The following is excerpted from Chapter 18: Software Defined Radio from the Book, RF & Wireless Technologies
by Bruce Fette. If you order a copy of this book before December 31, 2007 you can receive additional 20% off. Visit www.newnespress.com or call 1-800-545-2522 and use code 91137.
This book excerpt from Chapter 18 of RF & Wireless Technologies provides a sample of the comprehensive information available in this text. The book aims to be a complete desk reference and features input from numerous industry-leading experts. This part covers receivers for software defined radio.
Part 1 introduces the basics of SDR.
Part 3 covers transmitters for software defined radio.
Ideally the designer of an SDR would like to put the data converters directly on the antenna. However, this is not a practical solution. In reality, some analog front end must be used before the ADC in the receive path and after the DAC in the transmit path that does the appropriate frequency translation. The most common of these architectures is the super-heterodyne architecture. Although it's many decades old, new semiconductor technology and high levels of integration have kept this architecture vitalized and in popular use both in the transmit and receive signal paths [5, 6].
Other architectures such as direct conversion both for transmit and receive are seeing some popularity in applications that are not as demanding. Currently, direct conversion (Tx and Rx) is found in user terminals for cellular communications as well as for Tx on the base station side. It is possible that future developments will enable direct conversion on the receive side as well. Until then, the superheterodyne architecture will continue to be used in one form or another.
High-performance SDR receivers are typically constructed from some variant of the superheterodyne architecture. A super-heterodyne receiver offers consistent performance across a large range of frequencies while maintaining good sensitivity and selectivity [7, 8]. Although not trivial to design, the possibility of combining wideband analog techniques and multiple front ends would allow operation across different RF bands. In the case of multicarrier applications, this could be done simultaneously if necessary.
Depending on the applications, one or more receive channels may be desired. Traditional applications may require only a single RF channel. However, applications that require high capacity or interoperability may require a multicarrier design. SDRs are well suited for multicarrier applications, since they employ a highly oversampled ADC with ample available bandwidth.
An oversampled ADC is one in which the sample rate is operating beyond that which is required to meet the Nyquist criterion , which states that the converter sample rate must be twice that of the information bandwidth. Since an SDR may not have advance knowledge of the bandwidth of the signal it will be used to receive, the sample rate must be appropriately high enough to sample all anticipated bandwidths.
Current ADC technology allows high dynamic range bandwidths of up to 100 MHz to be digitized. With this much bandwidth, it is also possible to process multiple channels. Figure 18.5 shows a typical multicarrier receiver example, and Figure 18.6 shows a spectral display.
18.5. Multicarrier CDMA example.
18.6. Multimode spectrum with IS-95 and narrowband carriers.
In this example, the sample rate of the ADC is set to 61.44 mega-samples-per-second (MSPS), which gives a Nyquist bandwidth of 30.72 MHz. If each RF channel is 1.25 MHz wide, then Nyquist indicates that the number of potential channels is about 24.5. In practice, by allowing for reasonable transition bands on the antialiasing filters, the typical available bandwidth is one-third the sample rate instead of the Nyquist one-half. Thus, the available bandwidth for our example is 20.48 MHz, which is just over 16 channels at 1.25 MHz.
Since the channel characteristics can be changed, it is easy enough to change the CDMA
example to a GSM example. In this case, both the digital preprocessing and the general-purpose DSP are reconfigured, respectively, by changing the digital channel filter from GSM to CDMA and by loading the new processing code into the DSP. Since GSM channels are 200 kHz wide, this example could easily be reconfigured as a 102-channel GSM receiver.
While both such examples would provide a lot of utility, perhaps a more interesting example would be to configure the receiver such that part of the channels could be CDMA while the other would be configured as GSM! Furthermore, if one of the configurations is at capacity and the other is underutilized, CDMA channels could be converted into several GSM channels or vice versa, providing the flexibility to dynamically reallocate system resources on an as-needed basis (a key goal of software-defined radio).
Not all SDR applications require more than one channel. Low-capacity systems may require only one carrier. In these applications, a high oversampling is still desired. If the channel is reprogrammable, it is possible that it may be as narrow as a few kHz or as wide as 5 to 10 MHz. In order to accommodate this range of bandwidths, the sample rate should be suitable for the highest potential bandwidth, in this case 10 MHz. From the multicarrier example, we would typically sample at least three times the bandwidth. In this example, a sample rate of 30.72 MSPS or higher would allow signal bandwidths from a few kHz up to 10 MHz to be processed. Aside from the fact that only one channel is processed, the single-carrier receiver has all of the capacities of that of a multicarrier receiver; it can be reconfigured as necessary.
SDR Receiver Elements
Referring to the single-carrier block diagram in Figure 18.7, while keeping in mind that this applies to the multicarrier example as well, a fully developed SDR will have all signal elements that are programmable.
18.7. Single-carrier Rx example.
The antenna is no exception, and unfortunately it is one of the weakest elements in an SDR . Since most antenna structures have a bandwidth that is a small percentage of its center frequency, multiband operation can become difficult. In the many applications where single bands of operation are used, this is not a problem. However, for systems that must operate across several orders of frequencies, the antenna must be tuned by some means to track the operating frequency to maintain operating efficiency.