# Kiến trúc phần mềm Radio P7

Chia sẻ: Tien Van Van | Ngày: | Loại File: PDF | Số trang:21

0
54
lượt xem
8

## Kiến trúc phần mềm Radio P7

Mô tả tài liệu
Download Vui lòng tải xuống để xem tài liệu đầy đủ

Antenna Segment Tradeoffs The antenna segment establishes the available RF bands. Although much research has been applied toward creating an “all-band” antenna, multiband radios generally require at least one antenna per decade of RF band (e.g., HF, VHF, UHF, SHF, etc.). In addition, the antenna determines the directional properties of the receiving system. Sectorized antennas, static beamforming arrays, and adaptive beamforming arrays (smart antennas) each have different spatial and temporal properties, the most significant of which is the pattern of transmit and/or receive gain....

Chủ đề:

Bình luận(0)

Lưu

## Nội dung Text: Kiến trúc phần mềm Radio P7

2. RF ACCESS 245 Figure 7-1 Candidate antenna configurations. Figure 7-2 Four software radio bands span the JTRS requirements. realize a full-band antenna. The RF range extended from 2 MHz to 2000 MHz, a ratio of 1000 : 1 or 3 decades. Figure 7-1 shows that this requires a tech- nology breakthrough, since the maximum relative bandwidth of the well- established designs is 10 : 1, or one decade. Through in-depth antenna studies
3. 246 ANTENNA SEGMENT TRADEOFFS conducted by Rockwell, Hazeltine, and others, it was determined that at least 3 bands are needed for this range. In fact, SPEAKeasy employed three bands as follows: (a) 2–30 MHz; (b) 30–400 MHz; and (c) 400–2000 MHz. To be precise, only band b was fully implemented in SPEAKeasy I and only bands a and b were implemented in SPEAKeasy II. For the foreseeable future, affordable RF access will probably be limited to octave coverage in the bands above 100 MHz. One configuration of antenna coverage that employs four conservatively designed bands is illustrated in Figure 7-2b. II. PARAMETER CONTROL From a systems-engineering perspective, one must allocate end-to-end per- formance to parameters of the appropriate segment. The use of wideband antennas that enable SDR levels of performance complicates the control of SNR, timing, and phase parameters as follows. A. Linearity and Phase Noise Wide bandwidth is sufficient for detection, but high SNR is necessary for good SDR algorithm performance. As the antenna bandwidth is increased, the thermal noise power increases linearly. Thus, the antenna channels must be filtered to select only those subsets of the band required to service subscriber signals. This is accomplished in the RF conversion and digital IF processing segments. Low phase noise is also critical for phase-sensitive channel modulations such as high-order QAM (> 16 states). Phased array antennas that form beams through the switching of delay elements can have high phase noise induced by switching transients, making high-order QAM impractical. B. Parameters for Emitter Locations In addition, precision timing or RF phase control may be necessary. For ex- ample, the commercial sector now has requirements for the location of mo- bile stations from which emergency calls are placed. The US E911 service requires location to within 125 meters. Network-based emitter location tech- niques include time-difference of arrival (TDOA) and angle of arrival (AOA) estimation using phase interferometry. TDOA [218] requires timing precision on the order of 100 ns, systemwide, to meet E911 requirements. Similarly, AOA [219, 220] requires phase measurements equivalent to a few degrees of angle uncertainty, which is equivalent to a few electrical degrees of phase error. Smart antennas generally derive some estimate of the direction-manifold of the received signals. This information can be translated into AOA. In ad- dition, TDOA techniques may be used alone or in conjunction with smart antennas to estimate the location of mobile subscribers. TDOA is particularly
4. PACKAGING, INSTALLATION, AND OPERATIONAL CHALLENGES 247 Figure 7-3 Yagi illustrates mechanical configuration issues. relevant to CDMA systems because they continuously estimate time of ar- rival (TOA) in order to recover the direct-sequence, spread-spectrum wave- form. The conventional rake receiver may be augmented with, for example, extended multipath tracking Kalman filters in order to improve the TDOA measurement [221]. The presence of multipath can degrade both AOA and TDOA measurements. III. PACKAGING, INSTALLATION, AND OPERATIONAL CHALLENGES Challenges facing the SDR systems engineer include the packaging of anten- nas with the desired capabilities into suitable hardware formats. For precision applications like emitter location, antenna arrays must be calibrated periodi- cally. In addition, the influence of the human body on the antenna patterns of hand-held units should be understood. Software techniques may mitigate some of these effects to yield a corrected, more idealized antenna response. A. Gain versus Packaging A typical UHF satellite antenna has a fractional bandwidth of less than one octave, but relatively high gain, as illustrated in Figure 7-3. This specific antenna from Dorne & Margolin uses crossed grounded elements for a ground plane, with a relatively complex Yagi array of receiving elements that enhance the gain. Since this antenna operates only over the satellite band between 240 and 318 MHz, the narrow relative bandwidth is not a limiting factor. The high gain is available only within about 20 degrees of the direction in which the antenna is pointing. In addition, such narrow bandwidths and beamwidths seriously limit RF access, or increase overall system cost. If, for example, the Yagi were the standard antenna for the 240–318 MHz band, the node would not be able to receive other communications in that band from any direction other than that in which the Yagi is pointing. Alternatively, one could provide six to ten parallel Yagi’s for omnidirectional coverage, but this increases cost
5. 248 ANTENNA SEGMENT TRADEOFFS Figure 7-4 Wideband antennas degrade over time. (a) Highly directional dish antenna; (b) Omnidirectional phased array. and is not needed because of limited satellite geometries. As the SDR engineer increases band coverage to satisfy the need for agile RF access, the likelihood of needing to point the antenna’s gain in more than one direction increases. Other antenna configurations that provide wide relative bandwidths with om- nidirectional coverage include the Adcock array shown in Figure 7-4b. This array provides 10 : 1 relative bandwidth. The parabolic dish also shown in the figure provides a decade of bandwidth. An alternative is to accept lower directional gain, using an antenna with greater relative bandwidth. This may not be physically possible in come cases. For example, the satellite link budget requires the 11 dBi of antenna gain for acceptable outage probability. B. Bandwidth versus Packaging The microstrip [222] patch antenna illustrated in Figure 7-5 provides a much more convenient physical structure, but with only moderate relative bandwidth. Such patch antennas might easily be embedded in a PDA or soldier radio. Several such antennas could be combined using an analog received signal strength indicator (RSSI) circuit to yield reasonable gain in most directions. Using a lower gain antenna reduces the link margin and therefore increases the outage probability proportionally. However, the SDR design process must entertain the use of such suboptimum antennas. That is, the SDR antenna may be suboptimal for a specific band, but may be optimal in terms of aggregate cost and quality of information services across the combination of bands and modes over which the radio operates. C. Antenna Calibration Commercial cellular systems historically have not required extensive antenna calibration. The narrow bandwidth of first- and second-generation air inter- faces allowed one to ignore the minimal distortion introduced by the antenna
6. PACKAGING, INSTALLATION, AND OPERATIONAL CHALLENGES 249 Figure 7-5 Microstrip and patch antennas provide small fractional bandwidth. Figure 7-6 Amplitude vs. frequency response of antenna in the field. response. Third-generation bandwidths of 20 MHz at 900 MHz carrier fre- quencies benefit from element calibration and real-time normalization. In ad- dition, smart antennas require normalization of both amplitude and phase responses in order to form accurate beams and/or nulls that enhance CIR. This section therefore provides a systems-level introduction to the antenna- calibration process. As the test data in Figure 7-6 shows, antennas are vulnerable to diver- gence from ideal responses, and to degradation over time. The scale of the figure is 10 dB per vertical division. Marks are provided at 3, 4, and 6 GHz in the horizontal dimension. The relatively deep notches in the amplitude re- sponse result in phase and amplitude distortion to the degree that subscriber signals span those artifacts. In the band-overlap region, one must select the subscriber signal from the appropriate channel. If each band has its own
7. 250 ANTENNA SEGMENT TRADEOFFS antenna, RF conversion, and wideband ADC, the choice of band in the overlap region may be made digitally. In addition, the spurious out-of-band response shows that a high-powered out-of-band signal can create distortion within the operating band of the antenna, degrading communications capability. The out- of-band energy can alias back into the passband through the digital sampling process. These variations from the ideal response may be compensated for through calibration of the antenna system. To correct the amplitude response, one first establishes a reference amplitude (e.g., 0 dB). The amplitude versus frequency response is then measured by tuning the calibration signal, noting the differ- ence from the reference amplitude. A narrowband calibration table is then created by stepping the known frequency-amplitude source by a small incre- ment, ±f. If Wa is the bandwidth accessed by the antenna, then N = Wa=±f is the number of points in the narrowband calibration table. For the notional antenna response of Figure 7-6, ±f of 100 MHz appears reasonable. The nar- rowband calibration table is indexed by the input frequency. The values in the table are the constants by which to multiply the observed amplitude in order to recover the reference amplitude. Narrowband signals are those for which a single amplitude calibration constant normalizes the signal. A single constant is a good approximation to the frequency response if the bandwidth of the signal is much smaller than the bandwidth of the deepest/narrowest notch. If the bandwidth of the signal spans multiple ±f points, then these wide- band signals should be normalized or “prewhitened.” The normalization process attempts to drive the normalized components to equal amplitudes across the band. Since signals that are uniform in the frequency domain are called “white,” the normalization process is sometimes called prewhiten- ing. This may be accomplished by transforming the signal to the frequency domain (e.g., by an FFT), multiplying the signal by the calibration table values, and transforming the signal to the time domain. Alternatively, the calibration table may be transformed to the time-domain and the signal may be convolved with impulse-responses from the wideband table. If the sub- scriber signal spans 2n + 1 values of the narrowband calibration table, then each entry of the wideband table should have 2n + 1 time-domain impulse response coefficients. The Fourier transform of the calibration table yields the impulse response stored in each entry of the wideband calibration table: y(t; f) = F(Cf"n , Cf"n"1 , : : : Cf , Cf+1 , : : : Cf+n ) # x(t; f) where # is the convolution operator. The antenna signal x(t; f) must be indexed into the wideband calibration table at point f = k ±f, which could be the frequency on which the subscriber signal is supposed to be transmitted. Doppler and frequency errors could in- troduce distortion errors. Generally, Doppler spread is much smaller than ±f, so these errors may be neglected.
8. PACKAGING, INSTALLATION, AND OPERATIONAL CHALLENGES 251 Phase may be calibrated using an analogous approach. Let z = C! + n be an ideal data model, where ! is the ideal array response, n is the noise component, and z is a (complex) measurement. The structure of the calibration algorithm is given by: ! min $zi " ai C!(µi )$2 C k In this equation, (ai , µi ) are the known amplitude and phase angle of the source for the ith measurement, zi . The calibration table C, in this case, is a matrix, is constructed to minimize the total square error. Each element in an array antenna system must be calibrated and corrected using the calibration tables in real-time. Since the values in the calibration tables change only when the antenna is recalibrated, and since the size of the tables is not large and is well known and fixed, antenna calibration can be allocated to an FPGA or programmable ASIC. If well-known signals are present in the deployment environment, then antennas may be recalibrated in the field. Usually, how- ever, the system must be moved to a facility in which the antenna pattern may be recalibrated using precision sources and test equipment. This pro- cess should generally be undertaken when the antenna subsystem undergoes configuration changes. Movement of a large antenna to a new site may ne- cessitate recalibration using portable test equipment. Structural changes to a vehicle on which the antenna(s) are mounted may also necessitate recalibra- tion. D. Antenna Separation The physical separation of antennas can substantially control self-generated interference. Local oscillators from one band, for example, can leak into other bands. This can be particularly problematic for a SDR in a low band (e.g., SINCGARS) on a platform in which a fast-tuning LO is operating in a high band (e.g., JTIDS). If these two antennas are located in the same antenna enclosure or on the same mast, the JTIDS LO leaking through the antenna could cause interference in the SINCGARS band or on another low band. The benefits of physical separation may be estimated using a link budget spreadsheet. Consider, for example, the placement of an HF antenna with respect to a UHF antenna in a vehicular application. If these antennas are separated by 10 ft instead of 1 ft, the path loss of out-of-band spurs increases by 20 dB to "11 dB. Near-field effects and local reflections may reduce this to 5 to 10 dB. Skin currents in metal structures can also contribute to coupling and can cause passive intermodulation. Mounting the antennas as much as possible on opposite sides of the vehicle tends to suppress these effects. Separation among multiple vehicles can also be a problem for military ve- hicles. A military operations center, for example, may contain a half-dozen or more vehicles with a dozen or more radios operating in the 30 to 500 MHz RF