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Digital DX pileups

Phil Karn wrote:

K0VTY asked about digital voice in a DX pileup, implying that it would have to be worse than SSB. But imagine a modulation mode where you could actually separate and demodulate each individual transmission in a pileup of stations all transmitting to you in an uncontrolled fashion on the same frequency at the same time.
Such a mode actually exists! It's called spread spectrum, and that's exactly how CDMA cellular base stations operate.

Wayne replies:

I think Phil is understating the challenge of handling a DX pileup in digital modes, and overstating the capability of CDMA.

The CDMA technology commonly used in cellular networks can only support about 30 simultaneous users on a single RF channel.  To do that, each user's transmit power is rigidly controlled by the network, such that each of the 30 users has the same signal strength at the cell site.  The capacity (max. number of simultaneous users) goes way down without such rigid power control.  It is theoretically possible to implement if everybody got new radios purpose-built for this digital mode, but I doubt that any kind of power control scheme will ever be used in DX pileups.  DX pileups can have hundreds or even thousands of users calling at the same time.

CDMA technology requires each user to be ASSIGNED a unique PN code by a network access channel.  The access channel is a "logical channel" (a particular PN code), not a separate RF channel.  An access channel is feasible to implement on amateur satellites, where the satellite administers the access channel.  I doubt it would ever be widely used on terrestrial ham bands because a rare DX station is unlikely to administer an access channel, and a remote access channel would not have the same propagation as the DX station.

As a practical matter, future digital modes on ham radio must be accomplished using a "standard" computer (no specialized modem hardware) connected to a "standard" ham radio (no precise power control, frequency agility, or I/Q modulators).  It is reasonable to modify radios for the required IF bandwidth.

A few years ago it might have been acceptable to have a digital ham radio mode that requires an external modem and/or specialized radio equipment.  But technology and expectations have changed.  No future digital mode will achieve widespread use unless the mode can be implemented with standard computers and standards radios.  Furthermore, terrestrial digital modes should not require any type of controlling infrastructure.

Wayne Estes W9AE
Mundelein, IL, USA
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