FHSS vs. DSSS Presented by Ali Alhajhouj Presentation Outline • Introduce the issues involved in the system behaviors for FHSS and DSSS systems used in WLAN (Wireless LAN) and BWA (Broadband Wireless Access) • Advantages of FHSS v. DSSS • Disadvantages of FHSS v. DSSS Systems Behaviors • Some issues to study for FHSS and DSSS systems: – System collocation – Noise and interference immunity – Near/Far problem – Multipath immunity – Security – Bluetooth interference System collocation • The issue: How many independent systems may operate simultaneously without interference ? System collocation • For installations requiring big coverage and multiple collocated cells, it would be much easier to used FHSS • DSSS could be used but collocated cells should be non-overlapping cells at the radio level, thus requires using directional antennas: – Directional antennas means limited coverage – Requiring more systems to be installed – Difficult to design because of the collocation issue • This severe limitation of DSSS is in both 2Mbps and 11Mbps Noise and Interference Immunity • The issue: What is the capability to operate when other radio signals are present in the same band ? Noise and Interference Immunity • FHSS systems operate with SNR(Signal to Noise Ratio) of about 18 dB • DSSS systems operate with SNR 12 dB because it is using more efficient modulation techniques, called Phase-shift keying (PSK) Noise and Interference Immunity • All band interference – Noise • Because DSSS systems operate with lower SNR, for same level of transmitted energy, DSSS systems can operate over longer distances than FHSS – Interference • Since whole spectrum used by the radio is 83.5 MHz in FHSS, while it is 22 MHz in DSSS, there is a greater chance of having interference in DSSS than FHSS Near/Far problem • The issue: The problems generated to a receiver by other active transmitters located in its proximity, also known as Near/Far problem Near/Far problem • If the receiver is using FHSS, the worst case will be that “foreign” transmitter will block some hops. Forcing the FHSS system to work less than optimum conditions, but allowing it to work • In DSSS systems, the problem is more critical and signals generated by the “foreign” transmitter could not be heard at the receiver Throughput • The issue: What amount of data actually carried by the system ? Throughput • Throughput is defined as the average amount of data carried by the system • DSSS systems provide greater throughput than with non-synchronized FHSS • Since synchronized FHSS systems never collide, they provide more or less the same throughput as DSSS systems • With no synchronization, DSSS provides higher throughput than FHSS Multipath Immunity • The issue: environments with reflective surfaces generate multiple possible propagation paths between transmitter and receiver resulting in multiple copies of the original (transmitted) signal, shifted in time Multipath Immunity • DSSS systems are extremely sensitive to the multipath issue when compared to the FHSS systems (especially when operated at 11 Mbps) • To minimize multipath effects, point to multiple topology systems have to be limited to small geographical (such as offices) • For long distance systems, the use of directional antennas may help reducing the multipath sensitivity in DSSS systems, however, FHSS systems shows more resistance to multipath immunity. Security • The issue: protecting the transmission against eavesdropping Security • DSSS systems use spreading sequence of 11 chips, thus can modulate 14 channel defined in the IEEE 802.11 standard. • Since the carrier frequency is fixed for a given DSSS system and possible frequencies is limited, it would be easier for a listener to “tune in” on the DSSS transmission • This requires message encryption at the application layer. Security • In FHSS, frequencies may be selected by the user • In order for the bad guy to “tune in” on the FHSS transmission, a bad guy should know: 1. 2. 3. 4. # of frequencies selected in the system The actual frequencies The hopping sequence The dwell time • FHSS modulation acts as a layer 1 encryption process, therefore, there is no need for application level encryption. Bluetooth Interference • The issue: What is the system behavior in the presence of Bluetooth/ IEEE 802.15 collocated systems ? Bluetooth Interference • Bluetooth radio is a FHSS operating in the 2.4 GHz band ( with dwell time of 0.625ms) • When DSSS executes a transmission, it is using 22 MHz for the duration of the transmitted frame • When FHSS executes a transmission, it is using 1 MHz for the duration of the transmitted frame Bluetooth Interference • The chances of Bluetooth “hitting” the DSSS systems is much higher than the chances of hitting the FHSS systems • Therefore, the DSSS is more sensitive to Bluetooth interference rather than in the FHSS systems Conclusion • DSSS provides higher (11Mbps) capacity than FHSS, but is sensitive to many environmental factors such as (system collocation, multipath effects, near/far problem, and Bluetooth) – For long distance applications, use point to point topologies – For short distance applications, use point to multiple topologies • FHSS provides only 3 Mbps, but it is very robust technology ( works better in harsh environment that is characterized by large areas of coverage multiple collocated cells, noises & interference, multipath, Bluetooth, and etc.) – Best to use for big coverage areas with a big number of collocated systems and where directional antennas are impossible to implement. Therefore, it is more suitable for fixed broadband wireless access (BWA) End Any Question ?