Frequency Hopping Spread Spectrum (FHSS) vs. Direct Sequence Spread Spectrum (DSSS) 
by Sorin M. SCHWARTZ.
     Product Marketing Director, BreezeCOM
 
Scope
Wireless LANs (WLAN) made their triumphal entry in the local data communications arena! This white paper explains the principles of the radio technologies related to WLAN as well as the advantages and disadvantages of each one of them. 
Executive Summary
WLANs may be implemented using optical or radio technologies for the transmission of the signals through the air and both are defined in the recently ratified (June 26,1997) standard for WLAN, IEEE 802.11. However, as of today, most implementations available on the market are radio based. The radio technology on which WLANs are based is known as Spread Spectrum modulation and has its origins in the military. Spread Spectrum systems can coexist with other radio systems, without being disturbed by their presence and without disturbing their activity. The immediate effect of this elegant behavior is that Spread Spectrum systems may be operated without the need for license, and that made the Spread Spectrum modulation to be the chosen technology for WLANs.  

There are basically two types of Spread Spectrum modulations: Frequency Hopping (FHSS) and Direct Sequence (DSSS). As of today, most implementations on the market are Frequency Hopping based, and this white paper explains why, by comparing the performance of the technologies for a few parameters of crucial importance for WLANs:  

  • possibility to collocate systems
  • noise and interference immunity
  • operation in environments generating radio reflections
  • data transfer capacity (throughput)
  • size
  • power consumption (relevant for battery based note books)
  • price. 
 
A. Basic principles
Spread Spectrum
Spread Spectrum modulation techniques are defined as being those techniques in which:
 
  • The bandwidth of the transmitted signal is much greater than the bandwidth of the original message, and 
  • The bandwidth of the transmitted signal is determined by the message to be transmitted and by an additional signal known as the Spreading Code. 
Two main Spread Spectrum modulation techniques are used in the WLAN arena:
Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). By transmitting the message energy over a bandwidth much wider than the minimum required, Spread Spectrum modulation techniques present two major advantages for Wireless Local Area Networks (WLAN): 
     
  • Low power density relates to the fact that the transmitted energy is spread over a wide band, and therefore, the amount of energy per specific frequency is very low. The effect of the low power density of the transmitted signal is that such a signal will not disturb (interfere with) the activity of other systems' receivers in the same area. 
  • Redundancy relates to the fact that the message is (or may be) present on different frequencies from where it may be recovered in case of errors. The effect of redundancy is that Spread Spectrum systems present a high resistance to noises and interference, being able to recover their messages even if noises are present on the medium. 
Low power density and immunity to noises allow for license free use of the technology and made Spread Spectrum the technology of choice for (unlicensed) WLAN. One of the frequency ranges allocated for use with Spread Spectrum technology is 2.4 GHz to 2.4835 GHz, same as the Industrial, Scientific and Medical (ISM) band. Most of the WLAN products currently available on the market operate in the ISM band.  

Spread Spectrum modulation techniques are composed of two consecutive modulation processes:  

  • One executed by the message to be transmitted, and 
  • One executed by the spreading code (= the spreading process). It is this spreading process that generates the wide bandwidth of the transmitted signal.
 
FHSS 
In FHSS systems, the two modulation processes are as follows: 
 
Process 1
The original message modulates the carrier, thus generating a narrow band signal.
  Process 2  
The frequency of the carrier is periodically modified (hopped) following a specific spreading code.(In FHSS systems, the spreading code is a list of frequencies to be used for the carrier signal). The amount of time spent on each hop is known as dwell time.  
Redundancy is achieved in FHSS systems by the possibility to execute re-transmissions on frequencies (hops) not affected by noise.
 
DSSS
In DSSS systems, the two modulation processes are as follows: 
 
Process 1
The original message is modulated by the spreading code. In DSSS systems, the spreading code is a sequence of bits (known as chips), and the first modulation step is a XOR operation executed between the message and the spreading code (process known as "chipping"). The result of the first modulation step is that a "0" bit of message is converted into a chip sequence representing the "0" bit, and the "1" bit of message is converted into another chip sequence, representing the "1" bit. Instead of transmitting the original message bit, a chip sequence representing the bit will be transmitted.
Process 2
The sequences representing the message bits modulate the carrier signal.  
Redundancy is achieved in DSSS systems by the presence of the message bit on each chip of the spreading code. Even if some of the chips of the spreading code are affected by noise, the receiver may recognize the sequence and take a correct decision regarding the received message bit.
 
 
(Select image for large view)
Figure 1 - Signals used to modulate the carrier in FHSS and DSSS
(Dwell time in FHSS is represented as 3 x data bit duration. Spreading sequence in DSSS is represented as being 5 chip long) 
 
B.- Systems Behavior 
The following issues will be studied in parallel for FHSS and DSSS systems:
    1. Systems Collocation 
    2. Noise and Interference Immunity 
    3. The Near / Far problem 
    4. Multipath Immunity 
    5. Throughput 
    6. Form Factor 
    7. Power Consumption 
    8. Price
1.- Systems Collocation
The issue: How many independent systems may operate simultaneously without interference? 
In DSSS systems, collocation could be based on the use of different spreading codes (sequences) for each active system (CDMA = Code Division Multiple Access). On condition that the sequences used are highly distinguishable one from the other one (property known as orthogonality) each receiver will be able to "read" only the information dedicated to it (receiver and transmitter use same spreading code). CDMA could indeed be the solution, but orthogonal pseudo-random sequences are needed. The number of orthogonal pseudo-random sequences is limited and it is a function of the sequence length [number of chips (bits) in the sequence]. 
(The following table is taken from "Modern Communications and Spread Spectrum" by G.R. Cooper and C. D. McGillan) 
 
Length of sequence Number of available sequences  Number of possible collocated systems
 
15 2 2
31 6 6
63 6 6
255 16 16 
1,023 60 60 
 
For the collocation of 16 systems, 255 chip (bit) long sequences should be used. Every message bit should be represented by 255 bits! If message rate is 1 Mbps (minimum required in LANs), the rate of the transmitted signal would be 255 Mbps ! Expensive!  

Actual DSSS systems use 11 bit long spreading sequences making the use of CDMA impossible. System collocation is therefore based on the fixed allocation of bandwidth to each system (same as in narrow band systems).  

For the transmission of 11 Mbps (Msymbols per sec), IEEE 802.11 defines the need for a minimum distance of 30 MHz between the carrier frequencies of collocated DSSS systems.  

As the total available bandwidth is 83.5MHz (2.4GHz - 2.4835GHz) and as the distance between carriers has to be 30 MHz, only 3 DSSS systems may be collocated! 

For FHSS systems IEEE 802.11 defines 79 different hops for the carrier frequency. Using these 79 frequencies, IEEE 802.11 defines 78 hopping sequences (each with 79 hops) grouped in three sets of 26 sequences each. Sequences from same set encounter minimum collisions and they may be allocated to collocated systems. Theoretically, 26 FHSS systems may be collocated. However, as synchronization among independent systems is forbidden (synchronization would eliminate collisions), the actual number of systems that can be collocated is around 15. 
 
2.- Noise and Interference Immunity 
The issue: Capacity of the system to operate without errors when other radio signals are present in the same band. 
FHSS systems operate with SNR (Signal to Noise Ratio) of about 18 dB.  
DSSS systems, because of the more effective modulation technique used (PSK), can operate with SNR as low as 12 dB. 
2.1.- All band interference 
For a given level of all band interference (interference covering the whole spectrum used by the radio), DSSS systems can operate with lower signal levels and therefore, for same level of transmitted energy, DSSS systems can operate over longer distances.  

Let's remember however that the "whole spectrum used by the radio" is 83.5 MHz in FHSS (the whole ISM band) while for DSSS it is only 20 MHz (one of the sub-bands). The chances of having an interference covering a range of 20 MHz are obviously greater than the chances of having the interference covering 83.5 MHz! A 20 MHz wide interference may totally block a DSSS system, while it will block only 25% of the hops in a FHSS system. A FHSS system will work in these conditions at 75% of its capacity, but it will work! A DSSS system will not work at all! 

2.2.- Narrow band interference 
DSSS systems have to be able to receive the energy present in their "working band" which is about 20 MHz. The filters included in the radio interface allow all the signals present in the working band to enter the device. A narrow band interference signal (interference present around one single frequency) is accepted by the receiver, and if enough energy is present in it, the interfering signal will totally block the receiver.  

FHSS systems work with narrow band signals (located each time around a different carrier frequency) and therefore the filters included in the radio have a much narrower pass band.  

A narrow band interference signal present on a specific frequency, will block only one hop (or maybe a couple, if the interfering signal has a wider band). The FHSS receiver will not be able to operate at that specific hop, but, after hopping to a different frequency, the narrow filter will reject the interfering signal, and the hopper will execute reception without being disturbed... (In IEEE 802.11 the frequencies for consecutive hops are separated by at least 6 MHz in order to reduce to a minimum the chances of being disturbed by interference on two consecutive hops). 

 
3.- Near / Far problem
The issue: The problems generated in DSSS systems by transmitters located close to receivers of other systems are known as Near / Far problems. 
The interfering signals described above may be generated for example by the transmitter of one system (System A) located close to the receiver of a different system (system B). If receiver B is a DSSS one, the interference generated by transmitter A could totally block its activity. On the other hand, if receiver B is FHSS, the worst case will be that transmitter A will block SOME hops, forcing B to work in less than optimum conditions, but work ! 
 
4.- Multipath
The issue: Environments with reflective surfaces (such as buildings, office walls, etc.) generate multiple possible paths between transmitter and receiver and therefore the receiver receives multiple copies of the original (transmitted) signal. 
The effect of receiving multiple copies due to multipath will be analyzed separately in the frequency domain and in the time domain. 
4.1.- Effect of multipath as seen in the time domain 
The paths available for the transmitted signal to propagate through have different lengths and as a result, signal propagation time is different from one path to another and therefore the multiple copies (of the original signal) arriving at the receiver are shifted in time. [Remember the ghost (multiple) images in TVs? - it is the effect of multipath!]  

In DSSS systems, the chipping process generates a high rate transmitted signal. The symbols of this transmitted signal are much shorter / narrower (in time) than the symbols generated by a FHSS system transmitting the same data rate (see figure 1).  

Obviously, a narrow pulse (DSSS systems) is more sensitive to delays (shifts in time) than a wider pulse (FHSS systems) and as a result the FHSS systems have better chances to be undisturbed by the presence of multipath effects (A shift of 5% for a FHSS system, becomes - in a DSSS system operating with 10 chip long spreading sequence - a shift of 50%!). (see fig. 2) 

 
 
(Select image for large view)
Figure 2 - Effect of identical shift in time on signals received in FHSS and DSSS systems 
 
4.2.- Effect of multipath as seen in the frequency domain - fading 
The multiple copies of the original signal arrive at the receiver with different instantaneous amplitudes and phases. The mixing of these copies at the receiver results in having some frequencies canceling one another, while other frequencies will sum up. The result is a process of selective fading of frequencies in the spectrum of the received signal.  

(Supposing that the transmitted signal has the spectrum of fig.3a, the spectrum of the received signal could look as in fig.3b).  

FHSS systems operate with narrow band signals located around different carrier frequencies. If at a specific moment, the FHSS system is using a carrier frequency significantly faded as a result of multipath, the FHSS receiver could not get enough energy to detect the radio signal. (Narrow rectangle in fig.3b). The resultant loss of information is corrected by retransmitting the lost packets on a different hop (frequency) not faded by multipath. 

DSSS systems operate over wider bands, transmitting their signal over a group of frequencies simultaneously. As long as the average level within the wide rectangle in fig.3b is high enough, the DSSS receiver will be able to detect the radio signal. 
 
 
(Select image for large view)
Fig. 3 - Fading effects for FHSS and DSSS systems 
 
However, even if the signal could be detected at the radio level better than in the case of FHSS, problems could occur when trying to convert the received radio signal into data bits, because of the time shift of the signals, as explained above.  

In addition, if fading affects a whole band of frequencies, a DSSS system could be totally blocked, while FHSS systems still have chances to operate on some unaffected hops. 

5.- Throughput 
The issue: What amount of data is actually carried by the system (measured in bps). 
The RATE of a system is defined as the amount of data (per second) carried by a system WHEN IT IS ACTIVE. As most communications systems are not able to carry data 100% of the time, an additional parameter - the THROUGHPUT - is defined, as the AVERAGE amount of data (per second) carried by the system. The average is calculated over long periods of time. Obviously, the throughput of a system is lower than its rate.  

In addition, when looking for the amount of data carried, the overhead introduced by the communication protocol should be considered, too.  

(For an Ethernet network for example, the rate is 10 Mbps, but the throughput is 3 Mbps to 4 Mbps only!) 

5.1.- Single system throughput 
DSSS systems are able to transmit data 100% of the time, having a high throughput. For example, systems operating at 2 Mbps over the air carry about 1.4 Mbps of data. (The difference is caused by the overhead introduced by the protocol itself).  

FHSS systems can not transmit 100% of the available time. Some time is always spent before and after hopping from one frequency to another for synchronization purposes. During these periods of time, no data is transmitted. Obviously, for the same rate over the air, a FHSS system will have a lower throughput than an equivalent DSSS system.  

IEEE 802.11 defines operation at 1 Mbps and 2 Mbps over the air for both DSSS and FHSS based systems.  

BreezeCOM is the only company providing FHSS systems operating at 3 Mbps over the air, providing a throughput of more than 1.6 Mbps, better than the throughput of competing DSSS based WLAN products available on the market. 

5.2.- Aggregate throughput of collocated systems 
Based on the IEEE 802.11 specifications, the maximum number of DSSS systems that can be collocated is 3. These 3 collocated systems provide a brut aggregate throughput of 3 x 2 Mbps = 6 Mbps, or a net aggregate throughput of 3 x 1.4 Mbps = 4.2 Mbps. Let's note that because of the rigid allocation of sub-bands to systems, collisions between signals generated by collocated systems do not occur, and therefore the aggregate throughput is a linear function of the number of systems.  

FHSS technology allows the collocation of much more than 3 systems. However, as the band is allocated in a dynamic way among the collocated systems (they use different hopping sequences which are not synchronized), collisions do occur, lowering the actual throughput.  

The greater the number of collocated systems (APs), the greater the number of collisions and the lower the actual throughput. For small quantities of APs, each additional AP brings in almost all its net throughput; the amount of collisions added to the system is not significant. When the number of APs reaches 15, the amount of collisions generated by additional APs is so high that in total they lower the aggregate throughput. 

 
 
(Select image for large view)
Figure 4 - Aggregate throughput of collocated systems 
 
6.- Form Factor 
The issue: Dimensions of the radio implementation 
DSSS radios use PSK modulation, while FHSS radios use FSK modulation. PSK implementations are more complex (coherent demodulation, AGC..etc) and therefore require more implementation space.  

BreezeCOM's PCMCIA adapter for notebooks and hand held devices uses FHSS and includes all the required radio - including two antennas, for diversity - in the form of a single, compact PCMCIA card.  

Equivalent DSSS based products have the electronics implemented on the PCMCIA card, while the radio and antennas are implemented in a different box to be hanged on the note book's screen!! (The PCMCIA card and the radio part are connected through an external cable!) 

7.- Power Consumption 
The issue: The amount of energy required for the operation of WLAN adapters (PCMCIA cards) for portable stations such as note books, hand held bar code readers and similar devices has a direct impact on battery life . 
As DSSS radios are more complicated, their power consumption is greater, too.  

During transmission, when maximum power consumption occurs, Breezecom's SA-PC PCMCIA adapter (FHSS) draw only 365 mA, while equivalent DSSS PCMCIA cards need sensibly more power. 

8.- Price 
The issue: Is money a real issue? 
If the answer to the above question is affirmative, then let it be known that because of lower complexity, FHSS based systems are cheaper than equivalent DSSS based systems. 
 
C.- Vendors of Spread Spectrum WLAN systems 
Following is a partial list of WLAN system vendors and the technology (original or OEM) implemented in their products: 
 
FHSS  DSSS 
 
AMP AT&T (Lucent)
Apple Solecteck
BreezeCOM Telxon (Aironet) 
DEC Apple
Fujitsu
IBM
Motorola
NDC
Proxim
Pulse
Raytheon
RDC
Symbol
Telxon (Aironet)
Thomson - CSF
Xircom
WaveAccess
 
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