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Software-Defined Radio is a rapidly evolving technology that is receiving enormous recognition and generating widespread interest in the telecommunication industry. Over the last few years, analog radio systems are being replaced by digital radio systems for various radio applications in military, civilian and commercial spaces. In addition to this, programmable hardware modules are increasingly being used in digital radio systems at different functional levels. SDR technology aims to take advantage of these programmable hardware modules to build an open-architecture based radio system software. SDR technology facilitates implementation of some of the functional modules in a radio system such as modulation/demodulation, signal generation, coding and link-layer protocols in software. This helps in building reconfigurable software radio systems where dynamic selection of parameters for each of the above-mentioned functional modules is possible. A complete hardware based radio system has limited utility since parameters for each of the functional modules are fixed. A radio system built using SDR technology extends the utility of the system for a wide range of applications that use different link-layer protocols and modulation/demodulation techniques.
Keywords: SDR, SCA, OFDMA
1 . INTRODUCTION
Software-Defined Radio (SDR) Forum [www.sdrforum.org] defines SDR technology as "radios that provide software control of a variety of modulation techniques, wide-band or narrow-band operation, communications security functions (such as hopping), and waveform requirements of current & evolving standards over a broad frequency range." In a nutshell, Software-Defined Radio (SDR) refers to the technology wherein software modules running on a generic hardware platform consisting of DSPs and general purpose microprocessors are used to implement radio functions such as generation of transmitted signal (modulation) at transmitter and tuning/detection of received radio signal (demodulation) at receiver. SDR technology can be used to implement military, commercial and civilian radio applications. A wide range of radio applications like Bluetooth, WLAN, GPS, Radar, WCDMA, GPRS, etc. can be implemented using SDR technology. This paper provides an overview of generic SDR features and its architecture with a special focus on the benefits it offers in commercial wireless communication domain.
SDR technology enables implementation of radio functions in networking infrastructure equipment and subscriber terminals as software modules running on a generic hardware platform. This significantly eases migration of networks from one generation to another since the migration would involve only a software upgrade. Further, since the radio functions are implemented as software modules, multiple software modules that implement different standards can co-exist in the equipment and handsets. An appropriate software module can be chosen to run (either explicitly by the user or implicitly by the network) depending on the network requirements. This helps in building multi-mode handsets and equipment resulting in ubiquitous connectivity irrespective of underlying network technology used. SDR technology supports over-the-air upload of software modules to subscriber handsets. This helps both network operators as well as handset manufacturers. Network operators can perform mass customizations on subscriber's handsets by just uploading appropriate software modules resulting in faster deployment of new services. Manufacturers can perform remote diagnostics and provide defect fixes by just uploading a newer version of the software module to consumers' handsets as well as network infrastructure equipment.
1.1. FEATURES OF SDR
The key features of SDR technology are as follows :
Reconfigurability: SDR allows co-existence of multiple software modules implementing different standards on the same system allowing dynamic configuration of the system by just selecting the appropriate software module to run. This dynamic configuration is possible both in handsets as well as infrastructure equipment. The wireless network infrastructure can reconfigure itself to subscriber's handset type or the subscriber's handset can reconfigure itself to network type. SDR technology facilitates implementation of future-proof, multi-service, multi-mode, multi-band, multi-standard terminals and infrastructure equipment.
Ubiquitous Connectivity: SDR enables implementation of air interface standards as software modules and multiple instances of such modules that implement different standards can co-exist in infrastructure equipment and handsets. This helps in realizing global roaming facility. If the terminal is incompatible with the network technology in a particular region, an appropriate software module needs to be installed onto the handset (possibly over-the-air) resulting in seamless network access across various geographies. Further, if the handset used by the subscriber is a legacy handset, the infrastructure equipment can use a software module implementing the older standard to communicate with the handset.
Interoperability: SDR facilitates implementation of open architecture radio systems. End-users can seamlessly use innovative third-party applications on their handsets as in a PC system. This enhances the appeal and utility of the handsets.
Here we have given a brief overview of a basic conventional digital radio system and then explained how SDR technology can be used to implement radio functions in software. We have then explained the software architecture of SDR.
The various functional blocks in a generic digital radio transceiver (transmitter/receiver) system is depicted in figure 1. The digital radio system consists of three main functional blocks: RF section, IF section and baseband section. The RF section consists of essentially analog hardware modules while IF and baseband sections contain digital hardware modules.
Figure : Block diagram of a generic digital transceiver
The RF section (also called as RF front-end) is responsible for transmitting/receiving the radio frequency (RF) signal from the antenna via a coupler and converting the RF signal to an intermediate frequency (IF) signal. The RF front-end on the receive path performs RF amplification and analog down conversion from RF to IF. On the transmit path, RF front-end performs analog up conversion and RF power amplification.
The ADC/DAC blocks perform analog-to-digital conversion (on receive path) and digital-toÂ analog conversion (on transmit path), respectively. ADC/DAC blocks interface between the analog and digital sections of the radio system. DDC/DUC blocks perform digital-downÂ conversion (on receive path) and digital-up-conversion (on transmit path), respectively. DUC/DDC blocks essentially perform modem operations, i.e., modulation of the signal on transmit path and demodulation (also called digital tuning) of the signal on receive path.
The baseband section performs baseband operations (connection setup, equalization, frequency hopping, timing recovery, correlation) and also implements the link layer protocol (layer 2 protocol in OSI protocol model).
The DDC/DUC and baseband processing operations require large computing power and these modules are generally implemented using ASICs or stock DSPs. Implementation of the digital sections using ASICs results in fixed-function digital radio systems. If DSPs are used for baseband processing, a programmable digital radio (PDR) system can be realized. In other words, in a PDR system baseband operations and link layer protocols are implemented in software. The DDC/DUC functionality in a PDR system is implemented using ASICs. The limitation of this system is that any change made to the RF section of the system will impact the DDC/DUC operations and will require non-trivial changes to be made in DDC/DUC ASICs.
II METHODS AND MATERIALS
A software-defined radio (SDR) system is one in which the baseband processing as well as DDC/DUC modules are programmable. Availability of smart antennas, wideband RF front-end, wideband ADC/DAC technologies and ever increasing processing capacity (MIPS) of DSPs and general-purpose microprocessors have fostered the development of multi-band, multi-standard, multi-mode radio systems using SDR technology. In an SDR system, the link-layer protocols and modulation/demodulation operations are implemented in software.
If the programmability is further extended to the RF section (i.e., performing analog-to-digital conversion and vice-versa right at the antenna) an ideal software radio systems that support programmable RF bands can be implemented. However, the current state-of-the-art ADC/DAC devices cannot support the digital bandwidth, dynamic range and sampling rate required to implement this in a commercially viable manner.
Figure illustrates the architecture of software components in a typical SDR system. The system uses a generic hardware platform with programmable modules (DSPs, FPGAs, microprocessors) and analog RF modules.
The operating environment performs hardware resource management activities like allocation of hardware resources to different applications, memory management, interrupt servicing and providing a consistent interface to hardware modules for use by applications. In SDR system, the software modules that implement link-layer protocols and modulation/demodulation operations are called radio applications and these applications provide link-layer services to higher layer communication protocols such as WAP and TCP/IP.
A software defined radio is a radio transmitter/receiver that uses digital signal processing (DSP) for coding/decoding and modulation/demodulation. This allows much more power and flexibility when choosing and designing modulation and coding techniques. The C6700 series of digital signal processors have been chosen for this project. More specifically the TMDSK6713 evaluation board with the TMS320C6713 DSP chip will be used.
Due to hardware availability, both the transmitter and receiver will be implemented on the same DSP evaluation board. The system will be constructed and programmed entirely in Simulink using the embedded target TI C6000 Simulink library. Simulink will generate the code based off of the model designed and will then download it to the board through TI Code Composer Studio for testing.
Overall System Block Diagram
The input to the system will be digital data in a computer file. This data will be modulated by the transmitter and sent to the channel. The channel will cause interference to the signal in the form of attenuation, phase delay, and noise. At the receiver side, the signal will be demodulated and reconstructed to produce the original transmitted message.
The transmitter shown in Figure 3 will generate the signal that will be transmitted through the channel. Demultiplexing, quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM), and up mixing work together to create the transmitter signal output.
Figure 2: Transmitter Subsystem Detailed Diagram
Demultiplexing & Modulation
The demultiplexing block will take 8 bits of binary data and then break the 8-bit stream into four 2-bit streams. These 2-bit streams will each be fed into a QAM modulation channel. Once the QAM channels have modulated the input data, they will be passed into the OFDM block. The OFDM system will multiplex the QAM signals together to produce the final modulated output.
Mixing is done to meet the bandwidth requirements of the channel. The up mixer will increase the frequency of the OFDM signal by multiplying it by a greater carrier frequency. The OFDM signal will be imbedded in the carrier signal that the local oscillator produces. The output of the mixer will be in bandwidth of the channel.
Figure 3 shows the detail of the channel block from figure 1. The channel block will implement a model of an actual transmission channel. Different parts of the channel will model different channel effects on the output of the transmitter. These are channel gain, multi-path interference, and noise.
The channel attenuation will model the attenuation or possibly the gain effect that the channel will have on the transmitted signal. It this gain can vary with frequency and/or time.
The multi-path interference will model reflections of the transmitted signal. These reflections will arrive at the receiver at different times. Each one of these paths can have its own attenuation that can vary with time and frequency.
Noise will also be introduced into the signal. This noise is either specific to a limited frequency range (narrow band noise) or affects the whole spectrum of the transmitted signal.
Figure 4 - Receiver Subsystem Detail
The receiver subsystem shown in figure 4 will recover the sent message. To do this it needs to extract the carrier, symbol, and frame timing from the signal. It then will use this information to extract the message from the phase, frequency, and amplitude noise of the channel. The receiver has the following main parts: carrier synchronization, automatic gain control, demodulation, symbol synchronization, and frame synchronization.
The carrier synchronization subsystem will correct for frequency differences between the transmitter and the receiver. It will also correct for the phase delay introduced by the channel. It will do this using a phased locked loop (PLL) technique.
Automatic Gain Control
The automatic gain control system will correct for the time varying differences in channel attenuation. It will do this by adjusting the average power of the input signal to a known value.
The demodulation system consists of two parts: OFDM demodulation and QAM demodulation. The OFDM demodulation will demodulate the signal into its constituent QAM sub signals. The QAM demodulation will demodulate the QAM carriers back into the pulses that were used to modulate it.
The symbol synchronization will figure out the most appropriate time to sample the pulses coming from the QAM modulation. This will allow the most accurate information to be extracted from the pulse stream. The output of this block will then be fed into a multiplexer. The output of the multiplexer will be the digital data that was fed into the system to begin with.
The frame synchronization will synchronize the data frames of the system. This will align the start time of the message so that the digital data can be interpreted correctly. This will allow the compute file or message to be translated back into its original form.
III . CONCLUSION
Current market drivers such as future-proof equipment, seamless integration of new services, multi-mode equipment and over-the-air feature insertion in commercial wireless networking industry have resulted in widespread interest in SDR technology. The technology can be used to implement wireless network infrastructure equipment as well as wireless handsets, PDAs, wireless modems and other end-user devices. However, factors like higher power consumption, increased complexity of software and possibly higher initial cost of equipment vis-à-vis the benefits offered by the technology should be carefully considered before using SDR technology to build a radio system.
Summarizing, SDR is a promising technology that facilitates development of multi-band, multi-service, multi-standard, multi-feature consumer handsets and future-proof network infrastructure equipment.
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