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Lecture 1:Lecture 1:Introduction to Wireless Introduction to Wireless CommunicationsCommunications

May 25, 2008

896960896960Introduction to Algorithmic Introduction to Algorithmic Wireless CommunicationsWireless Communications

David AmzallagDavid Amzallagdavid.amzallag@bt.com david.amzallag@bt.com

www.cs.technion.ac.il/~amzallag/awcwww.cs.technion.ac.il/~amzallag/awc

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Course administrationCourse administration

– Designed for graduate students. Good undergraduate students are also welcome.

– PrerequisitesPrerequisites: Undergraduate courses in algorithms, theory of NPNP-completeness, and probability.

– Course materialCourse material: Lecture notes, slides, and papers.– Guest lectureGuest lecture: June 29, SchemaSchema (www.schema.com).– Final gradeFinal grade: 4-5 homework assignments (50%) and a final exam

(50%). May submitted in pairs

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The basicsThe basics

– Cellular systems accommodate a large number of users over a large geographic area, within a limited frequency spectrum.

– High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic are called a cell using frequency reuse mechanism.

– A base station provides coverage (communication capabilities) to users (mobile phones) within its coverage area.

– Users within the coverage area transmit and receive signals from the base station.

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The concept of frequency reuseThe concept of frequency reuse

– The basic premise behind cellular system design.– Exploits the fact that signal power falls off with

distance to reuse the same frequency spectrum at spatially separated locations.

– Thus, the coverage area of a cellular system is divided into nonoverlapping cells, associated with some set of channels.

– The same channel set is used in another cell some distance away.

– Of course, there are (co-channel) interferences.

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Reuse distanceReuse distance

– The spatial separation of cells that reuse the same channel set is the reuse distance.

– This should be as small as possible so that frequencies are reused as often as possible.

– When reuse distance decreases, co-channel interference increases.

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QuestionsQuestions

– How to set the cluster size?– How to determine the minimal reuse distance? – How to assign channels to base stations such that frequency reuse

is maximized?

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The size is matterThe size is matter

– For a long time cellular systems design were mainly driven by the high cost of base stations.

– Planning was done using a small number of cells to cover an entire city (or region).

– Base stations were placed on tall buildings or mountains and transmitted at very high power with cell coverage areas of several km’s. These are the macrocells.

– Approximately signal power radiated uniformly in all directions. The hexagonal shape was birth.

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But now smaller is betterBut now smaller is better

– Cellular systems in urban areas now mostly use smaller cells with base stations close to street level that are transmitting at much lower power.

– Microcells (urban street, up to 1km long with base stations above rooftops at 25m height) and picocells (in-building base station with antennas in the middle of the room, on the ceiling).

– Why smaller? Because of the need for higher capacity in areas with high user density and the reduced size and cost of base station electronics.

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Smaller cells and network designSmaller cells and network design

– Planning task is now much more complicated.– Mobiles traverse a small cell more quickly – handoff must be

processed faster.– Location management becomes more complicated.– Good propagation models are hard to find.– Hexagonal cell shapes are no longer good representative, in

general (squares? triangulars? Probably not).

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Radio Network Controller (RNC)Radio Network Controller (RNC)

– All base stations in a given geographical area are connected via a high-speed communications link (also called access network) to a mobile telephone switching office (MTSO, or RNC).

– Among RNC responsibilities – Allocating channels within each cell, – Coordinating handoff between cells when a mobile traverse a cell

boundary, – Routing calls to and from mobile users,– Routing voice calls through the public switched telephone network

(PSTN, like Bezeq) or provide Internet access.

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What’s Radio Access Network (RAN)?What’s Radio Access Network (RAN)?

User Equipment RAN (e.g., UTRAN) Core Network

External Networks

Mobile Phones BTS RNC

Mobile Phones BTS RNC

SCP

MSC

EIR

SGSN

SMS-GMSC

GMSC

HLR

GGSN

INTERNET

PSTN/ISDN

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How to design access networks in future?How to design access networks in future?

– Star topology is the most acceptable architecture for access networks until 3G days.

– High bandwidth capability yields expensive cables in future.

– New topology is now required.– How to assign base stations to RNC?

How many RNCs are needed? Any constraints?

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How a cellular telephone call is made?How a cellular telephone call is made?

– A new user located in a given cell requests a channel by sending a call requestcall request to the cell’s base station over the control channelcontrol channel.

– The request is relayed to the RNC, which accepts the call request if a channel is available in that cell. Otherwise request is Otherwise request is rejectedrejected.

– A call handoff is initiated when the base station or the mobile in a given cell detects that the signal power is approaching threshold.

– In that case, the base station informs the RNC that the mobile requires a handoff, and the RNC then queries surrounding base stations for good new host.

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How a cellular telephone call is made? How a cellular telephone call is made?

– If so, then the RNC coordinates a handoff between the original base station and the new one.

– If no channels are now available in the cell of the new base station then the handoff fails and the call is terminated.

– A call will also be dropped if the signal strength between a mobile and its base station falls below the minimum needed threshold.

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Multiple access techniques Multiple access techniques

– Used to allow many mobile users to share simultaneously a finite amount of radio spectrum.

– What is duplexingduplexing? Duplexing may be done using frequency or time domain.

– Frequency division duplexingFrequency division duplexing (FDD) provides two distinct bands of frequencies for every user: downlinkdownlink and uplinkuplink.

– In FDD, and duplex channel actually consists of two channels and a duplexer, in both mobile and base station, to allow synchronization.

– In FDD frequency separation is needed between downlink and uplink.

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Multiple access techniquesMultiple access techniques

– Time division duplexingTime division duplexing (TDD) uses time instead of frequency to provide both downlink and uplink channels.

– In TDD, multiple users share a signal radio channels by taking turns in the time domain.

– Individual users are allowed to access the channel in assigned time slots. Each duplex channel has both downlink and uplink time slots.

– Time separation between downlink and uplink is necessary.

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Frequency division multiple access (FDMA)Frequency division multiple access (FDMA)

– Each user is allocated (on demand) a unique frequency band or channel.

– In FDD systems users are assigned a channel as a pair of frequencies (uplink and downlink).

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FDMA FDMA

– The FDMA channel carries only one phone circuit at a time.– If an FDMA channel is not in use, then it sits idle and cannot be

used by other users to increase or share capacity. It is essentially a wasted resource.

– After the assignment of a voice channel, the base station and the mobile transmit simultaneously and continuously.

– FDMA systems have higher cell site system costs as compared to TDMA systems.

– Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA.

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Time division multiple access (TDMA)Time division multiple access (TDMA)

– Dividing the radio spectrum into time slots, and in each slot only one user is allowed to either transmit or receive.

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TDMA TDMA

– TDMA shares a single carrier frequency with several users, where each user makes use of nonovelapping time slots.

– Data transmission for users of a TDMA system is continues, but occurs in bursts. This results in low battery consumption, since the subscriber transmitter can be turned off when not in use (which is most of the time).

– Because of discontinuous transmissions in TDMA, the handoff process is much simpler for a subscriber unit, since it is able to listen for another base stations during the idle time slots.

– TDMA uses different time slots for transmission and reception, thus duplexers are not required.

– Guard time should be minimized.

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What is this?What is this?

– A combination of both methods. A channel gets a certain frequency band for a certain amount of time.

– Used in GSM.

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Code division multiple access (CDMA)Code division multiple access (CDMA)

– In CDMA systems, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal.

– Each channel has a unique code.– All channels use the same spectrum

at the same time.

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CDMA CDMA

– Many users of a CDMA system share the same frequency. Either TDD or FDD may be used.

– Unlike TDMA or FDMA, CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor in a linear manner.

– No coordination nor synchronization is necessary.