MSc Mobile Computing Systems 1 Professor Rolando Carrasco BSc(Hons), PhD, CEng, FIEE

2 Introduction (1) Mobile Computing Systems Types of wireless communication systems –Cellular System Intra-cell/Inter-cell operation Frequency Re-use Channel Assignment Strategies Handoffs –Interference and System Capacity Types of Interference Capacity/Interference Relation Improving Capacity in Cellular Systems

3 Introduction (2) Multiple Access in the Mobile Environment –Frequency Division Multiple Access (FDMA) –Time Division Multiple Access (TDMA) Capacity and Interference for FDMA and TDMA Commercial Applications for FDMA & TDMA –Spread Spectrum Multiple Access Techniques, Code Division Multiple Access (CDMA) General Concepts and Characteristics Capacity and Interference in CDMA –Other Multiple Access Techniques

4 Mobile Computing Systems Rely on radio transmission as the final link between terminals –Finite resource, spectrum available is strictly limited –Multipath propagation, fading & interference –Terminals ability to move, complicates the system The term mobile: –Any radio terminal, that can be moved during operation –Radio terminal that is attached to a high speed platform (cellular telephone inside a vehicle) The term portable: –A radio terminal that can be hand-held & used at walking speed

5 Radio Transmission Impairments MSC Fixed Networks PSTN, ISDN BISDN,...

6 Mobile Computing Systems Mobiles users communicate through fixed base stations (BS) BSs are controlled by the radio network controller (RNC in 3G) RNC allows the system to contact the fixed backbone network

7 Types of wireless communication systems type of transmission used is evaluated: –Analogue: transmitting unbroken electromagnetic waves, closely correspond to the waveforms produced by the original sounds. –Digital:convert information (e. g. voice or data) into a series of coded pulses, transmitted at a fast rate. Depending on the direction of the transmission and the simultaneity of the communication –Simplex: communication in one direction only. –Half-duplex: two-way communication over the same radio channel. A user can only transmit or receive (no simultaneity). –Full-duplex: two simultaneous but separate channels to achieve a two-way communication

8 Types of wireless communication systems A final classification can be made depending on the type of service provided: –Paging systems –Cordless –Adhoc –Cellular

9 Paging Systems Systems that send brief messages (numeric, alphanumeric or voice) to a subscriber. message is called a page and is sent in one direction only (simplex transmission) messages are broadcasted to inform the subscriber about the attempts made by other users to contact them or to receive news headlines, faxes or other types of information.

10 An example Paging System

11 Cordless Fully duplex, using a radio channel to connect a portable handset to a dedicated Base Station. Connected to fixed network via a specific telephone number Connection over short distances –1G cordless = few metres –2G cordless, DECT = few hundred metres

12 Mobile Adhoc Network (MANET) Mobiles communicate bouncing off each other. They are not fixed to using a BS to connect to fixed network Useful in disaster situations

13 Cellular Mobile Communication system The cellular concept is a system level idea where many low power transmitters replace a single high power transmitter, covering a large geographic area, each covering a portion of the service area called a cell

14 Cellular Concept MSC Fixed Networks PSTN, ISDN BISDN,...

15 Frequency Re-use Each BS is allocated a different set of carrier frequencies Each cell has a usable bandwidth associated with these carriers No. of carrier frequencies available is limited It is therefore necessary to re-use the available frequencies many times in order to provide sufficient channels for the required demand This process is called frequency re-use All the cells with a different set of frequencies form a cluster

16 Frequency Reuse & Channel Assignment Frequency reuse concept. Cells with the same letter use the same set of frequencies. B A D C E G F B A D C E G F B A D C E G F Co-Channel Interference Fixed Channel Allocation Dynamic Channel Allocation Centralised or Distributed A b 5 f 4 d 6 b 1 -b 4 d 1 -d 5 C e2 -e 6 G f 1 -f 3 A b 1 -b 3 D C E G F A b6 D C E G F

17 Channel Assignment Strategies The way the channels are assigned inside a cell affects the performance of the system –especially when a change of BSs occurs Fixed Channel Allocation Schemes (FCA) Dynamic Channel Allocation (DCA) Hybrid Channel Allocation (HCA)

18 Fixed Channel Allocation (FCA) –channels are divided in sets –allocated to a group of cells & reassigned to other groups, according to some reuse pattern –Different considerations are taken before the assignment of the channels (i.e. signal quality, distance between BSs, traffic per BS) –they are fixed (i.e. a cell can not use channels that are not assigned to it) –assignment of frequency sets to cells when the system is designed & does not change unless restructured –Any call attempt within the cell can only be served by the unused channels in that cell –If all the channels in that cell are busy, the service is blocked –simple method but does not adapt to changing traffic conditions –introduction of new BSs supposes frequency reassignment for the complete system BS 1 BS 3 BS 4 BS 2 10 Channels

19 Dynamic Channel Allocation (DCA) BS 1 BS 3 BS 4 BS 2 40 Channels –Channels are placed in a pool –assigned to new calls depending on the carrier to interference ratio (CIR) and other criteria. –Each time a call is made the serving base station requests a channel from the RNC –The switch then allocates a channel to the requested cell following an algorithm that takes into account the likelihoodof future blocking within the cell the frequency of use of the candidate channel the reuse distance of the channel, and other cost functions. –The RNC only allocates a given frequency if that frequency is not presently in use in the cell or any other cell which falls within the minimum restricted distance of frequency reuse to avoid interference –reduces the likelihood of blocking, which increases the trunking capacity of the system, since all the available channels in a market are accessible to all of the cells

20 Dynamic Channel Allocation (DCA) Require the RNC to collect real-time data on –channel occupancy –traffic distribution –radio signal strength indications (RSSI) of all channels on a continuous basis This increases the storage and computational load on the system but provides the advantage of increased channel utilisation and decreased probability of a blocked call Allocation of channels is more complex since additional information is needed, but is also more flexible to traffic changes (i.e. non-uniform traffic).

21 Hybrid Channel Allocation (HCA) a combination of both FCA and DCA some channels are pre-assigned others are shared dynamically One of these approaches is based on the principal of borrowing channels from a neighbouring cell when its own channels are occupied Known as the borrowing strategy RNC supervises such borrowing procedures & ensures that the borrowing of a channel does not disrupt or interfere with any of the calls in progress in the donor cell

22 Allocation Comparison FCA better for high uniform traffic loads –Max reusability of channels is always achieved DCA performs better for non-uniform traffic loads –allocation of channels is flexible FCA schemes behave like a no. of small groups of servers DCA provides a way of making these small groups of servers behave like a larger server, which is more efficient. FCA call must always be handed off into another channel –same channel is not available in adjacent cells. DCA the same channel can be used if interference does not occur.

23 Allocation Comparison variations in traffic that are typical of microcells are not well handled in FCA. DCA techniques perform better in microcells Implementation complexity of DCA is higher than FCA. –FCA:each cell has a number of channels and the channel selection is made independently –DCA: the knowledge of occupied channels in other cells is necessary (i.e. heavy signalling load). –A great deal of processing power to determine optimal allocations is also required.

24 Allocation Control Centralised fashion –channels are assigned by a central controller, usually the RNC Distributed fashion –Channels are selected either by the local BS or by the mobile BS control: BSs keep info about current available channels in its vicinity. –Updated by exchanging data between BSs. In a mobile control system the mobile chooses the channel based in its local CIR measurements (i.e. lower complexity but less efficiency). FCA is suitable for a centralised control system. DCA is applicable to a centralised or decentralised control system