1 Telecommunication Systems and Applications (TL - 424) Part A: Spread Spectrum Systems Dr. ir. Muhammad Nasir KHAN Department of Electrical Engineering Swedish College of Engineering and Technology March 20, 2014
Hopping codes (Cont d) Orthogonal hopping codes are designed such that no hit will occur: the channels in the hop-set are never occupied by two or more signals at the same time. True orthogonality requires accurate hop clock synchronization between the different users. In a practical systems with channel and multi-path delays, this cannot be maintained perfectly, and partial hits will occur Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 2 / 1 2
Hopping codes (Cont d) Random hopping codes choose at random a channel from the hop-set independent from the choices made by other users. Now it is quite likely that a channel is used by multiple users at the same time: collision. The probability of a collision is related to the hop-set size N and the number of users that share the hop-set. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 3 / 1 3
Hopping codes (Cont d) For random hop-set of size N and K active users, the probability of a hit for a certain user is given by: p h = 1 pr(no hit) ) K 1 = 1 ( 1 1 N ( ) K 1 N, for N >> 1 For unsynchronized users and due to channel multi-path delays, the probability of a (partial) hit increases substantially. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 4 / 1 4
Bit Error Probability in FH-SS How is the BER affected by occasional collisions?: Assume a perfectly synchronised FH-SS system in an AWGN channel: ( 2E Let the BER be P e in case of no collision(e.g., for BPSK P e = Q b N o )) In case of a collision, which happens with probability p h, the data ill be lost and the BER is P e = 1 2. Now we find for the average BER for the FH-SS system with collision: = (1 p h ) P e + 1 2 p h P FH e This is a simplified situation, since usually systems are not synchronised. A packet may be partially overlapped by one or more users in different ways. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 5 / 1 5
FH-SS (Cont d) In FH-SS modulation, a modulated signal is transmitted of which the transmit frequency is regularly changed according to a hopping code known by the receiver. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 6 / 1 6
FH-SS Reception (Cont d) At the receiver, the FH-SS against is de-hopped to retrieve the data: This system is robust against interference and multi-paths, since: Other FH-SS signals with a different code or narrowband interference only rarely cause a collision: fixed frequency interference is occasionally hit or can be avoided. signal strength variations due to multi-paths are averaged out over the hop-set. the robustness is determined by the processing gain which is equal to the size of the hop-set. G p = BWFH SS BW data = N. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 7 / 1 7
FH-SS Reception (Cont d) At the receiver, the FH-SS against is de-hopped to retrieve the data: requires the same hopping code at the receiver. requires perfect synchronisation. This system is robust against interference and multi-paths, since: Other FH-SS signals with a different code or narrowband interference only rarely cause a collision: fixed frequency interference is occasionally hit or can be avoided. signal strength variations due to multi-paths are averaged out over the hop-set. the robustness is determined by the processing gain which is equal to the size of the hop-set. G p = BWFH SS BW data = N. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 7 / 1 8
Employing FH-SS By applying FH-SS, spectral efficiency is traded for increased robustness due to the introduced processing gain. Similar trade-offs can be made as for DS-SS, e.g., the spectral efficiency can be increased by: 1 Increasing the symbol rate but using the same total bandwidth. the processing gain is decreased due to smaller hop-set. less robust. 2 Increasing the modulation level at the same symbol and chip rate. robustness is maintained: same processing gain. increase of signal power may be required. 3 Allow multiple users to share the same bandwidth: FDMA Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 8 / 1 9
Narrowband vs FHMA vs CDMA In CDMA/FDMA, the spectral resources are shared among multiple users based on different SS codes. Each user is identified by its own code. The cross-correlation between the codes has to be low (ideally zero orthogonal codes. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 9 / 1 10
Exercise Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 10 / 1 11
Applications Spread-spectrum techniques have implications in a number of applications from networking to broadcasting. Spread spectrum techniques may be used in areas such as wireless communications where the last mile of fibre to the home may be a significant investment for rural locations. Spread spectrum provides a method of circumventing additional infrastructure cost in the form of Wireless Local Area Networks (WLANS). These wireless networks are primarily located in four major industries Healthcare, Manufacturing (factory floors), Banking, and Education Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 11 / 1 12
Applications (Cont d) In the healthcare industry, WLANS can be used to give doctors up-to-the-minute information on patients by accessing databases through laptops and some hand-held devices. Additionally, patient health levels through blood pressure and heart rate may be monitored at a distance. Factories use WLANS to give up-to-the-minute information to the docks regarding inventory in addition to quickly realigning production to meet incoming orders and specifications. Similar incarnations of WLANS are present in the financial industry where down time of the network is critical. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 12 / 1 13
Applications (Cont d) Information may be updated or transferred in banks without shutting down the network through the use of wireless local area networks. While down time in educational institutions is not a major concern, the classroom becomes more distributed through the implementation of WLANS. Distance learning becomes a great deal more feasible at very low costs as a result of WLAN technology. In addition to the areas listed above, spread spectrum has applications in a number of other areas such as amateur radio, radio frequency identification in areas such as key fobs for cars, and position location. Dr. ir. Muhammad Nasir KHAN (SCET) Part A: Spread Spectrum Systems March 20, 2014 13 / 1 14