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Multiband Transceivers - [Chapter 5] Software-Defined Radios
Software-Defined Radios

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Multiband Transceivers - [Chapter 5] Software-Defined Radios

  1. 1. Multiband RF Transceiver System Chapter 5 Software-Defined Radio 李健榮 助理教授 Department of Electronic Engineering National Taipei University of Technology
  2. 2. Outline • Introduction • Mobile Generations • Traditional Hardware-Defined Radio (HDR) • Ideal Software-Defined Radio (SDR) • Basic SDR Architecure • Hybrid Analog and Digital Radio Architecture • Wideband Downconversion in the SDR Department of Electronic Engineering, NTUT2/16
  3. 3. Introduction • In the 20th century, most radios are hardware defined with little or no software control (hardware defined radio, HDR). Fixed in function for mostly consumer items. A short life and are designed to be discarded and replaced. • SDR uses programmable digital devices (DSPs or FPGAs) to perform the signal processing necessary to transmit and receive baseband information at radio frequency. Offers greater flexibility and potentially longer product life. Can be upgraded very cost effectively with software. A major challenge is to equal the efficiencies of hardware solutions. The developer will want to be shielded from the details hardware and complete all development in a unified environment using a single high- level language. Department of Electronic Engineering, NTUT3/16
  4. 4. Generation of Mobile Communications • 1980s: 1st generation of mobile cellular Uses analog modulation techniques to transmit and receive analog voice only information between mobiles and base stations. • 1990s: 2nd-generation (2G) systems They were known as “digital” because they encoded voice into digital streams and used digital modulation techniques for transmission. • 2000s: IMT 2000 standard (defined 3G-compatible systems) Support up to 2 Mbps data connections. A means to provide new services to customers and to provide much needed capacity via better spectrum utilization. Of the 3G standards, the 3GPP Universal Mobile Telecommunications System (UMTS) is strongest in Europe (not universal). The 3GPP2 CDMA2000 standard and the TDMA-based GSM-EDGE systems will be successful in North and South America, while Japan has its own WCDMA system similar to UMTS. Department of Electronic Engineering, NTUT4/16
  5. 5. 3G SDR Applications • All of the 3G systems are potential SDR applications. • SDR offers the potential to solve many of the problems caused by the proliferation of new air interfaces. • Base stations and terminals using SDR architectures can support multiple air interfaces during periods of transition and be easily software upgraded. • Intelligent SDRs can detect the local air interface and adapt to suit the need; this capability will be valuable for frequent inter- country travelers. Department of Electronic Engineering, NTUT5/16
  6. 6. Traditional Hardware Radio Architecture • Conventional dual conversion superheterodyne transceiver: This design has been around since the 1930s. The analog superheterodyne radio has experienced a marvelously successful history; it was used in 1G mobile phone terminals (e.g., AMPS) and is sure to endure in lowcost broadcast radios. De-mod. Modulator Baseband analog receive Baseband analog transmit LO1 LO2 IF IF LNA PA RF combiner Department of Electronic Engineering, NTUT6/16
  7. 7. Ideal Software Defined Radio (I) • The analog functions are restricted to those that cannot be performed digitally. (Antenna, RF filtering, RF combination, receive pre- amplification, transmit power amplification and reference frequency generation) 123 LNA PA RF combiner Reference LO Digital processing resources e.g., Digital signal processors FPGA’s Reconfigurable communications processors Microprocessors Memory Operating systems Drivers Inter- processor comms CORBA Resource management Management and control Baseband digital user data Hardware Middleware Application software Framework Digital subsystemAnalog subsystem API Department of Electronic Engineering, NTUT7/16
  8. 8. Ideal Software Defined Radio (II) • Analog conversion stage right up as close as possible to the antenna. • The separation of carriers and up/down frequency conversion to baseband, channel coding and modulation functions are performed by the digital processing resources. • Frameworks using an open API into the middleware will make applications development more portable, quicker, and cheaper. • The ideal architecture is commercially feasible for limited low data rate HF and VHF radios but is not yet practical for any generation of cellular mobile phone technology. Department of Electronic Engineering, NTUT8/16
  9. 9. Basic SDR Architecture • For 3G mobile and many other multiuser radio technologies, the ideal SDR is not yet a practical or cost-effective reality. Direct sampling of wideband RF frequencies at high SNR (>90 dB) is not yet technically possible. • Decide where the radio stops being hardware defined and where it starts being software defined. • Considering normal commercial requirements (principally cost effectiveness), it is apparent that SDR implementations of 3G wireless need purpose-built hardware to be successful. Department of Electronic Engineering, NTUT9/16
  10. 10. 2G Radio Architecture • Compared with current generations, 1G and other equivalent analog radio systems trade off complexity for bandwidth utilization. That is, they are less complex and consume more bandwidth. AMPS consumes 30 kHz for a voice user. • A major requirement of the 2G standards was to increase bandwidth efficiency in a increase in complexity. The 2G Groupe Speciale Mobile (GSM) standard achieved this by implementing a digital standard that allowed for time division multiplexing, multiple access, and other relatively sophisticated techniques to improve system capacity. GSM occupies 200 kHz for its 8 voice users. The added features can produce an approximately 3 to 4 times capacity improvement. Department of Electronic Engineering, NTUT10/16
  11. 11. Hybrid Radio Architecture (I) • The analog fixed function HDR survived right through to the 1960s and 1970s, making its way into color television transmission, private mobile radio, and even parts of 1G cellular mobile radio. • The complexity of a color television receiver and a 1G mobile terminal stretched this analog technology to the absolute limit. Analog circuits consume more space and power and are more subject to performance variations as a result of environmental factors (e.g., temperature). • The emergence of low-cost ADCs, DACs, and DSPs in the 1980s and the need for more efficient RF bandwidth utilization shifted radio architecture development away from purely analog to hybrid analog and digital systems. Department of Electronic Engineering, NTUT11/16
  12. 12. Hybrid Radio Architecture (II) • Ppopular with early 1990s hybrid radios (e.g., 2G BTS). IFA1, typically 140 or 70 MHz Each filter ensures that acceptable selectivity and image rejection are achieved. IFA2, typically 10.7 MHz. De-interleaving and error correction (e.g., Viterbi decoder) Modulation and filter Demodulation and equalization Channel decoding Voice decoding Network interface voice packet extraction Voice encoding Channel encoding DAC FRx PA RF combiner Digital information bits LNA IFA1 IFA2 LP ADC LO2Rx LO2Tx LO1Rx LO1Tx LO3Rx LO3Tx fs Per carrier analog Rx chain Per baseband Rx channel Per carrier analog Tx chain Per baseband Tx channel GSM 900 BTS: RX: 880–915 MHz TX: 925–960 MHz. Department of Electronic Engineering, NTUT12/16
  13. 13. Hybrid Radio Architecture (III) • The single carrier case is expanded to a multicarrier system by adding RF carrier transmit and receive chains. Channel decoding Voice decoding Demodulation and filter Network interface voice packet extraction Multichannel information digital bits Voice encoding Channel encoding Modulation and filter ADC DAC FRx IFA1 IFA2 IFA3 LNA PA fsLO2Rx LO2Tx LO3Tx LO3RxLO1Rx1 LO1Rx2 LO1RxN LO1Tx1 LO1Tx2 LO1TxN RF combiner FTx Tx chain 1 Tx chain 2 Tx chain N Rx chain 1 Rx chain 2 Rx chain N Baseband 1 Rx Baseband 2 Rx Baseband N Rx Baseband 1 Tx Baseband 2 Tx Baseband N Tx Multicarrier 1990s digital radio Department of Electronic Engineering, NTUT13/16
  14. 14. Basic SDR Block Diagram Wideband capability, designed to replace many narrowband analog receive or transmit frequency conversion chains. Network interface voice packet extraction Multichannel information digital bits Channel encoding Modulation and filter Vocoding Channel decoding Demodulation and filter Interpolation filter Interpolation filter Interpolation Filter decimation Interpolation Filter decimation ADC DAC RF combiner LNA PA FRx IFA1 IFA1 BP1 BP2 BP3 IFo IFo fs LORX1 LORX2 LOTX1 LOTX1 FTx DSUM NCO NCO Analog Rx chain Analog Tx chain Wideband analog front end Down conversion Up conversion Hardware defined subsystem Software defined subsystem Digital frequency conversion and baseband processing resources Carrier 1 Carrier 2 Carrier N Q I Q I (A) (C) (B) Digital IF The hardware subsystem details some lower-level physical components (PA, LNA, ADC…) Department of Electronic Engineering, NTUT14/16
  15. 15. Wideband Downconversion • The wideband front end converts or shifts an entire segment of spectrum to a suitable intermediate frequency—IFD, “the digital IF”—prior to digitization. A segment of the GSM 900-MHz band A popular choice for IFD is 70 MHz due to the COTS availability of satellite/ microwave frequency converters. The spectrum of the required shifted to baseband by software prior to demodulation. Carriers Carriers N 2 1 1 2 N N 2 1 1 2 N A(f) A(f) +f (MHz) ~ 900 MHz0 0 0 -f (MHz) -f (MHz) -f (MHz) +f (MHz) +f (MHz) (A) (B) ~ 70 MHz = IF e.g., carrier 2 ~ 70 MHz RF IFD BB Department of Electronic Engineering, NTUT15/16
  16. 16. Summary • In this chapter, the concepts of HDR, SDR, and the hybrid analog and digital radio architecture were introduced. • The HDR is often fixed in function but offering higher performance than SDR solution. • The SDR offers greater flexibility and can be upgraded very cost effectively with software. A major challenge is to equal the efficiencies of hardware solutions. • The SDR may be a good solution for wideband or multi- band/multi-mode applications. Department of Electronic Engineering, NTUT16/16