Re: [44net] OFDM Modem

Hopefully this isn't too far off topic. I've just completed testing of an OFDM modem using IP over DVB-T2. It uses an SDR transmitter and a commercial DVB-T2 receiver to implement the RFC 4326 Unidirectional Lightweight Encapsulation (ULE) for Transmission of IP Datagrams over an MPEG-2 Transport Stream (TS). ULE is supported in the Linux kernel for DVB receivers. https://tools.ietf.org/html/rfc4326 It's a full-duplex modem capable of up to 50 Mbps (in both directions) in an 8 MHz bandwidth. The current test bed consists of an Ettus B200 SDR transmitter, PCTV 292e DVB-T2 USB receiver, Kuhne down converters for 13cm and 9cm, Microlab BK-26N diplexer and RFSpace TSA600 Vivaldi antenna. http://www.w6rz.net/IMG_0119.jpg http://www.w6rz.net/traceroute.png The transmitter is based on the DVB-T2 transmitter in GNU Radio and uses this OOT module for the ULE protocol. https://github.com/drmpeg/gr-ule To reduce the latency, I've merged the DVB-T2 blocks to avoid having so many buffers between blocks. https://github.com/drmpeg/gr-dvbt2ll Current test frequencies are 2305 and 3429 MHz with an 8 MHz bandwidth. The bit-rate is 28.6 Mbps (symmetrical). It's intended to replace commercial WiFi equipment for amateur WAN interlinks. Advantages: 1) Full-duplex. Adding power amplifiers, preamps, diplexers/duplexers is easy. 2) Frequency agile. Can work on any band above 420 MHz. 70cm through 5cm direct TX from the SDR and millimeter frequencies with an up-converter. 3) Bandwidth agile. 5, 6 ,7 and 8 MHz bandwidths. 4) May be legal on 70cm. Although I haven't implemented it yet, a small portion of the bandwidth can be used to send a low-rate video stream (for example, a still picture of your call sign for ID). This would classify the emission as digital ATV, not data. Disadvantages: 1) Latency is a bit high. It's currently 100 ms (200 ms round trip). This is a function of buffering in GNU Radio and the USB 3.0 connection to the SDR. An FPGA implementation of DVB-T2 and a different SDR architecture could solve this. 2) Cost. It's difficult to compete with commercial WiFi equipment. However, lower cost components can be used instead of the "Cadillac" test bed I constructed. For example, a ADALM-PLUTO at $99 could be used instead of the Ettus B200 for transmit. Two antennas instead of a diplexer and lower cost down-converters than the Kuhne units. 3) Requires a Linux computer to run it. An Odroid XU4 may be adequate, but I haven't tested it. 73, Ron W6RZ _________________________________________ 44Net mailing list 44Net(a)hamradio.ucsd.edu http://hamradio.ucsd.edu/mailman/listinfo/44net