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FH vs DSSS

Technical Comparison of Frequency Hopping and Direct Sequence Spread Spectrum

By Green Bay Proffessional Packet Radio

 


Some terms used:

DSSS - Direct Sequence Spread Spectrum. A RF carrier and pseudo-random pulse train are mixed to make a noise like wide-band signa l.
FHSS - Frequency Hopping Spread Spectrum. Transmitting on one frequency for a certain time, then randomly jumping to another, and transmitting again.
FEC - Forward Error Correction. Common mean of error detection and correction in data networks.
multipath - When the RF signal arrives at the receiving antenna after bouncing through several paths. Degrades received sign al significantly.
DSP - Digital Signal Processor. See Texas Instruments DSP web site.
narrowband - A radio frequency signal that occupies a small amount of space
IQ - The two channels used in quadrature modulation, one in-phase (I), and one shifted 90 degrees from the I channel. (Q)
MAC - Medium Access Control. Part of the radio device managing the protocol and the usage of the link. Decides when to transmit and when to receive, creates the packets headers and filters the received packets.


  • FHSS radios are more susceptible to narrowband noise and interference than DSSS systems, so they will need to retransmit the same packet often. FHSS system will ignore most leaky microwave oven radiation, but with a DSSS system, a interfering signal could overload the receiver and bring the entire link down.

    FHSS will still overcome some narrowband interference, to a certain level, by just retransmitting the packet again on the next frequency hop. The DSSS system will have to use its FEC to handle the corrupted data, if it's even correctable.

  • Packet size on average in FHSS systems is only one fifth that of a DSSS packet. 400 bytes typically for a FHSS packet versus 1500 or 2400 bytes for a DSSS one, so large FHSS packets will need to be fragmented. FHSS also suffers from higher MAC latencies. All of this adds up to almost a ten times advantage in raw data throughput for a DSSS system as compared to that in a FHSS one.

    That being said, FHSS systems are the cheapest, and easiest to implement. Their RF circuits can use efficient non-linear, class C amplification, all with a nominal bandwidth of 1 MHz. This is especially useful in portable operations where these circuits tend to draw less current.

    DSSS systems, on the other hand, require a DSP, linear IQ modulation, IQ spin control, and linear (class A or AB RF power amplification. Along with precise linear amplification of all up/down converters and low noise amplifiers. All these circuits also need to have 22 MHz of bandwidth. Ouch.

  • FHSS radio systems also appear to do better indoors and in severe multipath environments. This is because the frequency hopping scheme can defeat multipath by just hopping to a new frequency. The wavelength of that new frequency changes just enough to alter the signal path, and therefor change any multipath interference that might occur.
  • DSSS are much more useful in outdoor and non-cluttered environments. Their processing gain makes them preferable in that situation, where they will have better receiver sensitivity.
  • A wireless network's data rate is limited by the Ethernet data rate. The more protocol converters and overhead you add to the system, the worse your throughput gets.
  • DSSS systems currently have a 70 - 80% market share and that is expected to grow.
  • DSSS use their bandwidth more efficiently can result in a much higher data throughput than FHSS systems. (almost a ten times improvement in real life situations).
Here are some additional notes on FHSS systems from Chuck Phillips, N4EZV, from the ARRL Spread Spectrum Sourcebook.
The advantage of using frequency hop signals instead of conventional narrowband signals or other wideband signals include the following:
  1. Processing gain.
  2. Jamming resistance.
  3. Traffic privacy.
  4. Low probability of intercept
  5. Multiple access capability
  6. Short synchronization time.
  7. Mulipath rejection, and
  8. Near-far performance.
These features are described in the following paragraphs.
Processing Gain
Frequency hop systems generally possess a large processing gain which allows the systems to operate with a low signal-to-noise ratio at the input of the receiver. The processing gain for frequency hop signals is:
    Processing Gain = RF Bandwidth / Information Bandwidth  
For example, a frequency hop signal that has a 10 MHz RF bandwidth and an information bandwidth of 1 kHz has a processing gain of 40 dB.
    Processing Gain = 10 MHz / 1,000 = 10,000     10 * log10 10,1000 = 40 dB  
Thus, the output signal-to-noise ratio of the frequency hopping demodulator will be 40 dB higher than the receiver input signal-to-noise ratio. This assumes no loss in the demodulator.
Jamming Resistance
Since a frequency hop signal generally has numerous frequency slots, the only time a narrow band jammer affects signal reception is when the signal hops to a frequency slot that is occupied by the jammer. If the frequency hopper has 500 frequency slots and a narrow band jammer interfers with the signal reception from one of the 500 slots, the only 1/500th of the signal might be jammed. Therefore. the low average power density combined with the pseudorandom frequency hopping make these signals difficult to intercept.
Multiple Access Capability
Frequency hop systems can be used for multiple access systems; time division, frequency division and code division multiple access systems can all employ frequency hop signals.

Frequency division multiple access systems assign a frequency band for each user. While most frequency division multiple access systems employ narrowband signals, wideband signals could also be used.

Short Synchronization Time
Frequency hop systems generally require a significantly shorter time to acquire synchronization that other types of systems having the same bandwidth. In frequency hop systems the receiver can usually synchronize with the transmitted signal within a small fraction of a second. Direct sequence systems, for instance, require about a second to achieve synchronization. For some applications like voice communications, the shorter acquisition time is highly desirable. If one or two seconds is required to synchronize, the transmitter has to be keyed at least two seconds before the voice will be received at the receiver. Therefore, a voice reply would be delayed by at least two seconds. By using frequency hop signals, the receiver can synchronize within a fraction of a second and no noticable delay is encountered for voice.
Multipath Rejection
When the transmitted signal is propagated towards the receiver, several paths may exist which may cause interference due to phase cancellation at the receiver. This is called multipath propagation. If the signal is propagated via the ionosphere, the path delays can range from tens of microseconds to several milliseconds. Similar multipath delays can exist at VHF and UHF frequencies due to reflections from buildings, towers, and other reflective materials.

If the hopping rate is adequately high, then the receiver listens on a new frequency slot before the interfering paths have a chance to interfer with the direct path. For slow frequency hoppers, the path propagation times are too fast to allow the receiver to reject the interference. Thus, to be effective, the hopping rate must be kept higher than the inverse of any interfering path delay time.

Frequency Diversity
Frequency hopping systems provide frequency diversity since many frequencies are used in the system. If proper data coding is used, a severe fade at any one particular frequency will have little effect on the data transmission. At HF frequencies, signals fade independently of one another if the frequency separation is several kilohertz. For frequency hop systems to provide frequency diversity, the minimum separation between frequency slots should be greater than several kilohertz. Typically, the separation used is much larger than a few kilohertz, ranging from 20 kHz minimum separation for voice or data communications. Thus, frequency diversity is easily provided for.
Near-Far Performance
Frequency hop systems provide better near-far performance that direct sequence systems. Near-far performance describes the behavior of the spread spectrum system with other users both near and far away from the intended users.
Traffic Privacy
Frequency hop systems provide a great degree of traffic privacy. The low probability of intercept combined with pseudorandom frequency hopping make these signals difficult to demodulate for unintended receivers. If additional security is desired the intelligence may be further encrypted using additional techniques.
Low Probability of Intercept
Frequency hop signals have a low average power density which can make these signals difficult to intercept. While the instantaneous power level of the frequency that is transmitted is high, the average power of that frequency is equal to the instantaneous power divided by the number of frequency slots. For example, a frequency hop system with 500 frequency slots transmits a specific frequency only a small fraction of the time (1/500th). If the instantaneous power from the transmitter is 100 watts, the average power in any one frequency slot is:
    Average power per frequency slot = 100 watts / 500 slots = 0.2 watts  
A frequency hop systems jumps to many different carrier frequencies in a time interval and filters the carrier frequency with the intermediate frequency filter. Users outside the filter's bandwidth are rejected and only the proper signal is demodulated. Since the I.F. filter passes only a narrow bandwidth, potential interferers are more easily rejected.

If the other users of the frequency band are near the frequency hop receiver and a frequency hop transmitter is far away, a frequency hop system can more easily reject the nearby interference than a direct sequence system can.

 
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