International Journal of Advances in Engineering Science and Technology 01 www.sestindia.org/volume-ijaest/ and www.ijaestonline.com ISSN: 2319-1120 High Data Rate QPSK Modulator with CCSDS Punctured FEC channel Coding for Geo-Imaging Satellite Rahul Gupta, Sohan M. Patel, Dipti R. Patel, D. R. Gandhi SATCOM & Navigation Payload Area Space Applications Centre, Indian Space Research Organization rahulgupta@sac.isro.gov.in, gupta.89.rahul@gamil.com Abstract - The GISAT (Geo-Imaging Satellite) mission of ISRO (Indian Space Research Organization) is to provide state-of-the-art high resolution imaging of earth from the Geo-stationary orbit [1]. The large amount of this imaging data is to be sent in real time on earth. Therefore, a high data rate QPSK modulator at 200 Mbps is designed. CCSDS (Consultative Committee for Space Data Systems) standard punctured FEC (Forward Error Correction) convolutional channel encoding is also used at baseband, prior to modulation to compensate the channel losses. This paper describes the design of punctured (rate 7/8) FEC dual channel encoding and QPSK modulator. The results of above HDRM (High Data Rate Modulator) hardware developed for GISAT mission are also described and compared with the theoretical and simulated results. Keywords - GISAT, HDRM, CCSDS, Punctured FEC, QPSK modulator, PLL. I. INTRODUCTION ISRO (Indian Space Research Organization) is responsible for designing and launching of communication, Imaging and remote sensing satellites for Indian sub-continent region applications. ISRO is using the low earth orbits (LEO) for remote sensing spacecrafts while geostationary earth orbit (GEO) is used for communication and meteorological satellites. ISRO is engaged in meteorological observations since 1982 through INSAT-1 series and subsequently INSAT-2 & 3 series as well as the exclusive KALPANA spacecraft [1]. Though they have high temporal resolution, spatial and spectral resolutions need to be improved for several new applications. GISAT (Geo-Imaging Satellite) is configured to address these new requirements. GISAT consists of a state of the art meteorological payload as a main payload, designed for enhanced meteorological observation and monitoring of land and ocean surfaces for weather forecasting & disaster warning on a continuous basis. The allotted frequency band [2] in Ku-band is of 200 MHz with center frequency as 12.35 GHz. An on-board data transmitter is proposed to transmit high resolution imaging data from Geo-stationary satellite to the earth stations. One of the major subsystems of on-board data transmitter is HDRM (High Data Rate Modulator) which modulates the imaging data at IF (Intermediate Frequency) proposed as 375 MHz. Channel encoding scheme is also proposed to compensate the channel losses in the RF signal during transmission from GEO to earth stations. But due to the limitation of available bandwidth, QPSK modulation with higher rate punctured FEC (Forward Error Correction) convolutional channel encoding at baseband signal, prior to modulation, is planned to use. This paper explains the design, simulation and measured results of developed HDRM with above described configurations. II. HDRM SPECIFICATION The major specifications [2] of developed HDRM are shown in the Table 1. The difference in the I and Q channel s gains during signal generation in the modulator, is referred to as Amplitude Imbalance. The Phase Imbalance is the phase difference between the measurement vector and the reference vector. This imbalance is also introduced by the transmitter imperfection.
IJAEST, Volume 3, Number 1 Rahul Gupta et al. 2 TABLE 1 SPECIFICATIONS OF HDRM Sr. No Parameter Specification 1 Input Data Rate 200 Mbps 2 Input Data Format NRZ - L 3 Input Data Interface LVDS 4 Modulation scheme QPSK 5 Carrier Frequency 375 MHz 6 Differential Encoding NRZ - M 7 Channel Coding Convolutional FEC Rate 7/8 8 Output Power -10 dbm max. 9 Amplitude Imbalance < 0.5 db 10 Phase Imbalance < 3 degree 11 Error Vector Magnitude (EVM) < 7% 12 Carrier Suppression < -25 db All these result to the measured symbol position at constellation diagram, different from the ideal symbol. The root-mean-squared (RMS) value of this difference, normalized by reference vector is called the Error Vector Magnitude (EVM) and expressed in percentage. III. HDRM DESIGN IMPLEMENTATION The main function of HDRM is to modulate the data stream containing the high resolution imaging payload data at 200 Mbps. Some of the critical parameters, in designing the HDRM, like delay, rise time and timing tolerances due to temperature variations etc. are associated with imaging data to HDRM interface. Therefore better timing margin for HDRM is maintained by interfacing the data from the imaging payload through two parallel lines each at 100 Mbps data rate. The complete HDRM implementation at block level is shown in fig.1 and these are discussed in detail in subsequent sections. Fig.1. HDRM Block level Implementation A. LVDS INTERFACE AND REDUNDANCY
3 High Data Rate QPSK Modulator with CCSDS Punctured FEC channel Coding for Geo- Imaging Satellite The data and Clock from the Imaging unit is received through 100Ω impedance LVDS (Low Voltage Differential Signal) interface. The differential interface is selected to reject the common mode noise and other interfering signals those might be picked up by the interfacing harness wires which carry the signals from the Imaging unit to the HDRM units. LVDS interface is selected keeping in account its power efficient and high data rate operations. The LVDS interface between Imaging unit and HDRM with termination impedances is shown in fig.2 Fig.2. LVDS Interface between Imaging unit and HDRM The HDRM is designed with the provision of selecting data from one of the two imaging units at an instant, based on the tele-command signal given to HDRM. This configuration of keeping cross interfacing between the two Imaging units and two HDRMs increases the overall redundancy of subsystems and reduces the failure possibilities of the overall payload. B. DIFFERENTIAL ENCODER In a typical suppressed carrier PSK (Phase Shift Keying) system, there is no absolute phase reference, so phase ambiguities in the modulated data and regenerated carrier may occur at the receiver end. The phase ambiguities must be resolved in order to properly decode the received data. A differential encoder provides this function. The differential encoder transforms the input data stream into an indication of transitions rather than absolute ones or zeros. Fig.3. Differential encoder in HDRM Let A n is the input data bits in NRZ-L format at the n th instant and C n is the differentially encoded data bits in NRZ-M format at the same instant. C n = A n A n-1 (1) Similar differential decoding algorithms are to be used after the demodulator in receiver to resolve the phase ambiguity. A one-bit delay and a modulo-2 adder are shown in fig.3 to implement the differential encoder. C. CCSDS PUNCTURED CONVOLUTIONAL RATE 7/8 FEC CHANNEL ENCODING
IJAEST, Volume 3, Number 1 Rahul Gupta et al. 4 The convolutional coding is one of the two major types of channel coding. Convolutional coding is always performed after the differential encoding to avoid unnecessary link performance loss [3]. A convolutional encoder consists of an n-stage shift register, where clocked data is input and a number of modulo-2 adders (XOR gates) are connected to different stages of the shift register according to a chosen polynomial. The shift register length is called the constraint length of convolutional code. Fig.4. CCSDS recommended Rate ½ Convolutional FEC encoder As per the CCSDS channel coding blue book [3], the basic convolutional code is a rate ½, constraint-length (n) 7 transparent code which is well suited for channels with Gaussian noise. Rate ½ convolutional encoders have been widely used in satellite communication systems due to their moderate complexity and acceptable coding gain. The polynomial as per CCSDS standard [3] for constraint length 7 and rate ½, is (133 0, 171 0 ) which is shown in fig.4. The rate ½ convolutional codes can be modified to higher coding rates using a puncturing pattern P(r) to achieve an increment in bandwidth efficiency as shown in fig.5. Puncturing removes some of the symbols before transmission, providing lower overhead and lower bandwidth expansion than the original code, but with slightly reduced error correcting performance [3]. Fig.5. Punctured Convolutional FEC encoder Block diagram as per CCSDS The CCSDS channel coding blue book defines the puncturing patterns for different code rates as per the Table 2. TABLE 2 PUNCTURE CODE PATTERNS FOR CONVOLUTIONAL CODE RATES [3] Puncturing Pattern* C1: 1 0 C2: 1 1 C1: 1 0 1 C2: 1 1 0 Code Rate Output Sequence C1(t), C2(t) denote values at bit time t 2/3 C1(1) C2(1) C2(2)... 3/4 C1(1) C2(1) C2(2) C1(3)... C1: 1 0 1 0 1 C2: 1 1 0 1 0 C1: 1 0 0 0 1 0 1 C2: 1 1 1 1 0 1 0 5/6 C1(1) C2(1) C2(2) C1(3) C2(4) C1(5)... 7/8 C1(1) C2(1) C2(2) C2(3) C2(4) C1(5) C2(6) C1(7)... * 1 = transmitted symbol; 0 = non-transmitted symbol In HDRM, to utilize the limited bandwidth of 200 MHz, rate 7/8 FEC convolutional channel encoding scheme is used. To implement the rate 7/8 FEC, clock of higher frequency (100*8/7MHz) is required which is generated using a PLL (Phase Locked Loop) acting as a clock manager outside the FPGA. Considering the radiation effects in the GEO orbit, Radiation hardened Actel FPGA (RTAX250S) is used to realize the entire baseband signal processing functions including channel encoding. All the Verilog-HDL (Hardware Descriptive Lan-
5 High Data Rate QPSK Modulator with CCSDS Punctured FEC channel Coding for Geo- Imaging Satellite guage) programs for digital processing logic are developed in-house and successfully implemented using Actel Libero IDE [4] (Integrated Design Environment). Punctured data is framed in a register at another derived clock of lower frequency (100/7MHz). This framed data is serially shifted out at the PLL generated higher frequency clock. D. QPSK MODULATION Double Balanced Mixers are conventionally used for modulating the carrier source with data. A TCXO at 375 MHz is used as carrier source. A 2-way Quadrature (90 o ) power divider is used to generate carriers for I and Q channels which require carriers with orthogonal phases. The double balanced mixers modulate the carrier with digital data having sufficient driving current. The encoded data is passed through Op-Amp (Operational Amplifier) circuit which works as a driver for mixer. Double balanced mixers provide very high carrier suppression which makes the data demodulation easier. Both the mixers output are combined in a 2-way power combiner as shown in fig.1, which results the QPSK modulated RF output. IV. SIMULATION AND TEST RESULTS HDRM was simulated in MATLAB as per the fig.1 before the hardware development. Then after verifying the simulated results near to theoretical results, the DVM (Design Verification Model) of HDRM is designed and measurements are carried out using standard Agilent High Data Rate PRBS data generator and Cortex HDR (High Data Rate) Demodulator. The EVM is calculated theoretically [5] using the measured amplitude and phase imbalances. The results are shown in the Table 3. TABLE 3 RESULTS OF HDRM DVM Parameter Specification Result 1 Output Power -10 dbm max. -10 dbm 2 Amplitude Imbalance < 0.5 db 0.4 db 3 Phase Imbalance <3 degree 1.6 degree 4 Error Vector Magnitude (EVM) < 7% 3.2% 5 Carrier Suppression < -25 db -27 db The complete measurement test set-up is shown in fig.6. The DVM results are also approaching towards the simulated results as shown in fig.7.
IJAEST, Volume 3, Number 1 Rahul Gupta et al. 6 DVM Fig.6. DVM Measurement Test Set-up Fig.7 (a). MATLAB Simulated Spectrum Fig.7(b). DVM Measured Spectrum V. FURTHER SCOPE Although the designed DVM hardware gives quite appreciating results but considering the size and weight constraint in on-board systems, the design can further be modified to smaller size and more features. The driver and double balanced mixers circuit can be replaced by a Hybrid Quadrature modulator on chip to reduce the size and parts count on-board. Further the design can be modified using the same resources to develop a DCM (Dynamic Coding Modulator) which would be flexible enough to support the variable data rates, different punctured convolutional encoding schemes and variable modulation schemes, those can be switched among themselves on-board itself. VI. CONCLUSION As the GISAT being a highly ambitious mission for Indian Space Research Organization, the design of this onboard HDRM would be a milestone for future higher data rate missions. The results of the DVM of designed HDRM are quite encouraging for future improved implementation methodologies.
7 High Data Rate QPSK Modulator with CCSDS Punctured FEC channel Coding for Geo- Imaging Satellite REFERENCES [1] Preliminary Design Review GISAT Communication Payload-Volume 1, Document No. ISRO-SAC-GISAT COM PL-PDR-01, July 2012. [2] Preliminary Design Review GISAT Communication Payload-Volume 3, Document No. ISRO-SAC-GISAT COM PL-PDR-01,, July 2012. [3] CCSDS Blue book 101.0-B-6, Telemetry Channel Coding, Issue 6, October 2002. [4] www.tecnomic.com [5] Apostolos Georgiadis, Gain, Phase Imbalance, and Phase Noise Effects on Error Vector Magnitude, IEEE Trans. Veh. Technol., vol. 53, No 2, pp. 443-449, Nov. 1993.