This paper describes the realization of a circuit that transmits a data stream, through spatial modulation in the 2.45 GHz frequency band. The development of the transmitter includes RF circuit design with components such as a PLL synthesizer, Tx-DAC and IQ-modulator. A microcontroller, integrated into the circuit and programmed in C, is at the heart of the system. 

In this hardware system, developed specifically for spatial modulation, data is BPSK modulated and transmitted through an RF switch connected to two antennas. It can differ for every symbol which antenna is used, according to an extra series of information bits that are to be transmitted. Here the number of the selected antenna encodes the extra information bit per symbol, which not only results in a doubling of the data rate but also realizes diversity. Spatial modulation allows these features with only a single hardware transmit chain, resulting in low-cost and low-complexity hardware. At the receiving side, the extra information bits are decoded by assessing the channel used for each symbol. 

This practical system has been thoroughly tested by means of different measuring campaigns. The measurement results show that spatial modulation is correctly demodulated at the receiving side and forms an effective way to realize affordable MIMO systems

Van Torre, Patrick

Verhaevert, Jo

English

Ghent University Academic Bibliography

Realization and MIMO-link Measurements of aTransmit Module for Spatial ModulationJo Verhaevert1, Patrick Van Torre11Industrial Technology & Construction, Ghent University, Valentin Vaerwyckweg 1, B-9000 Gent,Jo.Verhaevert@UGent.Be, Patrick.VanTorre@UGent.BeAbstract—This paper describes the realization of a circuitthat transmits a data stream, through spatial modulation inthe 2.45 GHz frequency band. The development of the trans-mitter includes RF circuit design with components such as aPLL synthesizer, Tx-DAC and IQ-modulator. A microcontroller,integrated into the circuit and programmed in C, is at the heartof the system.In this hardware system, developed specifically for spatialmodulation, data is BPSK modulated and transmitted throughan RF switch connected to two antennas. It can differ for everysymbol which antenna is used, according to an extra series ofinformation bits that are to be transmitted. Here the numberof the selected antenna encodes the extra information bit persymbol, which not only results in a doubling of the data rate butalso realizes diversity. Spatial modulation allows these featureswith only a single hardware transmit chain, resulting in low-costand low-complexity hardware. At the receiving side, the extrainformation bits are decoded by assessing the channel used foreach symbol.This practical system has been thoroughly tested by means ofdifferent measuring campaigns. The measurement results showthat spatial modulation is correctly demodulated at the receivingside and forms an effective way to realize affordable MIMOsystems.Index Terms—RF-design, MIMO, modulation, spatial modu-lation.I. INTRODUCTIONAs more and more wireless applications arise, with moreand more users who each demand a higher data rate, thecontinuous development of more and more advanced wirelesstechnology is required. On top of that, the radio spectrum andavailable bandwidth is limited. Transmitting more and moredata within the same bandwidth by increasing the spectralefficiency forms a great challenge.Different techniques exist to obtain a higher data rate withinthe same bandwidth. MIMO (Multiple Input, Multiple Output)systems effectively increase the data rate, but require fullhardware transmit or receive chains connected to each antenna,resulting in complex and expensive systems.One of the latest techniques to economically increase spec-tral efficiency, is spatial modulation, where a MIMO systemcan be realized with only one transmit chain [1]. A streamof additional information bits, synchronous to the modulatedsymbols, selects a transmit antenna number as a functionof these information bits. Based on its channel estimationinformation, the receiver is able the decide which antennatransmitted each particular symbol, effectively recovering thestream of extra information bits. This transmission schemeoffers significant advantages in comparison to a SISO (SingleInput, Single Output) system, at a very limited extra cost.In this paper, the design and practical implementation ofa transmitter for spatial modulation is described with twotransmit antennas. Using BPSK (Binary Phase Shift Keying)modulation and switching between antenna 1 or 2 based onthe extra information bits to be transmitted, the data rateand spectral efficiency are doubled effectively. Section IIhandles the hardware design for the transmit circuit, whereasSection III focuses on the control software and the processingat the receiver. Section IV documents the measurement results,illustrating that the transmit design is realized successfully.Finally, the conclusions are listed in section V.II. TRANSMITTERFigure 2 contains the block diagram of the transmitter. Thecentral part is the Silicon Laboratories C8051F320 microcon-troller [2], which is clocked by an external 24 MHz crystaloscillator. In the final lay-out the microcontroller is placed atthe backside of the PCB, to limit the interference to the RFcomponents, which are all on the opposite side of the board’sground plane.Microcontroller DAC QAM-modulatorSynthesizer SwitchBalunAmplifierOutput 1 Output 2Fig. 2. The block diagram of the transmitter.The DAC (Digital to Analog Convertor) of the block dia-gram is an AD9761 [3], providing analog baseband signals tothe modulator. In order to modulate the data, an AD8346 IQ-modulator [4] is used, allowing QAM- or phase-modulation.The Mini-Circuits DSN-2520A-219A+ RF frequency syn-thesizer [5], a hybrid component including a PLL and VCO, isemployed to accurately generate the carrier frequency withinthe 2.45 GHz frequency band. The external reference crystalFig. 1. Full circuit diagram of the 2-antenna spatial modulation transmitter.oscillator FOX-HC736R-20 [6] operates at a frequency of 20MHz, with ±25 ppm stability. The balun in the block diagramtransforms the synthesized carrier into a differential signal,as required for the balanced operation of the IQ-modulator’sinternal circuitry. The final power amplifier HXG-242+ has anamplification of 13.6 dB, specified at 2.36 GHz [7]. Driven bythe extra information bits, the RSW-2-25P+ [8] switches theRF-signal between both transmit antennas, providing the Spa-tial Modulation. The latter switching operation is performedduring the zero-crossings of the transmitted pulses, in orderto prevent abrupt transitions and hence limit the transmissionbandwidth.The design of the PCB is realized using Cadsoft Eagle [9],for the circuit diagram and board design. The diagram of thecomplete circuit is displayed in Figure 1. In the bottom leftcorner of it, the microcontroller as heart of the circuit can beseen. The microcontroller is connected with its neighbour thefrequency synthesizer, which is also controlled by it. At thebottom right, the DAC can be seen, which is connected viathe IQ-modulator above at the right and the balun to the leftof it, to the RF-switch. This switch is also controlled via themicrocontroller and is connected to the antennas via SMA-connectors.The PCB implementation is shown in Figure 3. Although allRF paths are kept as short as possible, inevitably some signalradiation of the PCB occurs, given the small wavelength of12 cm at 2.45 GHz. To shield the PCB, an aluminum enclosureis used, with only the grounded SMA antenna connectors at theoutside (at the right on Figure 3). Please note that the powersupply enters the casing by means of a feed-through capacitor,allowing only the DC voltage and not the RF-signals to crossthe shielding.Fig. 3. The realized PCB with complete circuit of the transmitter.III. SOFTWARE AND PROCESSINGThe microcontroller controls the complete system and gen-erates I- and Q-signals via the DAC. In order to detect thedifference between both antennas in the received signals, pilotsymbols are used. Those are a fixed sequence of pseudorandom symbols, which is known at the receiver side. Becausethe transmit antennas are separated in space, the transmittedsignals have phase and amplitude differences at the receiverside. Based on channel estimation the receiver can estimatefrom which antenna exactly each symbol has been transmitted.The receiving part of the MIMO communication system isa Signalion HaLo430 [10] which transfers the sampled re-ceived signals, at 4 different receiving antennas, to Matlab forpostprocessing. The receiving antennas are the elements of aTip-Truncated Equilateral Microstrip Patch Antenna (ETMPA)array [11], [12]. The antenna elements are positioned at acenter-to-center distance of 92 mm, corresponding to 3/4λat 2.45 GHz.0 0.5 1 1.5 2 2.5x 10400.010.020.030.040.050.060.070.080.09All IQ samples, modulusSamples @ fs = 100 ksamples/sAmplitude (RX ADC value)Fig. 4. Pilots and data.The received Matlab data provides I- and Q-samples. Afterremoving the DC-offset, the signal is downsampled by a factor100, resulting in a sample rate of 100 ksamples/sec. In Figure4 the frames, consisting of 2 pilot sequences followed bythe data symbol sequence, are displayed. Clearly, the signalsfrom one antenna experience more fading than those from theother antenna, resulting in a detectable difference in amplitudebetween both received signals. Note that the detection of theselected antenna uses the amplitude as well as the phase ofthe received symbols and estimated channels, implicating thateven equal-gain channels can be distinguished, based on theirphase difference.In processing the received signal, coarse time synchroniza-tion is performed, searching for the sequence of pilot symbols,the data and the gap between them. The Oerder & Meyralgorithm [13], further provides fine time synchronization,indicating the optimal sample moments for the symbols (cor-responding to the center of the opening in an eye-diagram).IV. MEASUREMENT RESULTSIn this section, measurement results are shown, with thePCB shielded by the aluminum case. A Non Line-of-Sight(NLoS) configuration was selected, with the anechoic chamberin between transmitter and receiver and approximately 8 mseparation. The propagation path hence can be via reflectionagainst the wall, resulting in a specular component and a QuasiNLOS configuration. There is an orientation difference of 90◦between both whip antennas and for both antennas an angleof approximately 45◦ with respect to the ground. Also notethat no coaxial cables are used and the whip antennas areconnected directly to the SMA-connectors of the aluminumcase.1 2 3 4 5 6 7 8 9 1005101520253035Sample level time syncSample offset @ fs = 100 ksamples/sOerder & Meyer amplitudeFig. 5. Oerder and Meyr time synchronization.The Oerder & Meyr algorithm described earlier results inFigure 5, where a maximum occurs at the best sampling time,being every sixth sample in this case. The signal is nowdecimated by keeping only every sixth sample.−0.015 −0.01 −0.005 0 0.005 0.01 0.015−0.015−0.01−0.00500.0050.010.015QuadratureIn−PhasePilot 1, uncorrected scatterplotFig. 6. Uncorrected scatterplot for pilot 1.−0.05 0 0.05−0.08−0.06−0.04−0.0200.020.040.060.08QuadratureIn−PhasePilot 2, uncorrected scatterplotFig. 7. Uncorrected scatterplot for pilot 2.Even a small frequency offset between the oscillators at thetransmitter and receiver results in a rotation of the receivedconstellation over 360◦, and hence needs to be compensatedfor. The error on the crystal frequency at the transmitter sideis specified at 25 ppm, whereas an error of 1 ppm on afrequency of 2.45 GHz already results in a frequency error of2450 Hz. The uncorrected scatter plot in the IQ-plane is givenin Figure 6 for pilot 1 and for pilot 2 in Figure 7. Please notethe weaker received amplitude level of pilot 1 in comparisonto pilot 2.−0.015 −0.01 −0.005 0 0.005 0.01 0.015−0.015−0.01−0.00500.0050.010.015QuadratureIn−PhasePilot 1, removed frequency offset of 3683 HzFig. 8. Scatterplot with compensated frequency offset for pilot 1.The next step in the processing is now estimating thisfrequency offset and compensating for it, in order to eliminatethe constellation rotation. The software iteratively compensatesthe received signal for frequency offsets within a given rangeand selects the result with the least residual rotation in theIQ-plane. In the particular case documented here, a frequencyoffset of 3683 Hz is removed and the results are depicted inFigure 8. The frequency offset of the second pilot sequenceis also compensated for with the same offset and the resultingconstellation is shown in Figure 9.−0.05 0 0.05−0.08−0.06−0.04−0.0200.020.040.060.08QuadratureIn−PhasePilot 2, removed frequency offset of 3683 HzFig. 9. Scatterplot with compensated frequency offset for pilot 2.0 50 100 150 200 250 300−25−20−15−10−505101520Pilot 1 phase noiseSymbol nr.DegreesFig. 10. Phase noise for pilot 1.0 50 100 150 200 250 300−20−15−10−505101520Pilot 2 phase noiseSymbol nr.DegreesFig. 11. Phase noise for pilot 2.The phase noise of the first pilot is given in Figure 10and for the second pilot in Figure 11. The phase noise iscentered around 0◦ with a variation around ±10◦ for bothpilot sequences, indicating accurate channel estimates, usablefor spatial demultiplexing. The remaining phase shift is alsoestimated and compensated for. These results are shown inFigures 12 and 13, with a similar spread of both pilots in thescatterplot, due to the phase noise.−2 −1 0 1 2x 10−4−2−1012x 10−4QuadratureIn−PhasePilot 1, phase−correctedFig. 12. Corrected scatterplot with phase offset for pilot 1.−5 0 5x 10−3−5−4−3−2−1012345x 10−3QuadratureIn−PhasePilot 2, phase−correctedFig. 13. Corrected scatterplot with phase offset for pilot 1.−0.5 0 0.5 1−1−0.8−0.6−0.4−0.200.20.40.60.81Spatial Demux  Antenna 1Antenna 2Fig. 14. Spatial demultiplexed signal on both antennas.The final step is the demultiplexing of the data signal inorder to detect the spatially modulated bits, corresponding tothe values 1 and −1 for transmit antennas 1 and 2, respectively.This results in Figure 14, showing a symmetric received signalconstellation and a correct demodulation of BPSK. Giventhe dramatic difference in amplitudes for signals originatingfrom different transmit antennas, as indicated by the scaleof Figures 12 and 13, the scatterplot in Figure 14 indicatesan error free determination of the antenna from which eachindividual symbol was transmitted. In the latter figure, thereceived BPSK constellation is plotted with compensation forthe channel gains corresponding to each individual symbol,resulting in a normal BPSK constellation, despite the largedifference in complex channel gains. The results indicate thatit is perfectly possible to capture the correct informationbits, even at lower signal-to-noise ratios than present in thisparticular experiment.V. CONCLUSIONSA Spatial Modulation Transmitter was successfully designedand practically implemented on a printed-circuit board, trans-mitting a BPSK signal on a 2.45 GHz RF-carrier and spatiallymodulating this signal on two antennas. The full designincluding all key components is documented, as well as the fullschematic diagram. The measurement results indicate a correctoperation of the system. The transmitted signals are receivedand demodulated correctly. The data can be correctly detectedas well as spatially demultiplexed. Both the bits conveyed inthe BPSK symbols as well as the bits encoded in the selectedantenna numbers are detected at a very low bit error rate.ACKNOWLEDGMENTThe authors want to express their gratitude to ElectronicsEngineering student Joren De Witte for the fruitful cooperationand the interesting work. Without his efforts, the practicalimplementation would not have been realized.REFERENCES[1] N. Serafimovski, A. Younis, R. Mesleh, P. Chambers, M. Di Renzo,C.-X. Wang, P. M. Grant, M. A. Beach, and H. Haas, PracticalImplementation of Spatial Modulation, IEEE Transactions on VehicularTechnology, vol. 62, no. 9, pp. 4511-4523, Nov. 2013.[2] Silicon Laboratories, C8051F320/1 [Rev. 1.1], Austin, 2003.[3] Analog Devices Inc., Dual 10-bit TxDAC+ with 2x Interpolation FiltersAD9761 [Rev. C], Norwood, 2003.[4] Analog Devices Inc., 0.8 GHz to 2.5 GHz Quadrature ModulatorAD8346 [Rev. A], Norwood, 2005.[5] Mini-Circuits, Frequency Synthesizer DSN-2520A-219+ [Rev. A], NewYork.[6] FOXElectronics, Model: FXO-HC73 SERIES [Rev. 28/11/2007], FortMyers, 2008.[7] Mini-Circuits, Ultra High IP3 Amplifier Module HXG-242+ [Rev. A],New York.[8] Mini-Circuits, High Isolation Switch RSW-2-25P+ [Rev. G], New York.[9] Eagle PCB design software, http://www.cadsoftusa.com/[10] Signalion GmbH, HaLo 430 Hardware In The Loop MIMO Prototyping& Monitoring Platform, Dresden.[11] L. Vallozzi, H. Rogier, and C. Hertleer, Dual Polarized Textile PatchAntenna for Integration Into Protective Garments, IEEE Antennas andWireless Prop. Lett., vol. 7, pp. 440-443, 2008.[12] M. Scarpello, L. Vallozzi, H. Rogier, and D. Vande Ginste, High-Gain Textile Antenna Array System for Off-Body Communications,International Journal of Antennas and Propagation, vol. 2012, doi:[10.1155/2012/573438].[13] M. Oerder and H. Meyr, Digital Filter and Square Timing Recovery,IEEE Transactions on Communications, vol. 36, pp. 605-612, 1988.

Realization and MIMO-link measurements of a transmit module for spatial modulation

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Realization and MIMO-link measurements of a transmit module for spatial modulation

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