Spread Spectrum Rule Recommendations
By Phil Karn, KA9Q
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 (Eb/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 Eb/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.
- Eb/N0 is independent of bandwidth and data rate.
- The required Eb/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 Eb/N0: 5 dB in fading.
- Typical mobile transmit power: 1-3 mW!
- 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 Eb/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 Eb/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.