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The digital technologies and the advanced computer systems emphasized and started shifting from digital hardware to software implementation of systems widely known as Software Defined Radio (SDR). In SDR's the channel bandwidth, rate, and modulation are all flexibly determined through software. In this paper the need for software defined radios, its advantages and its various applications are described. The software defined radio and its various modulation techniques like PSK, QPSK, QAM, MSK and OFDM are discussed. Any input signal can be either analog or digitally modulated using these modulation techniques and the transceiver is able to demodulate the modulated signal in order to retrieve the transmitted information.
Software Defined radio or just Software radio (SR) is a technological innovation that is coming of age for wireless communications of many types.The term Software Defined radio was coined by Mitola in 1991, to refer to a class of reprogrammable of reconfigurable radios. The initial goal of SDR is to replace as many analog components and hardwired digital VLSI devices of the transmitter-receiver as possible with programmable devices which will include Air interface, Data converters (ADC/DAC) Modulation and coding schemes.etc.Hence, the role of modulation techniques in an SDR is very crucial since modulation techniques define the core part for any wireless technology . Software Defined Radio is an advanced radio communication technology in which modulation and demodulation of radio signals is performed in software. The communication link consists of three components Transmitter, Channel and Receiver.A wide range of radio applications like Bluetooth, WLAN, GPS, Radar, WCDMA, GPRS, etc. can be implemented using SDR technology. Software radio is the technique of getting code as close to the antenna as possible. Thus it turns radio hardware problems into software problems.
SDR technique can receive various kinds of modulation signal just by programming of software based on DSP due to its system flexibility system. Therefore, SDR can be considered as an important radio technique for 4G system and driving modes of the various radio network such as cellular, digital multimedia broadcasting, W-LAN (wireless local area network) and W-PAN (wireless personal area network),etc (Nkjima et al., 2001). In this paper review of various modulation techniques like PSK, QPSK, MSK & QAM, OFDM are discussed.
2. NEED OF SOFTWARE DEFINED RADIO
In the past two groups of people using different types of radio systems (GSM, CDMA, and TDMA) were not able to do communication due to incompatibility problems. There was need to communicate for two groups of people with different equipments which can be solved using software programmable radios (SDR) because its architecture is flexible .The user can switch from one air interface format to another in milli seconds ,using Global positioning system (GPS) or satellite transmission.. In military communications the U.S. SpeakEasy programme formed some of the initial basis for the SDR Forum. A radio which can change not only its scrambling or encryption codes on an ad hoc basis, but one which can also change its modulation format, channel bandwidth, data rate, and voice codec type is clearly an exciting operational prospect. An adaptable radio of this type could both foil an enemy's attempts at eavesdropping and be configured to match operational requirements or conditions (e.g., propagation characteristics). Such a system clearly has huge potential benefits in the theatre of war.In case of civilian mobile communications any system that allows an operator or service provider to offer enhanced benefits or services relative to competing operators clearly has huge potential. If the existing GSM infrastructure hardware had been designed on software defined radio principles (and if it possessed sufficient processing power when it was installed in the early 1990s), then the cost of deploying 3G would be a much small.
. 3. ADVANTAGES OF SOFTWARE DEFINED RADIO
However, there are benefits that software radios can do and haven't been possible before: SDRs have several advantages over today's hard-wired radios.
Multifunctionality- SDR can support infinite variety of service capabilities in the system. The same set of hardware i.e. the radio set may be used to transmit, receive and process different communication signals which represents air interface standards. They can talk and listen to multiple channels simultaneously. Software Radios can adapt to changing network environments.
Multicarrier - Depending on the applications, one or more receive channels may be desired. Applications that require high capacity or interoperability may require a multi-carrier design.
Global Mobility - A number of communication systems which exist today are IS-136, GSM, IS-95/CDMA1 (2G). So the ability of radios to operate with all of these standards (2G, 3G) in different regions of the world has increased the growth of software radio .
Ease of manufacture- RF components are much hard to standardize and may have varying performance characteristics. Hence digitization of signal can result in a design which incorporates fewer parts hence reduced inventory of manufacture. A SDR consists of much fewer hardware parts than a traditional radio as most of the processing is done in software using digital signal processing (DSP), application specific integrated circuits(ASIC) or Field programmable gate arrays(FPGA)
Software replaces hardware as much as possible and leads to reduction in cost, Increases versability, Equipment /infrastructure 'recycling'. Because of their modularity and flexible software architecture, SDRs will be an effective low-cost solution for both manufacturers and end users.
4. APPLICATIONS OF SOFTWARE DEFINED RADIO
SDR is able to support applications like WLAN, radar, Global Positioning system (GPS), radar, WCDMA, Bluetooth and GPRS. Software Communications Architecture concept came out of the Joint Tactical Radio System, JTRS (Defense Applications). During this project it was necessary to assemble software for the Software Defined Radio from a variety of different suppliers. Thus it was also necessary to re-use software whenever possible. According to that the Software Communications Architecture was defined and was implemented. SDR Architecture is ideally suited for evolving 802.16 WIMAX Standards.
5. MODULATION TECHNIQUES OF SOFTWARE DEFINED RADIO
Different digital modulation schemes were employed in this study for adaptation according to need. These includes the Phase Shift Keying (PSK), Binary Phase Shift keying (BPSK), quadrature phase shift keying (QPSK), Minimum Shift Keying (MSK), Quadrature amplitude Modulation (QAM), orthogonal frequency-division multiplexing (OFDM)- which is selected adaptively by either the user or a master controller software module to match the transmitting and receiving environments.
Software radio is concerned with digital bandpass modulation or carrier modulation. In bandpass modulation a sequence of digital symbols are used to change the characteristics of a sinusoidal waveform. The three characteristics of a sine wave are amplitude, phase, and frequency so the basis modulation schemes are therefore amplitude modulation, frequency modulation, and phase modulation.
Phase Shift Keying (PSK) is a modulation technique in which the phase of the carrier wave is modified based on input signal to map data symbol to corresponding phase status. PSK is considered to be an efficient form of data modulation as it provides the lowest probability of error for a given received signal level, when it is measured over one symbol period. Satellite communication systems and Terrestrial microwave radio links and also utilize PSK as their modulation format. If the phase of the signal is changed in accordance with the digital information data, then the modulation scheme is called Phase Shift Keying. . A carrier signal may be represented as follows:
PSK Modulation scheme detection was implemented as of software radio system. Figure 1 shows a basic block diagram of a software radio receiver illustration how the recognition section is integrated into the radio. The software radio processing core uses a sequential processing block approach; each layer can be added or removed as required resulting in very flexible software radio architecture.
The BPSK wave is generated by multiplying between the digital signal data and the carrier wave . Binary Phase Shift Keying (BPSK) allows binary information to be contained in two signals with different phases. The two phases normally used are 0 and Input data 0or 1 is directly converted to phase 0 or respectively as shown in the following equation (Harada & Prasad 2002).
S(t) = A cos ( 2Ï€fct + Ï€.dk) (2)
Fig.1 schematic block diagram of radio system
Figure 2 shows how digital information and carrier wave can be combined to form the BPSK transmission waveform.
Fig.2 Graphical depiction of the BPSK wave .
Even though there is only one transmission waveform, the BPSK signal can be viewed
as two different signals as follows:
, 0tTb (for binary1) (3) , 0tTb (for binary 0) (4)
where, A = Amplitude, fc = Center frequency
The above Equations represent signals which are called antipodal implying that they are equal and opposite. The two signals have the same frequency and energy and only the phase is modulated which leads to a constant waveform envelope as opposed to a varying waveform envelope that would be found in an amplitude modulated signal. The BPSK scheme it can be said that modulation is simple but the information rate or data rate is not good.
BPSK is not a commonly used modulation technique due to its bit rate, however Quadrature phase shift keying (QPSK) which is only slightly more complex than BPSK allows the bit rate to be doubled . QPSK is the most widely used phase modulation scheme and has applications that range from voice-band modems to high-speed satellite transmissions (Wilson 1996).
The QPSK signals are defined as follows:
Si(t)=Acos(2Ï€fct+Ó©i), 0â‰¤tâ‰¤T, i=1,2,3,4 (5)
The four available phases are therefore Ï€/4, 3Ï€/4, 5Ï€/4, and 7Ï€/4. Four combinations of dibits (two bits) can be represented as shown in the phasor diagram or signal constellation figure 3.
Fig.3 QPSK signals phasor representation.
QPSK can also be referred to as Quadriphase shift keying or 4-PSK. As outlined by Wilson (1996) .
QPSK is frequently used for several reasons:
The signals are easily formed by sign (not sine) modulation of a carrier or quadrature versions of the carrier. The signals have constant amplitude and can be amplified by nonlinear devices. Also have Reasonable level of bandwidth conservation.
The QPSK modulation process is shown in figure 4.Itconsists of serial to parallel converter, a pair of product modulators, a supply of two carrier waves in phase quadrature and a summer . Serial to parallel converter represents each successive pair of bits of the incoming binary data stream m (t) as two separate bits, with one bit applied to the in-phase channel of the transmitter and other bit applied to the quadrature channel. Hence for a given transmission bandwidth, a QPSK system carries twice as many bits of information as the corresponding binary PSK system.
Binary wave m(t)
Serial to parallel converter
Ac cos (2ÐŸ fc t)
Ac sin (2ÐŸ fc t)
In - Phase Channel
Fig.4 QPSK Modulation Process.
QPSK demodulation process shown in figure 5 consists of two correlators connected in parallel. One correlator computes cosine of carrier phase, and other computes sine of carrier phase. Hence comparing the signs of the two correlators output by means of decision devices, a unique resolution of one of the four transmitted phase angles is made. Parallel to serial converter interleaves decisions made by in-phase and quadrature channels or receiver hence constructs a binary data stream which in absence of noise is identical to original data at transmitter input.
Parallel to serial converter
In - Phase Channel
Fig.5 QPSK Demodulation Process
QAM is Quadrature Amplitude Modulation refers to QPSK with Amplitude Modulation. Quadrature Amplitude Modulation (QAM) is a modulation scheme which is carried out by changing (modulating) the amplitude of two carrier waves. The carrier waves are out of phase by 90 degrees, and are called quadrature carriers. QAM (Quadrature Amplitude Modulation) is chosen to be the modulation scheme of the designed software defined radio system noting that this modulation is widely used for data transmission applications over bandpass channels such as FAX modem, high speed cable, multi-tone wireless, and satellite channels.
In particular, digital cable television and cable modem utilize 64-QAM and 256-QAM [12-14].
QAM modulation process shown in figure 6 involves the use of two separate product modulators that are supplied with two carriers of same frequency but differ in phase by -900 .The multiplexed signal s (t) consist of sum of these two product modulator outputs as
S(t)=Acm1(t) cos (2ÐŸ fc t) +Acm2(t) sin(2ÐŸ fc t) (6)
Where m1 (t) and m2 (t) are two different message signals applied to product modulators.
The two paths to the adder are typically referred to as the 'I' (inphase), and 'Q' (quadrature), arms. QAM restores the balance by placing two independent DSBSC, derived from m1 (t) and m2 (t) in the same spectrum space as one DSBSC. The bandwidth imbalance is removed It is used because of its bandwidth conserving properties.
Multiplexed signal s(t)
Fig.6 QAM Modulation Process
QAM Demodulation process shown in figure 7 in which the multiplexed signal is applied simultaneously to two separate coherent detectors that are supplied with two local carriers of the same frequency, but differ in phase by -900.Hence the carrier is recovered in correct phase and frequency and multiplied with the QAM signal. Hence I and Q signal outputs are recovered.
Fig.7 QAM Demodulation Process
The constellation diagram for 16-QAM is shown in figure 8 .This scheme has twice the bit rate of QPSK with the same bandwidth. QAM is therefore a popular digital modulation technique. A disadvantage of QPSK and QAM is the spectrum splatter that is caused by abrupt changes at bit intervals. MSK, which can be thought of as a continuous phase shift keying technique, amends this problem.
Fig.8 16-QAM constellation diagram.
MSK is a type constant envelope modulation. A specific form of FSK known as Minimum Shift Keying Minimum shift keying (MSK) is a continuous phase or frequency modulation scheme. It is a special form of continuous phase (CP) FSK arises when the change in carrier frequency from symbol 0 to symbol 1 or vice versa is equal to one half of bit rate of the incoming data. It possesses properties such as: constant envelope, spectral efficiency, good BER performance, self synchronizing capability .
MSK continues from OQPSK by weighting each I channel and Q channel bit with a half Period of a cosine or sine waveform respectively .
Serial to Parallel
Fig.9 MSK Modulator
Figure 9 shows MSK Modulation process. The output of the first multipliers will be I(t) and Q(t).The two channels are then modulated onto orthogonal carriers and added together to produce the MSK signal.
Fig.10 MSK Demodulator
Figure 10 shows MSK demodulation process. The output of the first multiplier for the I channel will be:
Only the first term is required so a low pass filter is used to reject the two higher terms. After the signals have been filtered they are then multiplied by the cosine and sine functions (each with period of 4T). If their is little or no noise and channel impairments the threshold detector which has a zero threshold level can directly resolve the binary data.
GMSK is Gaussian minimum shift keying. GMSK can be viewed as either frequency or phase modulation.A variant of MSK called GMSK yields improved efficiency and increased output power. The spectral efficiency of MSK is further enhanced by filtering the baseband signal of square pulses with a Gaussian filter. GMSK is spectrally tighter than MSK. It is same as MSK except instead of half sinusoid as a pulse shape a Gaussian pulse shape is used instead.
GMSK is used in several mobile systems around the world. Global system for mobile (GSM), Digital European cordless telephone (DECT), Cellular digital packet data (CDPD) etc.
GMSK premodulation filter has an impulse response given by
hG (t)= exp (8)
HG(f)=exp(-Î±2f2), Î±= =
Orthogonal Frequency-Division Multiplexing (OFDM) a type of frequency-division multiplexing (FDM) scheme and is utilized as a digital multi-carrier modulation method. In an OFDM modulation, all orthogonal subcarriers are transmitted simultaneously. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion In other words, the entire allocated channel is occupied with the aggregated sum of the narrow orthogonal sub-bands. Thus, since it is a combination of multiple carriers, the OFDM-modulated signal can be considered to be composed of a great number of independent identically distributed (IID) random variables . OFDM is the modulation scheme which is mainly used in wireless communication. OFDM is a modulation scheme that is used instead of modulation such as BPSK and QAM in order to allow transmission of independent signal carriers simultaneously.The orthogonal property maintained in OFDM eliminates the inter symbol interference which is a common problem in wireless communication . OFDM communications system is very useful for the high data rate transmission.It is highly scalable, allowing expansion or reduction of the signal bandwidth to accommodate the dynamic creation or removal of signal carriers hence widely used in a variety of important applications such as mobile radio and digital broadcasting. Each subcarrier can be modulated differently, typically using bi-phase shift keying (BPSK), quadrature phase shift keying (QPSK), or quadrature amplitude modulation (16QAM or 64QAM).Different advantages of OFDM waveforms include robustness in a multipath propagation environment and good tolerance of delay spread.Figure12  is shown to identify the type of modulation. A tree structure has been proposed that combines an initial normality test to differentiate between multi-carrier signals such as OFDM and single-carrier signals with a combination of other tests and methods to further extract parameters and determine the exact type of modulations to apply the necessary demodulation method to the signal. The I-Q diagrams for transmitted signal and the received signal for the model are in Figure 13.
OFDM, n carrier
Fig.12 Tree Structure for modulation classification
Fig.13 I-Q diagrams of (a) transmitted and (b) received signals for OFDM-16QAM modulation 
We have reviewed and summarized different modulation techniques for software defined radio, its advantages, need and modulation and demodulation schemes. Users will be greatly benefited from the equipment built using the SDR platform as they support multiple communication standards with the same given hardware. There will be no need of buying multiple equipments for multiple purposes as one equipment can take care of all the things. In the near future all the mobiles, talkies that are used will become more and more flexible and tend towards SDR platform rather than the traditional architectures being used now. A day will come where a single transceiver can act as all in one and can be used as a mobile phone which can support both GSM and CDMA, which can also be connected to a WLAN access point, which has got Bluetooth connectivity, which can receive FM signals and work as FM radio etc. SDR's enable developers to build smarter radios with decision-making capacity. Cognitive Radio (CR), recognized by many industry experts as the next step in SDR technology, is an area where such benefits can be realized.