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. 

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. 

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. 

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.

Systems Collocation 

Noise and Interference Immunity 

The Near / Far problem 

Multipath Immunity 

Throughput 

Form Factor 

Power Consumption 

Price

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]. 

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. 

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. 

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! 

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). 

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 ! 

The effect of receiving multiple copies due to multipath will be analyzed separately in the frequency domain and 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) 

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. 

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. 

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!) 

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. 

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. 

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!) 

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. 

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.