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  1. 1. Software Defined Radio For Next Generation Networks 1. Introduction1.1 Definition: Software-Defined radio (SDR) Forum [] defines SDR technologyas "Radios that provide software control of a variety of modulation techniques, wide-band ornarrow-band operation, communications security functions (such as hopping), and waveformrequirements of current & evolving standards over a broad frequency range.”In short,software modules running on a generic hardware platform of DSPs ( Digital Signalprocessor) and general purpose microprocessors can implement radio functions such asmodulation/demodulation, signal generation, coding and link-layer protocols. This helps inbuilding reconfigurable software radio systems where dynamic selection of parameters ispossible.1.2 Features of Software Defined Radio: Regardless of the means by which the radio is reconfigured, a fully implemented SDRwill have the ability to navigate a wide range of frequencies with programmable channelbandwidth and modulation characteristics. The following list outlines some of the possibledynamic characteristics of an SDR: Multiband Multicarrier Multimode Multirate Variable bandwidth Ubiquitous Connectivity INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  2. 2. Software Defined Radio For Next Generation NetworksMultiband: The traditional radio architectures operate on a single band or range of frequencies.Applications like cellular communications, government and nongovernment agencies workon wide range of frequencies. A normal radio is designed to operate in one specified band.But a multiband radio can operate on two or more bands either sequentially orsimultaneously.Multicarrier: A multicarrier also called multichannel radio can simultaneously operate on morethan one frequency. This may be within the same band or in two different bands. This isgenerally seen in a base station that may be servicing many users at once or a user terminalthat may be processing both voice and data on different carriers.Multimode: An SDR has the ability to work with many different standards and be continuouslyreprogrammed. Multimode implies the ability to process several different kinds of standards.Examples of standards are AM FM, GMSK, and CDMA and many more. These modes maybe implemented sequentially or simultaneously.Multirate: Multirate is closely related to multimode. A Multirate radio is one that can processdifferent parts of the signal chain at different samples rates, as in a multirate filter. It can alsowork in different modes that require different data rates. An example is a radio that canprocess GSM at 270.833 kSPS (kilo Symbols Per Second) or CDMA at 1.2288 MCPS (MegaChips Per Second). This can also be done sequentially or simultaneously on different carriers.Variable Bandwidth: A traditional radio works in a fixed channel bandwidth with help of a analog filtersuch as an SAW (Surface Acoustic Wave) or ceramic filter. An SDR on the other hand uses INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  3. 3. Software Defined Radio For Next Generation Networksdigital filters where the bandwidth can be altered on the fly. Additionally, digital filters cancompensate for transmission path distortion.Ubiquitous Connectivity:If the terminal is incompatible with the network technology in a particular region, anappropriate 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 usedby the subscriber is a legacy handset, the infrastructure equipment can use a software moduleimplementing the older standard to communicate with the handset.1.3 An explanation of SDR: A software-defined radio (SDR) system is a radio communication system which cantune to any frequency band and receive any modulation across a large frequency spectrum bymeans of a programmable hardware which is controlled by software.An SDR performs significant amounts of signal processing in a general purpose computer, ora reconfigurable piece of digital electronics. The goal of this design is to produce a radio thatcan receive and transmit a new form of radio protocol just by running new software.Software radios have significant utility for the military and cell phone services, both of whichmust serve a wide variety of changing radio protocols in real time.The hardware of a software-defined radio typically consists of a super heterodyne RF frontend which converts RF signals from (and to) analog IF signals, and analog to digitalconverter and digital to analog converters which are used to convert a digitized IF signal fromand to analog form, respectively. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  4. 4. Software Defined Radio For Next Generation Networks1.4 Ideal concept: The ideal scheme would be to attach an analog to digital converter to an antenna. Adigital signal processor would read the converter, and then software would transform thestream of data from the converter to any other form.An ideal transmitter would be similar. A digital signal processor would generate a stream ofnumbers. These would be sent to a digital to analog converter connected to a radio antenna.The ideal scheme is not practical, however.1.5 The need for SDR:SDR presents a new concept for mobile operators and allows them to have a simpler, moreefficient network and in many cases, a guaranteed evolution path to future technologies. Inmost cases, current base stations need to satisfy several prerequisites to be considered fordeployment.Base stations need to be wideband enough to be able to run several air interfaces in the samefrequency band. For example, running GSM and UMTS simultaneously at the samefrequency in the same hardware platform will provide significant cost savings for operators inemerging markets.• Not be constrained to a single waveform. Existing equipment must be upgradable to futureair interfaces, either by software or by adding a new baseband processing card to the installedhardware platform. The majority of infrastructure vendors now claim to supportLTE through a card addition to their baseband units.• Have upgradable processing capabilities for future air interfaces. This is especiallyapplicable to LTE, which is expected to require additional processing at the base station dueto its low latency, higher bandwidth requirements. Existing base stations need to be hardwareupgradable to more powerful processors.• Be environmentally friendly. Power consumption is receiving increasing interest asoperators aim to operate “green” networks. Moreover, savings power in the base station willresult in far lower operational expenditures. Mobile operators have traditionally relied onrunning several independent equipment racks for different air interfaces. However, a singlehardware platform that can run all of these simultaneously can limit power costs greatly. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  5. 5. Software Defined Radio For Next Generation NetworksIn order to satisfy all of the above, base stations need to be reconfigurable and have flexiblehardware platforms. A form of SDR is the only solution in the long term and operators arenow coming to realize that they can benefit now and in the future by deploying a flexiblebase station.1.6 Scope of the SDR: The public cellular network is a market oriented service. It is extremely sensitive topublic demands. Consider the introduction of a new network standard in a particular domain.And suddenly the market response changes towards another standard which has differentcommunication standard and signal processing process. In this case it would be beneficial forany manufacturing company to be able to respond quickly to such situations. Softwaredefined radio systems have the potential to allow short time to respond to the changes in themarket. Wireless network operators face deployment issues while rolling out new services orfeatures to realize new revenue-streams since this may require large-scale customizations onsubscribers‟ handsets. SDR technology supports over-the-air upload of software modules tosubscriber handsets. This helps both network operators as well as handset manufacturers.Network operators can perform mass customizations on subscriber‟s handsets by justuploading appropriate software modules resulting in faster deployment of new services.Manufacturers can perform remote diagnostics and can perform fixes just by uploading anewer version of the software module to consumers‟ handsets as well in a base station. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  6. 6. Software Defined Radio For Next Generation Networks 2. ARCHITECTURE OF SDR2.1 Software Defined Radio Implementation Platforms:Real time software defined radio design can be implemented using a variety of digitalhardware namely • Field programmable gate arrays. • Digital signal processors. • Application specific integrated circuits • General purpose processors.DSP: The DSP platform is essentially a microprocessor based system optimized for digital signal processing applications; DSPs can be programmed repeatedly with a high level language such as C, MATLAB. Modifications and upgrades to the design are made through these high level languages, thus reducing the design times for each iteration. The flexibility offered by the digital signal processor comes at the cost of efficiency. When there are several computations to be performed, parallel executions of these computations will slow down the rate at which data is processed and this leads to the use of more than one DSP. This solution is limited since synchronizing several DSPs is difficult.FPGA: A field programmable gate array is a general purpose integrated circuit that is programmed by the designer rather than the device manufacturer. A unique feature of FPGA is that it can be reprogrammed, even after it has been deployed into a system. Field programmable gate array is programmed by downloading a configuration program (bit stream) into the static on-chip random access memory. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  7. 7. Software Defined Radio For Next Generation Networks This is similar to the object code of a microprocessor, in which the bit stream is the product of compilation tools that translate the high level abstractions produced by a designer into equivalent but low level executable code. Field programmable gate arrays were designed for multilevel circuits to handle complex circuits on a single chip. Since they are reprogrammable, their configurations can be easily changed to upgrade systems or correct system bugs, making it ideal for prototyping. Field programmable gate arrays are now used in various configurations, as in multimode systems, and are very useful in meeting the needs of a software defined radio implementation.ASIC: Application specific integrated circuits (ASICs) implement the system circuitry in fixed hardware, resulting in the most optimized implementation of the circuit in terms of speed and power consumption. However, ASIC design requires sophisticated circuit design and layout software tools. Also, as the name implies, their use are for specific application and not subject to modification at a later date. A general purpose processor is similar to DSP as a hardware platform in the design of software defined radio. Like DSP, it offers flexibility and ease of design. Radio functionalities can be implemented in high level languages such as C and C++. Designers can use the familiar approaches of object oriented programming and debugging to develop real time software radio systems. This increases productivity significantly and thus reduces system development time. Digital signal processor is the most generalized type of hardware that can be programmed to perform various functions, while ASIC is the most specialized and can be used only in specific application. Field programmable gate arrays offers a compromise in flexibility between ASIC and DSP platforms. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  8. 8. Software Defined Radio For Next Generation Networks In general, these hardware components constitute design spaces that trade flexibility, processing speed, and power consumption among other things. There should be a tradeoff between the maximum flexibility and high power consumption of DSP platforms to minimum flexibility and less power consumption of ASICs compared to FPGAs, which have good hardware optimization. Recently, FPGAs have become increasingly popular due to their ability to reduce design and development cycle time. Furthermore, latest FPGAs come with intellectual property (IP) cores, which are used for specific applications. There are other advantages of using FPGAs instead of DSPs for signal processing in commercial telecommunication systems. The power consumption is lower, the size is smaller, quicker to use and the costs are much lower in comparison to DSPs. Since the chip can be reused after fixing the bugs or upgrading a system, they are ideal for prototyping and testing the circuit design. Since FPGAs are reprogrammable, one chip can be configured to perform more than one function and the configurations can be changed during run time.2.2 WIRELESS COMMUNICATION SYSTEM MODEL FOR SDR Block diagram of a generic digital transceiver. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  9. 9. Software Defined Radio For Next Generation Networks The digital radio system consists of three main functional blocks Radio Frequency Section Intermediate Frequency Section Baseband Section Radio Frequency Section: The radio frequency (RF) section is responsible for transmitting and receiving the RF signal and converting the RF signal into an intermediate frequency (IF) signal. The RF section consists of antennas and analog hardware modules. The RF front-end is designed in such a way to reduce interference, multipath and noise. The RF front-end on the receive side performs RF amplification and down conversion from RF to IF. On the transmit side, the RF section performs analog up conversion and RF power amplification. Intermediate Frequency Section: The ADC/DAC performs analog-to-digital conversion on the receive path, and digital to- analog conversion on the transmit path. These blocks interface between the analog and digital sections of the radio system. Usually, the above conversion takes place in the IF stage. Digitizing the signal with an ADC eliminates the last stage in the conventional model, where problems such as carrier offset and imaging are encountered. As the names imply, the digital down converter (DDC) and digital up converter (DUC) perform digital down conversion on the receive path and digital up conversion on the transmit path, respectively. Digital filtering and sample rate conversion are often needed to interface the output of the ADC to the processing hardware at the receiver. The same happens in the reverse direction in the transmitter, where digital filtering and sample rate conversion are necessary to interface the digital hardware to the DAC that converts the modulated waveform to an analog waveform. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  10. 10. Software Defined Radio For Next Generation Networks Baseband Section: The baseband section performs operations, such as error correction, equalization, frequency hopping, modulation, demodulation, spreading, and dispreading and timing recovery. Forward error correction is a method of obtaining error control in data transmission in which the transmitter sends redundant data and the receiver recognizes only the portion of the data that contains no apparent errors. Equalization is done to counteract the inter symbol interference in the channel. Frequency hopping and spreading is used to minimize unauthorized interception or jamming of the communication system by repeated switching of frequencies during radio transmission using a specified algorithm. 2.3 ARCHITECTURES OF RECEIVER AND TRANSMITTER Ideally the designer of an SDR would like to put the data converters directly on the antenna. However as stated previously, 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 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 basestation side. It is possible that future developments will enable direct conversion on the receive side as well. Until then, the super- heterodyne architecture will continue to beused in one form or another. 2.3.1 Receiver High performance SDR receivers are typically constructed from some variant of the super- heterodyne 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 INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  11. 11. Software Defined Radio For Next Generation Networksfront ends would allow operation across different RF bands. In the case of multicarrierapplications, this could be done simultaneously if necessary.MulticarrierDepending on the applications, one or more receive channels may be desired. Traditionalapplications may only require a single RF channel. However applications that require highcapacity or interoperability may require a multi-carrier design. SDRs are well suited formulti-carrier applications since they employ a highly oversampled ADC with ample availableandwidth. An oversampled ADC is one in which the sample rate is operating beyond thatwhich is required to meet the Nyquist criterion which states that the converter sample ratemust be twice that of the information bandwidth. Since an SDR may not have advanceknowledge of the bandwidth of the signal it will be used to receive, the sample rate must beappropriately high enough to sample all anticipated bandwidths.Current ADC technology allows high dynamic range bandwidths of up to 100 MHz to bedigitized. With this much bandwidth, it is also possible to process multiple channels. Thefigure below shows a typical multi-carrier receiver example. In this example, the sample rateof the ADC is set to 61.44 Mega-samples-per-second (MSPS), which gives a Nyquistbandwidth of 30.72 MHz. If each RF channel is 1.25 MHz wide then Nyquist indicates thatthe number of potential channel is about 24.5. In practice, by allowing for reasonabletransition bands on the anti-aliasing filters, the typical available bandwidth is one-third thesample rate instead of the Nyquist one-half. Thus the available bandwidth for our example is20.48 MHz, which is just over 16 channels at 1.25 MHz. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  12. 12. Software Defined Radio For Next Generation Networks Figure 1 . Multi Carrier ExampleSince the channel characteristics can be changed, it is easy enough to change the CDMAexample to a GSM example. In this case, both the digital pre-processing and the generalpurpose DSP are both reconfigured respectively by changing the digital channel filter fromGSM to CDMA and by loading the new processing code into the DSP. Since GSM channelsare 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 examplewould be to configure the receiver such that part of the channels could be CDMA while theother would be configured as GSM! Furthermore, if one of the configurations is at capacityand the other is under-utilized, CDMA channels could be converted into several GSMchannels or vice-versa providing the flexibility to dynamically reallocate system resources onan as needed basis, a key goal of software defined radio!Single carrierNot all SDR applications require more than one channel. Low capacity systems may requireonly one carrier. In these applications, a high oversampling is still desired. If the channel isreprogrammable, it is possible that it may be as narrow as a few kHz or as wide and 5 to 10 INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  13. 13. Software Defined Radio For Next Generation NetworksMHz. In order to accommodate this range of bandwidths, the sample rate should be suitablefor the highest potential bandwidth, in this case 10 MHz. From the multi-carrier example, wewould typically sample at least 3 times the bandwidth. In this example, a sample rate of 30.72MSPS or higher would allow signal bandwidths from a few kHz up to 10 MHz to beprocessed. Aside the fact that only one channel is processed, the single carrier receiver has allof the capacities of that of a multi-carrier receiver; it can be reconfigured as necessary. Figure 2: Single carrier Rx exampleSDR Receiver ElementsReferring to the single carrier block diagram above (but keep in mind that this applies to themulti-carrier example as well), a fully developed SDR will have all signal elements that areprogrammable.The antenna is no exception and unfortunately, the antenna is one of the weakest elements inan SDR [1]. Since most antenna structures have a bandwidth that is a small percentage of itcenter frequency, multi-band operation can become difficult. In the many applications wheresingle bands of operation are used this is not a problem. However for systems that mustoperate across several orders of frequencies such as the SpeakEasy discussed earlier, theantenna must be tuned by some means to track the operating frequency to maintain operatingefficiency. While it is true that just about any antenna can be impedance matched to theactive electronics, there is usually a sacrifice in the link gain potentially resulting in anantenna loss whereas most antenna designs should actually provide a modest signal gain.Therefore tuning the electrical length of the antenna is desired over simply changing thematching of the antenna. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  14. 14. Software Defined Radio For Next Generation NetworksNext in the signal chain is the band select filter electronics. This element is provided to limitthe range of input frequencies presented to the high gain stage to minimize the effects ofintermodulation distortion. Even in the case where intermodulation is not a problem, it ispossible that strong out of band signals could limit the amount of potential gain in thefollowing stages resulting in limited sensitivity. This is especially true for receivers tunednear television and audio broadcast services where transmit power levels can exceed 100 kW.This can be especially problematic for multi-carrier receivers where many orders of signalmagnitude must be dealt with. If all of the signals are of interest, then it will not be possibleto filter the stronger signals and the resulting receiver must have a relatively large signaldynamic range.Most receivers require a low noise amplifier or LNA. A SDR should ideally incorporate anLNA that is capable of operating over the desired range of frequencies. In addition to thetypical low NF and high IP3, it may be desirable to have the ability to adjust the gain andpotentially scale the power down (often NF and IP3 track bias current) when possible thiswill allow for a variety of signal conditions that exist across the bands of operation.Mixers are used to translate the RF spectrum to a suitable IF frequency. While only 1 mixer isshown in this diagram, many receivers may use two or three mixer stages, each successivelygenerating a lower frequency. [Note: Receiver IF‟s are not always lower than the RF signal.A common example is found in HF receivers where the desired RF signal may only be a fewMHz. In these cases, they are frequently mixed UP to IF frequencies of 10.7 MHz, 21.4 MHz,45 MHz or higher IF frequencies because of the availability or performance of the requiredcomponent.] Each successive stage also takes advantage of filtering that is distributedthroughout the chain to eliminate undesired images as well as other undesired signals thatmay have survived the mix down process. The filtering should also be appropriate for theapplication. A traditional single carrier receiver would generally apply channel filteringthrough the mixer stages to help control the IP3 requirements of each stage. Analog channelfiltering is not possible in the case of a multi-carrier receiver where the channel bandwidthsare not known in advance. Therefore, mixing process must preserve the entire spectrum ofinterest. Likewise our single carrier SDR application must also preserve the maximumpossible spectrum in case the SDR requirements need the full spectrum. In this case, it isprobable that our single carrier example may be processing many carriers, even if only one isof interest. As with the LNA, it would be desirable for the mixer in an SDR to have an INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  15. 15. Software Defined Radio For Next Generation Networksadjustable bias. As with the LNA, this bias could be used to properly set the conversion gainand IP3 of the device to correspond to the desired signal conditions.Some receiver architectures utilize a quadrature demodulator in addition to, or instead of amixer. The purpose of the demodulator is to separate the I and Q components. Once theyhave been separated, the I and Q paths must maintain separate signal conditioning. In thedigital domain this in not a problem, however, in the analog domain, the signal paths must beperfectly matched or I/Q imbalances will be introduced potentially limiting the suitability ofthe system. Many SDR receivers avoid this problem by utilizing „real‟ sampling (as opposedto complex sampling) as shown in the single carrier example and use a digital quadraturedemodulator in the digital pre-processor that will provide perfect quadrature.The local oscillator is used to generate the proper IF when mixed with the incoming RFsignal. Generally a local oscillator (LO) is variable in frequency and easily programmable viasoftware control using PLL or DDS techniques. There are cases where the LO may notrequire frequency hopping. One such example is for receiving multiple carriers within a fixedband. In this case, the LO is fixed and the entire band is block converted to the desired IF. Itoften may be desirable to change the LO drive level to optimize spurious performance undera variety of signal conditions.Quite often the IF amplifier is in the form of an AGC. The goal of the AGC is to use themaximum gain possible without overdriving the remainder of the signal chain. Sometimes theAGC is controlled from an analog control loop. However, a digital control loop can also beused to implement difficult control loops not possible using analog feedback. In multi-carrierapplications, use of an AGC may at best be difficult. If insufficient dynamic range isavailable in the receiver (determined largely by the ADC), reduction in gain from a strongsignal may cause weaker signals to be lost in the noise floor of the receiver. In applicationssuch as this, a digital control loop for the gain is ideal. The control loop can be used asnormal as long as no signals are at risk to being lost. However, if a weak signal is detected inthe presence of a very strong signal, the decision could be made to allow a limited amount ofclipping rather than reduce the gain and risk total loss of the weak signal. Conditionalsituations like this are much easier tocontrol with a digital control loop than with an analogloop, allowing much greater control of total conversion gain of the receiver.The ADC is used to convert the IF signal or signals into digital format for processing. Quiteoften the ADC is the bottleneck and selection of the ADC is often a driving factor that INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  16. 16. Software Defined Radio For Next Generation Networksdetermines the architecture of the SDR [1, 9, 10]. Often times, the designer is forced to selectthe best available ADC, realizing that under many conditions the ADC may be over specified.Still other times, air interface standards may not be directed towards multi-carrier receiversand require a much better ADCs than are required when deployed in the field, simply becauseof the test methodology specified by the standard. For the ADC it may be desirable to changethe sample rate, input range and potentially the active bandwidth.The digital pre-processor can take many forms. For very high sample and data rates, this isusually implemented as either an FPGA or ASIC. These circuits by nature are quite flexiblein their functions and range of parameters. An FPGA can of course be programmed for anyfunction desired. Typically, an FPGA would be programmed to perform the quadraturedemodulation and tuning, channel filtering and data rate reduction. Other functions such asRF power measurement and channel linearization are possible. All of these elements areeasily generated using a variety of digital techniques and are readily programmed by loadinga variety of coefficients to the FPGA. By doing this, a single chip configuration can be usedto generate a digital pre-processor capable of tuning the entire range of the ADC Nyquistband and filtering a signal with bandwidths from a few kHz to several MHz. When multiplechannels are required, the design can be repeated to fill the FPGA. If a lower cost option isrequired, a variety of ASICs are available that perform these functions. They are oftenreferred to as channelizers, RSPs or DDCs.The final element in the SDR is the DSP. Since this is general purpose DSP, it can beprogrammed for any required processing task. Typical tasks include equalization, detection,rake receiver functions and even network interfacing to name a few. Because they are fullyprogrammable, they can be used for just about any signal processing task as well ascontrolling all of the features in the other elements of the block diagram. As DSP clock ratesincrease, DSPs may well take over many of the functions within the digital pre-processors.2.3.2 TransmitTransmit functions like the receive are typically based on some form of super-heterodyne ordirect conversion. The figures below illustrate these two options. The multi-carrier option isbest suited to single and multi-carrier applications while the direct conversion offers anexcellent, low cost solution for single carrier applications. As integration technologyimproves, multi-carrier direct conversion may become a possibility, however, such a transmit INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  17. 17. Software Defined Radio For Next Generation Networksconfiguration requires sideband suppression about 15 dB better than the spuriousrequirements to prevent images on one side of the center frequency from overpowering apotentially weak carrier on the other. Figure 3 : Multi – channel transmit with single up-convert super-heterodyne Figure 4 : Single carrier direct conversion transmitIn either application, a DSP or baseband ASIC is used to generate the modulated basebanddata. This data is fed either directly to a pair of baseband DACs (I and Q) for direct RF INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  18. 18. Software Defined Radio For Next Generation Networksmodulation or to a digital processor responsible for digitally translating them to a suitabledigital IF. Depending on the application, the DSP alone or in conjunction with digitalprocessor can be used to digitally pre-distort the baseband data in such a manner thatdistortion products generated later in the signal chain will be cancelled.If an IF stage is employed, the baseband data generated by the DSP must be up-convertedeither digitally with an FPGA or ASIC (also know as TSPs or DUCs) or alternately with atraditional mixer or modulator to the desired IF. This traditional technique is being replacedby digital means because of the added flexibility offered through digital logic and theavailability of good cost effective digital to analog converters. As with the related receivefunction, the purpose of this device is to shape the bandwidth of the desired channel and thenup-convert by digital means to the desired IF frequency. If multiple channels are required,they can be synthesized on one chip. After translation, each of the channels can be summedtogether and interpolated to the desired data rate and then sent to a DAC. If desired, digitalpre-distortion can be added in conjunction with the DSP to correct for distortion later in thesignal chain.Either a mixer or a modulator is used for frequency translation to the final RF frequency.Ifdirect RF modulation employed, an RF modulator will be used. If an IF is used (eitherdirectly from a DAC or a traditional IF up-conversion), a mixer will be used to translate tothe final RF frequency. As with the receive mixer/demodulator, it may be desirable tochange the bias levels or the drive level of the data or LO levels to optimize distortion.As with the receive LO, the transmit LO is variable in frequency and easily programmablevia software control using PLL or DDS techniques. Here too, it may be desirable to changethe LO drive level to optimize spurious performance under a variety of signal conditions. Aswith the single band operation of the receiver, there may be cases where a fix LO is required.Such an example would be for operation within a single band where tuning is accomplishedwithin the TSP, DUC or FPGA.As with the receive path the data converter or DAC is often the bottleneck. However sincedynamic range requirements for the transmit signal path are much lower (typically 25 to 45dB) than the receive path, component selection is not quite is difficult. Many DACs areavailable that facilitate a wide range of adjustments include gain and offset correction so thatI/Q imbalances in the transmit signal chain can be minimized. Other desired features includedata rate interpolation and I/Q phase correction. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  19. 19. Software Defined Radio For Next Generation NetworksFinally, power gain is achieved through a pre-amp and PA. Aside from the fact that thesedevices must operate across a wide range of frequencies, it is desirable to adjust the RFoutput power. There could be regulatory issues that require some frequencies to betransmitted at lower power than others. While the PA gain is usually fixed, the pre-amp maybe in the form of a VGA. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  20. 20. Software Defined Radio For Next Generation Networks 3. FUTURE EVOLUTION OF SDRThe infrastructure market currently indicates that additional cost savings and efficiency arerequired in networks; both are possible with SDR solutions. However, mobile operators arenow seeking assurance that SDR base stations will require little capital to upgrade to futurestandards. Effectively, this means that current SDR solutions will need to evolve in technicalefficiency in order for operators to truly regard SDR as a core component of their businessmodel.Chipset manufacturers and infrastructure vendors are working to make SDR a bettertechnology and are focusing on several issues, including: Higher bandwidth power amplifiers (PAs) to allow amplifier reuse for several technologies. Current amplifiers are cost and power efficient up to 20MHz for commercial use, a figure which is far too low to be used for more than one air interface. Higher computational power for baseband processing: Future technologies, - particularly LTE– will require higher bandwidth, faster data rates and lower latency, meaning that the computational burden on the base station will be several times higher than today‟s implementations. Chipset vendors are now working on faster DSPs and FPGA chipsets that will allow these. A faster communication bus between these hardware components is also being developed, as the communication between the hardware components is now proving to be the processing bottleneck.Baseband processing to take place in RF. This is the ultimate SDR concept, meaning that allaspects of the base station are implemented in software and the power amplifier will be theonly component that remains in hardware. This will mean total re-configurability andupgradeability but is currently not possible for 3G frequencies and certainly not cost efficient.Research organizations have performed this up to 800MHz but are now discovering that thereis a critical limit in moving to higher frequencies, due to the unavailability of such highbandwidth RF components and the computational complexity required. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  21. 21. Software Defined Radio For Next Generation Networks 4. Advantages & Disadvantages In Deploying SDRADVANTAGES It is believed by many that the successful deployment of SDR will revolutionize the field of communication. One of the advantages of SDR is that it can be changed quickly to support multiple standards. The ability to configure devices, which may be used by many communication systems (e.g., cellular phones, wireless-fidelity (WI-FI) transceivers, frequency modulation (FM) and analog modulation (AM) radios, terminals of satellite communications), will be remarkable. With SDR, the same piece of hardware will be configured to perform different functions. The re-configurability of the platform will ensure hardware reusability. System re- programmability allows hardware reuse until a new generation of hardware platforms is available. This will provide cost and time savings. Manufacturers will not be limited to reduced hardware platform set. As a consequence, mass production will allow lowered costs. Another advantage of SDR would be the possibility to improve the software in successive steps, and the correction of software errors and bugs discovered during the operation. In addition, SDR can enhance the interoperability of different systems in many applications such as the military, law enforcement, or search and rescue teams. Incompatibility of radio systems that has always hindered the seamless operation of the military, the law enforcement agencies and many rescue teams, will be eliminated. With the increase of channel data rates through multiplexing and spectrum spreading, SDR could be used in cellular networks, GSM based PCS network, and future generation systems network. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  22. 22. Software Defined Radio For Next Generation Networks A new approach to wireless base station design using SDR has the potential of offering significant benefits such as reduced size, complexity, and power consumption. More importantly, SDR can support a variety of air interface standards, modulation schemes and protocols, simultaneously. Some commercial telephone service providers have begun expressing interest in the SDR economic benefits in long term. While SDRs offer benefits as outlined above, there are drawbacks in the design and implementation of SDR. Those include: (1) The difficulty of designing software for various target systems or standards. (2) The difficulty of designing air interfaces to digital signals and algorithms for different standards. (3) The problem of poor dynamic range in some communication systems design.DISADVANTAGES The difficulty of writing software for various target systems The need for interfaces to digital signals and algorithms Poor dynamic range in some SDR designs High power consumption and processing power INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  23. 23. Software Defined Radio For Next Generation Networks 5. APPLICATIONSIn military: • Secure, encrypted communications • Real time flexibilityIn commercial: • International connectivity • Enabled virtual private networksIn Civilian radio applications like • Bluetooth • WLAN • GPS, Radar • WCDMA • GPRS INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  24. 24. Software Defined Radio For Next Generation Networks 6. Conclusion The mobile technology has been evolving very rapidly. As new and more complexcommunication standards are developed transceiver architectures will also grow. Newdesigns will increase the complexity and the cost. However, software radio technology aimsto group these architectures and make them work on single platform in the last decadesemiconductor technology achieved impressive gains and now it is up to the SDR to furtherincrease the performance in terms of flexibility and efficiency. There are two main issues arecost and power. Without low power, user devices will not be able to take full advantage ofSDR technology. Despite these challenges, the current state of performance is more thansufficient for engineers and manufacturers to seriously begin to investigate the possibilities ofSDR. INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012
  25. 25. Software Defined Radio For Next Generation NetworksBIBLIOGRAPHY:www.sdrforum.comwww.ti.com INTERNATIONAL SCHOOL OF TELECOM & TECHNOLOGY MANAGEMENT AUG - 2012