11 Passband modulator signal spectrum Demodulator Shown in Fig. 12 is the demodulator architecture. The signal from the A/D converter, y1(n), at a sample rate of 8 000 Hz and length (L + S)*(N + P)*R, is down-converted from a real passband signal into a complex baseband signal, y2(n). The complex signal, y2(n), is also used for timing and frequency recovery. The frequency offset, f, is used in the downconverter, and the timing recovery, r, is used in selecting the first symbol in the cyclic prefix. The down converter output, y2(n), is decimated by R into (L + S)*(N + P) samples, y3(n). The synchronization symbols are then removed and converted from serial to parallel into L by (N + P) symbols, y4(m, n). Note that at this stage in the demodulator there is one sample per symbol, so the terms “symbol” and “sample” can be interchanged. The cyclic prefix is removed resulting in L by N symbols, y5(m, n), at a sample rate of: (11) A complex FFT is then applied to y5(m, n) resulting in L by N symbols, y6(m, n). Then a detector recovers the symbols using a differential method, which eliminates the need for recovering the carrier phase, but still requiring recovery of the carrier frequency. Frequency is recovered for all of the subcarriers at the same time and is not required for the individual carriers. The detection is done individually on each of the N subcarriers. The symbols out of the detector are mapped into log 2(M)*L by N bits, y7(m, n), using the same mapping as the modulator. The bits are descrambler using the inverse process as that used in the modulator resulting in log 2(M)*L by N bits, y8(m, n). The bits are then finally converted from parallel to serial resulting in log 2(M)*L*N bits, y9(n). Details of the individual blocks are provided below.
28 BER for the different SLs with respect to the energy per bit 29 Spectrum of a PACTORIII signal at SL6 with all 18 tones active 2 Typical communication protocol (TBUS) 3 Global Link Network (GLN) General overview The GLN is a network of cooperating coastal radio stations (CRSs) offering data access for the maritime mobile service. Due to the increasing demand for email transfer and internet access on sea going vessels and the decreasing use of narrow-band direct-printing (NBDP) and radio telex, these radio stations now offer data services on shortwave. Organizational structure All CRSs are operated by independent companies. These companies have joined together to form the GLN. They use common technology and common modulation. The CRS are free to offer their own additional services depending on local requirements. If the connection to the network control centre (NCC) fails for political, military or other reasons, every station is able to operate independently. In such cases the CRSs may also offer long-distance communications outside the main communication networks. Technical structure The GLN is based on the so called Pactor IP-Bridge (PIB). PIB enables transparent data connections based on the TCP/IP protocol over 2k4 radio channels in all the maritime MF/HF bands. PIB may be used for any type of data service with a maximum transfer speed up to 5 600 bit/s compressed. All network servers operate with a Linux OS and additional software packages which ensure a high fail-safe performance. NCC The NCC is operated under an agreement with the CRSs. It is responsible for data bases, accounting, backup, data security and development. The NCC also operates a mail server for small stations without their own data infrastructure. The NCC offers basic data services like weather information, email online compression, web mail, tracking and crew email to all customers of the GLN network. CRS The CRSs hold one or more radio channels on standby for automated data links between vessels and the internet. They may offer additional services like data transfer (FTP), credit card services, web