January 2016 Archives

100 KHz Data Channels

In amateur radio we have 100 KHz wide data channels at a maxiumum of 19.2 kbaud (not kbit, kbaud indicates a symbol rate, not the actual data transferred or the binary serial data rate) at 222 MHz and above. However, very few radios actually support this data mode.

One approach to solve this problem is by using frequency-division multiplexing and inverse multiplexing. Inverse multiplexing separates a single high-speed serial data stream into a specific sequence of several slower serial data flows which pass over multiple data links of some form and are resequenced and combined at the opposite end of the link back into a high-speed data stream.

Using frequency-division multiplexing allows multiple voice channels to share a common group channel as if they were added up on the frequency axis. six 300 - 4,000 Hz voice channels are stacked to create a 300 - 24 KHz baseband voice group. That group when applied to an FM modulator with a modulation index of 1 in an occupied bandwidth of approximately 100KHz as calculated by Carson's Rule ( 24 KHz + 24 KHz = 48 KHz; 48 KHz * 2 = 96 KHz). (FCC Part 97 states that modulation index may not be greater than one.)

By splitting the resistive hybrid combiners from an external V.90 or V.92 modem (33.6 K or 56 K), the modem can be duplexed to use a four-wire interface with a separate TX and RX audio pair instead of a two-wire interface with both RX and TX combined. By capturing the independent audio streams using sound cards or DSP, the streams can be combined into four or just with simple processing. The incoming audio is captured at 8,000 samples per second with a 8-bit sample depth. The audio is mixed / stacked / combined using simple processor math or DSP, resulting in an audio stream that extends from 300 Hz to 24KHz. This is within the capabilities of 48KHz or 96 KHz sound cards, and there are USB varieties of each today. The resulting combined audio stream is applied directly to the modulator of a radio, creating the 100KHz wide modulated signal. To receive the signal, one need only remove the IF filters from the FM radio and replace the 20KHz filters with 100 KHz filters. The modulator may require specific modifications; some PLL loops operate at 50 KHz and may be incapable of handling the broad bandwidth.

The reason why 0-300 Hz is not used is to prevent issues with DC coupling in the modulator or demodulator. This also allows for introducing a low-frequency signaling component or clock synchronization carrier without introducing large amounts of AM, FM, and/or PM interference of any combination thereof.

The V.90 modem has a carrier frequency of 3429 Hz and runs at QAM256, encoding eight bits of information per resulting symbol. The V.90 and V.92 modem cores do not exist in an open-source format and contain related patents. Therefore, this is a circumstance where the modem function is effectively offloaded to dedicated hardware.

Combining the data flows from the six modems requires a form of inverse multiplexing. Rather than re-engineer and re-implement this in an format specific to this application, one may use Multilink PPP to effectively manage the modems and modem connectivity as well as perform frame disassembly and assembly.

Since digital switching is not involved, it is not possible to achieve 56 K or 64 K data rates. Instead, the maximum analog rate is the limit at 33.6 kbit/s per modem. This aggregates to 201.6 kbit/s, with one byte per modem being sent per symbol transmitted, (i.e.: 3429 * 6 = 20574 but the actual baud rate is 3429 Hz). Six bytes are transmitted per symbol time aggregated across all modems.

If one transmitter is located at 420.050 and another further up the band at 446.550 MHz, then duplexing may be possible without using cavity filters.

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This page is an archive of entries from January 2016 listed from newest to oldest.

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