Telemeasured Performances of a DSP based CDMA Software Defined Radio Abstract Marco Bagnolini, Cristian Alvisi, Alberto Roversi, Andrea Conti, Davide Dardari and Oreste Andrisano A tele-measurement experience on a DSP-based CDMA modem is presented. A brief description of the hardware and software realization is reported; the possibility to change the transmission parameters loading and running via internet on the DSP platform, different executable programs is described. In addition the possibility, that has a remote user, to obtain the measured parameters by either the DSP receiver (like Bit Error Rate, Frequency Offset between the modulation and demodulation carriers) or the measurement instruments involved in the transmission chain is shown. Finally the tele-measured performances are reported. Introduction This work is placed in the CNIT/ASI project Study, Design and Realization of Reconfigurable Satellite Network for Multimedia Applications with Guaranteed Quality of Service [1]. The main target of the project is the experimentation of software radio techniques in the context of configurable satellite networks with assured Quality of Service. Figures 1,2 depicts the hardware modem prototype, for wideband satellite transmission over geo-stationary networks, realized within the project. A Software Radio approach is especially useful in integrated terrestrial-satellite networks, where several standards have to coexist on the same devices: software reconfiguration should lead to the future implementation of multi-standard terminals, able at working worldwide. The software radio technology offers this advantages by means of digital signal processing techniques for which many functionalities are implemented via software with costs less than hardware realization. The software reconfiguration should be done by remote downloading. The reconfiguration by remote downloading has the advantage that can be performed in fully transparent way for the user by download software into hardware platform; in some cases, is useful to have this possibility for example when the transmitter and the receiver sides are divided by very long distances (thinking at the distance involved in a radio digital communications bridge or a digital satellite communication). 1
To external interface Clock Circuitry PLL SCI RESET Micro Controller SCIF SCIF BSC Serial port 2 Serial port 0 PLL DSP 1 Serial port 1 RESET HPI EMIF To external interface From external RX_FIFO interface DP-SRAM 2 (32 bit, 125 MHz) RS 232 To PC SRAM (16-bit) SRAM (16-bit) FLASH JTAG/H-UDI Connectors Voltage Reset Generators Circuitry DP-SRAM 1 (32 bit, 25 MHz) To external interface To external HPI Serial port 2 interface EMIF DSP 2 RESET Serial port 1 Serial port 0 PLL TX_FIFO To external interface Figure 1 : The CDMA modem hardware architecture. MCU DSP 1 DSP 2 Figure 2 : The CDMA modem prototype 2
Developed activity The software has been implemented on a commercial DSP-based platform manufactured by Texas Instruments. The hardware platform is composed by: two C6711 DSKs, based on TMS320C6711 DSP, that are used for digital base band implementation of transmitter and receiver sides; EVM TLV56919-5639 D/A converter, EVM THS 1206-1208 A/D converter. Other stages targeted at RF conversion are introduced. The architecture is designed in order to support an asynchronous Direct Sequence Spread Spectrum (DS/SS) CDMA transmission for multimedia applications. To avoid the phase offset introduced by the local oscillators, we chose a non-coherent D-QPSK modulation scheme [2]. Due to the digital signal processing limitations in the computational performance, the developed algorithms have to be characterized by a low complexity. Canonical synchronization algorithms based on the use of matched filter are not suitable for DSP realization, so the receiver algorithms for code acquisition is based on the sequential search of a synchronization symbol that are transmitted every N synch data symbols as described in [3]. The same methodology is used in the tracking algorithm to recovery time deviation due to different sample rates between transmitter and receiver sides. Besides a carrier frequency offset recovery scheme, software implemented in the receiver DSP program, permits to track frequency deviation in the range [-B s / 2, B s / 2], were B s is the symbol-rate, like described in [4]. The main feature of the software radio technology is to enable a multimode radio implementation that permits the use of different standards on the same hardware and it allows transmission parameters reconfiguration: the modem we present allows the users to configure the spreading factor N, the sample rate fs, which determine the symbol-rate B S and the parameters of the frequency offset recovery algorithm. The software radio remote transmission system, as illustrated in Figure 3, is composed of four elements : two target programs on DSK, two server tasks on PC connected via parallel port to related DSK (TX_Host, RX_host), a web server (Apache) to service the remote web clients requests, a Lab-view Web Server [5] that offers the Virtual Instruments Panel of the measuring instruments involved in the transmission chain (Figure 4). 3
Clients Internet CSITE24 Apache LabVIEW Web Server PORT 80 PORT 82 GPIB - Enet TX - Host Csite 168 LAN Fast Ethernet 802.3u Transmission Chain RX - Host Csite 169 DSK + D/A Converters DSK + A/D Converters Figura 3 : Remote programmable Software Radio Scheme The target programs (running on DSPs) implement respectively transmitter and receiver algorithms: each target program is started by related host program and communicates with it in order to receive the configuration parameters of the algorithm and transfer data information. The two host programs interact with web server Apache and related target program. They are waiting for a service request by the web server: when the request arrives each host sets the transmission parameters according to the web client request. Besides each host program loads on the corresponding DSP internal memory the parameters and the target program, and runs it. More precisely the web server Apache interacts whit host programs through CGI (Common Gateway Interface) that allow to service the remote web clients requests. CGI passes the transmission service request to the host, provides parameters selected by the user, waits for measurement results and displays them to the web client on a HTML page. 4
At the end of transmission the receiver host program also rebuilds the received file and calculates the BER that will be send to the web client. In case of transmission with frequency offset between transmission and reception local oscillator, the remote user can get in the same HTML page, the trend of the estimated frequency offset. In Figure 4, is reported the transmission chain and the measure instruments involved in the experience. The two base-band signals I,Q ways generated by the first DSP, are brought to a I&Q passive phase-modulator [6]. The modulator takes in input the two base-band channels and a frequency carrier provided by a signal generator (local oscillator). The modulated signal (IF signal) is sent to a Noise and Interference Test (NITS) [7] that allows to simulate an Additive White Gaussian Noise (AWGN) channel. At the reception side, the IF signal is down-converted by a I&Q passive phasedemodulator [8] that uses a second frequency carrier provided by a second signal generator. At last the two base-band I,Q ways are sampled by A/D converters to allow the DSP software elaboration. Along the transmission chain a digital oscilloscope and a spectrum analyzer are inserted to allow the telemeasures in the time and frequency domains: the base-band signals, the transmitted and received base-band signal constellations, the spectrum of the modulated signal without added noise and the spectrum of the signal plus AWGN inserted by the N.I.T.S. The analog signals are switched to different instruments through a switching matrix [9] using remote control. 5
DSK+ D/A Converter I/Q Baseband Quadrature modulator Modulated signal L.O. 70MHz GPIB-ENET/100 GPIB BUS Digital Oscilloscope LC534A Spectrum Analizer HP 70000 Switching Matrix Keithley 707A N.I.T.S. HP3708A L.O. 70MHz+ f DSK+ A/D Converter I/Q Baseband Quadrature demodulator Modulated signal + Noise Figura 4. Transmission chain and tele-measured signals When a remote client requires the control of an instrument (or the matrix), the LabVIEW Web Server [5] returns him a web page that reproduces the Virtual Instrument front panel. Remote users can perform the signals measurements driving the matrix and the instruments along the transmission chain simple clicking on a HTML page. Virtual Instrument front panel is the graphical interface provided by LabVIEW that allow to control an instrument through IEEE 488.2 protocol [10]. As illustrated in Figure 4, the instruments and the matrix are connected on a GPIB bus. The GPIB-Enet [11] make the telemeasurement feasible, also via WEB, providing a bridge between the IEEE 488.2 and TCP-IP protocols [12]. Figure 5 shows the telemeasured bit error rate (BER) referred to the AWGN channel and related to different Eb/N0 settled on the N.I.T.S. front panel. The green curve is in absence of frequency offset between carriers, the red one is referred to a normalized frequency offset δf/b s = 0,25. 6
Both the curves are related to a bit rate B r = 29.1 Kbit/sec and a spreading factor N =7. We can notice the presence of an irreducible error that gives a flat of BER performances which value is approximately 8.E 04. Figure 5. Bit error rate telemeasures (1) Figure 6 shows three estimated frequency offsets vs the number of synchronization symbols; the curves are reported for three different values of the parameter µ of the frequency offset recovery scheme, that fix up stability and convergence speed. The curves are related to a measure with bit rate Br = 29.1 Kbit/sec, and measured frequency offset between local oscillators of 7200 Hz. (1) The values were taken with a not immediately solvable calibration problem of the N.I.T.S Instrument 7
Figura 6. Estimated frequency offset vs number of synchronization symbols for different values of µ. References [1] - Study, Design and Realization of Re-configurable Satellite Network for Multimedia Applications with Guaranteed Quality of Service, First year activity report, Workpackage 1.1, October 2002. [2] - J. G. Proakis Digital Communications, Mc GRAW-HILL INTERNATIONAL EDITIONS [3] - A. Conti, D. Dardari, DSP-Based Satellite CDMA Modem: A Low Complexity Implementation, Wireless Personal Communications 24: pp. 123.139, 2003. [4] - A.Q. Hu, P.C.K. Kwok and T.S. Ng, MPSK DS/CDMA Carrier Recovery and Tracking Based on Correlatio Technique, IEE Electronics Letter, Feb. 1999. [5] - National Instruments LabVIEW 7 Express User Manual [6] - Minicircuits ZFMIQ-70ML http://www.minicircuits.com/dg03-210.pdf [7] - HP3708a Noise and Interference Test Set Operator s manual [8] - Minicircuits ZFMIQ 70D http://www.minicircuits.com/dg03-212.pdf [9] - Keithley model 707/A Switching Matrix [10] - IEEE Standard Codes, Formats, Protocols, and Common Commands For Use With IEEE Std 488.1-1987, IEEE Standard Digital Interface for Programmable Instrumentation Document Number: IEEE 488.2-1992 Institute of Electrical and Electronics Engineers, Inc. Staff Institute of Electrical and Electronics Engineers [11] - J. Humphrey, V. Malhotra, V. Trujillo, Developing Distributed GPIB Test Systems Using GPIB-ENET/100 and Existing Ethernet Networks, National Instruments, Application Note 103, www.ni.com [12] - W.R. Stevens et al., TCP/IP Illustrated, Vol. 1-3, Addison-Wesley Professional Series, 2000 8