, enabling multihop communications (ionosphere–water–ionosphere– land). HF BAND COMMUNICATIONS MODES 79 B. HF Air Interface Modes Morse code has been used since the 1800s for ship-to-shore and transoceanic communications. Machine-generated Morse code became popular with the emergence of microprocessors in the mid-1980s. PC-based software readily translates text into Morse. Voice transmission at HF uses amplitude modu- lation (AM) to accommodate the limited bandwidth of the HF channel. The simple double side band (DSB) AM creates two mirror-image replicas of the voice waveform—one above and one below the carrier, using twice the band- width required for the information content. Upper side band (USB) filters the lower of these two voice bands, suppressing any residual carrier. Lower side band (LSB) is the converse of USB. Vestigial side band (VSB) allows a small component of carrier to be transmitted, simplifying carrier r ecovery in the recei ver. Each of these modes is used in HF communications. Voice intelligibility requires only 3 to 4 kHz for the principal formants (sinusoidal information-bearing components of the speech waveform). Consequently, each of these modes may be digitally implemented with an ADC rate of typically 10 to 25 kHz using commodity DSP chips with modest processing power (10 to 25 million instructions per second—MIPS). Thus, the speech-processing niche was one of the first commercial applications of ADCs and DSPs. Morse code might be thought of as an on-off-keyed (OOK) data mode with the channel code information carried in the duration (pulse width) of the channel waveform—Morse dits are three to four t imes shorter than daa’s .Be- cause of the relatively low rate at which p eople can compose and send Morse code, it occupies a bandwidth approximately 5 Hz. This yields a plethora of such narrowband signals packed into the very busy HF bands. Other com- mon HF data modes include frequency shift keying (FSK). The FSK channel code consists of mark or space , corresponding to a negative or positive fre- quency shift, respectively. The frequency shift may be as small as a few tens of Hz. Data rates ranging up to 1200 bits per second require FSK shifts of several hundred Hz. An FSK channel symbol is also called a baud .Iten- codes one bit of information. During very short time intervals (from a few milliseconds to a few tenths of a second), the ionospheric transfer function is approximately constant. Higher data rates (e.g., 10 to 40 kbps) may be used for such short intervals to burst small amounts of data over long dis- tances using FSK modems. Both standard and burst FSK waveforms can be implemented using commodity DSP chips and low-speed/high-dynamic-range ADCs. HF Automatic Link Establishment (ALE) equipment [121] 18 probes the propagation path in a pre-arranged sequence to identify good frequencies on which to communicate. The ALE signals include “chirp” waveforms that linearly sweep the RF channel so that the receiver can estimate the channel transfer function. The two ends of the link negotiate choice of RF based on reception quality. 18 The examples of military communications equipment appearing in this chapter are from [121]. 80 THE RADIO SPECTRUM AND RF ENVIRONMENT TABLE 3-1 Software Radio Applications Parameters—Baseband and HF Software Radio Application Sampling Rate ( f s ) Dynamic Range (dB) HF Baseband .5–8 kHz 24–64 Modems 8–32 kHz 48–64 Music 20–100 kHz 60–96 HF-IF .2–10 MHz 72–120 HF RF 75 MHz 130 The research literature also describes a long-haul HF telecommunication system using direct-sequence s pread spectrum to achie ve a data rate of 100 kbps via a 10 MHz spreading sequence [119]. Low grazing angle, nearly optimal choice of transmit and receive frequency, and l ocation and other spe- cialized factors contributed to the success of this experiment, which appears infeasible for general HF communications. The serial modem [122] delivers 1200 to 2400 baud data on HF channels with high reliability. Recently, t he SiCom Viper [424] direct-sequence spread-spectrum radio has demonstrated data rates of 19.2 kbps and 56 kbps over skywave HF links on a routine ba- sis by employing cyclostationary techniques in t he receiver. This 1 to 2 M Hz spread-spectrum signal has an instantaneous SINR of about $ 50 dB, which it overcomes with processing gain. The software radio parameters of HF sampling rate and dynamic range depend on the point in the system at which the ADC/DAC operates from baseband through IF to RF, as illustrated in Table 3-1. C. HF Services and Pro ducts Amateur radio (ham), commercial broadcast, aeronautical mobile, amateur satellite, and timing/frequency standards are provided at HF as outlined in Fig- ure 3-2. HF antennas and power amplifiers often dominate the size, weight, and power of HF radio systems. Antennas matched to HF wavelengths are large—some research antennas extend for over a kilometer. M ilitary applica- tions employ circularly disposed array antennas for long-haul communications and location finding using triangulation. Reliable long-haul communications is also possible using small log-periodic antennas (e.g., 20 % 25 meters hor- izontally mounted on a 50 or 100 ft mast). W h ip antennas 8 to 15 ft long may also be inductively loaded to match HF wavelengths. And 2 to 10 meter loop antennas measure direction of arrival. Although software radios cannot change the laws of physics that cause HF antennas to be large, they can en- hance signals received using smaller, less optimally tuned antennas to achieve quality approaching that of the larger antennas. Mercury Talk [121] exemplifies the relatively short-range, low-power HF radios. With 2 watts of output power, this radio can close a voice link on a 10 km path. With its 3.5 watt output, it can close a Morse code link over a LOW-BAND NOISE AND INTERFERENCE 81 Figure 3-3 Radio noise and incidental interference. 160 km path. Thomson CSF of France makes the TRC331, another portable HF radio weighing less than 10 kg. Figure 3-2 lists additional narrowband communications standards such radios meet for military interoperability. III. LO W-B AND NOISE AND INTERFERENCE As illustrated in Figure 3-3 [from 5, p. 34-7], the lower radio bands—HF, VHF, and lower UHF—include significant sources of radio noise and interference. The incidental and unavoidable interference includes automobile ignitions, microwave ovens, power distribution systems, gaps in electric motors, and the like. Cellular bands are dominated by intentional interference introduced by other cellular users occupying the RF channel in distant cells. Unavoidable interference results when tens to hundreds of thousands of military personnel use their LVHF radios at the same time. Thus, high levels of interference characterize these congested low bands. The noise/interference levels are defined with respect to thermal noise: P n = kTB where k is Boltzmann’s constant, T is the system temperature ( T 0 is the refer- ence temperature of 273 K elvin), and B is the bandwidth (e.g., per Hz). 82 THE RADIO SPECTRUM AND RF ENVIRONMENT Figure 3-4 The LVHF communications band. In the microwave bands above 1 GHz this thermal noise 19 is a good approx- imation of the noise background. In urban areas, however, incidental urban interference dominates thermal noise until about 5 GHz. In the lower bands, atmospheric noise arises from the reception of lightning-induced electrical spikes from thunderstorms, etc. halfway around the world. Consequently, this noise component is much stronger in summer than in winter as illustrated in the figure. In addition, this noise has a large variance. The short-term (1 ms) narrowband (1 kHz) noise background varies at a rate of a few dB p er sec- ond over a range of from 10 to 30 dB, depending on the latitude, time of the year, and sunspot cycle. High-quality HF receivers track this noise background independently in each subscriber channel. IV. LOW VHF (LVHF) BAND C OMMUNICATIONS MODES The LVHF band from 28 to 88 MHz has traditionally been the band of ground armies because of the robust propagation offered among ground-based sub- scribers in rugged terrain. Amateur radio and the U.S. citizens’ band also use LVHF. The upper edge of this band is defined by the commercial broadcast band from 88 to 108 MHz. Wavelengths from 10.7 to 3.4 meters admit smaller antennas than HF, with a 1 4 wave dipole having a length of 3 to 10 feet, as summarized in Figure 3-4. Historically, LVHF military users have employed 19 Because of the equation, thermal noise is sometimes called kTB noise. [...]... and discrete analog and programmable digital radios These radio suites also monitor emergency channels using dedicated transceivers This includes simultaneous VHF and UHF operation Illustrative discrete radio products include general-purpose, single-channel ground-based radios and multichannel radio relays The AN/GRC-171(V) general-purpose ground-based radio, for example, delivers 20 W of RF power... with airborne radios This radio weighs 36 kg, operates between 225 and 400 MHz, and supports AM voice, AM secure voice, and FM air interfaces Rhode and Schwarz offer a multichannel radio relay in their Series 400 radio It produces 15 to 300 watts of power to relay from 12 to 40 channels Each channel may have 25, 12.5, or 6.25 kHz bandwidth This rack-mount radio is typical of military radio- relays D... discrete radios, within the price-performance envelope of the associated markets For the military avionics bands, this means two or more dedicated emergency broadcast receivers Since one of the features of SDR is the elimination of discrete radios, it may be difficult to obtain type certification for a single SDR to replace two discrete radios The reliability aspects of two or three discrete radios are... include digital multichannel radios such as the GRC-103 [128] and the FHM9104 [129] The AN/GRC-103 (V) radio relay operates from 220 MHz to 1.85 GHz It delivers 15–30 watts of power and supports 24-channel PCM, 63-channel delta modulation, and 4 to 60-channel FM/FDM for legacy radio compatibility This unit weighs 31 kg, not including antenna and cables The FHM9104 Digital Radio Link Terminal operates... 5 W when multiplexing 10 channels and weighs 45 kg D UHF SDR Table 3-3 shows software radio applications parameters derived from propagation and air interface mode considerations 95 SHF BAND COMMUNICATIONS MODES TABLE 3-3 Software Radio Applications Parameters—UHF Software Radio Application UHF-SHF FDM Cellular Radio UHF Air Nav UHF RF Sampling Rate (fs ) 1–25 2–75 2–25 5.4–10 Dynamic Range (dB) MHz... counter-countermeasures (ECCM) including FH The Jaguar-V from Racal Radio Ltd., UK [4, p 69] popularized LVHF FH This affordable manpack configuration produces power of 10 mW, 5 W, and 50 W with the Jaguar’s own advanced FH ECCM in a compact 6.6–7.5 kg package 86 THE RADIO SPECTRUM AND RF ENVIRONMENT TABLE 3-2 SDR Parameters—VHF Software Radio Application Dynamic Range (dB) 50–150 kHz 12–200 MHz 25–500... place of three discrete radios therefore offers reliability challenges Ground infrastructure radios have to transmit on both VHF and UHF at the same time in order to interoperate with military and civilian aircraft This keeps the cost of SDR implementations high General aviation markets are very pricesensitive A military avionics SDR priced at $10 k may be affordable, but the 92 THE RADIO SPECTRUM AND RF... wavelength VHF radios may maintain reception continuity across Fresnel zones using diversity in space (e.g., multiple antennas) and frequency (e.g., slow frequency hopping) with error control coding 84 THE RADIO SPECTRUM AND RF ENVIRONMENT 2 Reflections from Meteor Trails Each minute a dozen meteors penetrate the earth’s atmosphere, where they burn up This creates trails of ionized gas from which radio waves... 128 to 256 kbps accommodate other combinations of low and medium data-rate radio relay, depending on the mix of delta modulation, VCELP, CVSD, ADPCM, and other compressive coding waveforms E LVHF Services and Products As shown in Figure 3-4, LVHF supports broadcast, fixed, and mobile applications, radio astronomy, aeronautical radio navigation (74.8 MHz), and commercial FM broadcast (87.5–108 MHz) Antenna... transmission mode in which transmitter and receiver are not within LOS of each other Each radio has LOS access to a point in the troposphere from which radio waves are scattered beyond LOS Due to the weak coupling between the radio waves and the troposphere, the radios employ very large apertures (e.g., 10-meter diameter dish) with kilowatts of power Diversity reception is typically mandated, requiring that . duty cycle. Mobile cellular radio (MCR) operates in frequency allocations between 400 MHz and 2.5 GHz, with clusters at 900 and 1800 MHz. There are similar radio services such as special mobile radio (SMR) as. appearing in this chapter are from [121]. 80 THE RADIO SPECTRUM AND RF ENVIRONMENT TABLE 3-1 Software Radio Applications Parameters—Baseband and HF Software Radio Application Sampling Rate ( f s ) Dynamic. programmable digital radios. These radio suites also monitor emergency channels using ded- icated transceivers. This includes simultaneous VHF and UHF operation. Illustrative discrete radio products