What Is Spread Spectrum?

• ANY communication system that uses much more RF bandwidth than baseband bandwidth.
• Not really limited to "traditional" spread spectrum systems, e.g. direct sequence, frequency hopping, etc.
• Includes "narrowband forward error correction (FEC)" systems.
• Arguably applies to ordinary analog FM!

Why Use Spread Spectrum?

• For the same reasons we use FM!  And many more.
• Interference and noise rejection (capture effect).  10 dB for FM; digital spread spectrum systems have negative capture ratios (e.g. -15 dB).
• Simplifies spectrum management.
• Highly effective against multipath fading.
• Can dramatically increase capacity of spectrum in a frequency reuse environment.

Traditional Spectrum Management

• Bandwidth is precious - minimize its use.
• Carve up the spectrum into channels and fight over them.
• Give lip service to transmitter power control.

Why Tradition Is Wrong!

• Goes against well-established theory (Shannon, 1948).
• Users' demands are seldom constant - trunking inefficiencies and (re)allocation overheads are enormous.
• In a frequency reuse situation (i.e. almost all of amateur radio), interference is a fact of life.
• Increasing interference resistance inherently requires extra bandwidth.
• Interference resistance wins out over extra bandwidth.

These Ideas Are Not New!

• Shannon published theory in 1948.
• John Costas (K2EN) published Poisson, Shannon, and the Radio Amateur in 1959: "The results ... challenge the intuitively obvious and universally accepted thesis that congestion in the RF spectrum can only be relieved by the use of progressively smaller transmission bandwidths..."
• What's new is the digital technology now available to us.

Why Should We Encourage Amateur Spread Spectrum?

• Because it exists, and we're hams.
• Because the rest of the world is rapidly embracing it (GPS, cellular phones, Part 15.247).
• Because shared, congested amateur bands are a fact of life, and we should encourage spectral efficiency.

Can't Nearby Spread Spectrum Stations Blanket A Whole Band?

• Yes!  But the same is true in practice for narrowband stations - ever tried to share 20-meters with a kilowatt station next door?
• Efficient, power-controlled spread spectrum is actually a pretty benign neighbor.

Can't A Whole Bunch Of Spread Spectrum Stations Raise The Noise Floor?

• Yes!  But a whole bunch of narrowband stations can occupy every communications channel, which is even worse.
• Our licenses do not guarantee us access to the spectrum at all times - it's a dynamically shared resource, and sometimes the demand exceeds supply.
• Spread spectrum represents a way to increase spectral efficiency and thereby reduce the chance that demand will exceed supply.

How Do We Promote Efficiency?

• Encourage spread spectrum!
• Minimize power, not bandwidth.
• CDMA cellular shows minimizing power is the key to maximizing spectral efficiency.
• i.e. We should require automatic power control in amateur spread spectrum systems as a condition of relaxed bandwidth limits.
• Repeaters and directional antennas also minimize power.
• Other benefits: reducing RFI, biohazards, battery drain, etc.

Spread Spectrum Power

• By themselves, frequency hopping and direct sequence are "power neutral".  Over a nonfading, white-noise channel, they use the same total power as a narrowband signal.
• Spread spectrum on fading channels needs less fade margin.

Forward Error Correction

• By adding FEC we can actually reduce the total power (E_b/N₀) to send at a given rate!
• Think of "spectal density" as "QRM potential".
• We now win twice from a QRM perspective: first, because power is spread out thinly, second, because there is less total power.

What Is E_b/N₀?

• The ratio of the received energy per bit, in watts per second or joules, to the noise spectral density, in watts per hertz.
• Equal to the S/N (signal-to-noise) ratio only when the bandwidth is equal to the data rate.
• S/N ratio depends on bandwidth and data rate.
• E_b/N₀ is independent of bandwidth and data rate.
• The required E_b/N₀ is a modem's fundamental figure of merit - the lower the better.
• Inversely proportional to capacity in a spread spectrum environment.

Power Reduction With Forward Error Control

• FEC coding, a basic part of all modern spread spectrum systems, actually reduces the power required to send at a given rate.
• Gains of 7-10 dB are possible on nonfading channels, as much as tens of dB on fading channels and against interference.
• FEC inherently requires extra bandwidth, making it "spread spectrum-like" without actually spreading.

Example: UHF Mobile

• Qualcomm CDMA (IS-95) digital cellular uses direct sequence spread spectrum on both forward and reverse links.  1.25 MHz bandwidth.
• The forward link uses BPSK data modulation with a rate 1/2 FEC and lose power control.
• The reverse link uses 64-ary orthogonal data modulation with rate 1/3 FEC and tight power control (+/- 1 dB).
• Typical reverse link E_b/N₀: 5 dB in fading.
• Typical mobile transmit power: 1-3 mW!

Example: HF

• HF simulator tests of Clover II vs. STANAG 4285 (NATO standard military modem) by KE4BAD (QEX, Dec. 1994).
• Clover uses a 500 Hz bandwidth; STANAG 4285 uses 3 kHz.
• Both are reasonably efficient systems within their bandwidth constraints; both significantly outperform uncoded frequency shift keying (FSK).
• Clover requires at least 10 dB more E_b/N₀ than STANAG 4285 for the same error rate.

Recommended Rule Changes

• Delete existing spread spectrum rules (97.311).
• Waive exisiting 97.307 bandwidth limits for stations that use less than 100 watts and:
  • Use less than 1 watt, or
  • Automatically limit received E_b/N₀ at the intended receiver(s) to 20 dB.
• Maximize flexibility - do not require any particular form of modulation, coding, etc, or mandate a minimum processing gain.
• Resolve interference disputes in favor of the lower-powered station, regardless of mode.