Cellular Networks Why use cellular networks? What mobile radio services where provided before cellular? Use multiple low-power transmitters (100 W or less), because the range of such transmitter is small, an area divided into cells 1. each served by its own antenna 2. Served by base station consisting of transmitter, receiver, and control unit 3. Band of frequencies allocated 4. Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern)

Frequency Reuse 1. Adjacent cells assigned different frequencies to avoid interference or crosstalk 2. Objective is to reuse frequency in nearby cells Minimum distance between co-channel cells? (minimum distance between centers of cells that use the same band of frequencies(co-channels)) Number of frequencies assigned to each cell? (10 to 50) Transmission power control needed (to allow communication within a cell using a given frequency while limiting the power at that frequency that escapes the cells into the adjacent ones. ) Reuse factor? (N, number of cells in the repetition pattern)

Ways to increase the capacity Add new channels (if such exists…) Frequency borrowing from adjacent cells (dynamically or permanent) Cell splitting –cells in areas of high usage can be split into smaller cells Cell sectoring –cells are divided into a number of wedge-shaped sectors, each with their own set of channels Microcells–radius less than 1 km. Used at crowded streets, inside buildings, along highways,…

Cellular System Overview

Cellular Systems Terms Base Station (BS) –includes an antenna, a controller, and a number of receivers Mobile Telecommunications Switching Office MTSO (or Mobile Switching Center MSC) –connects calls between mobile units Two types of channels available between mobile unit and BS 1. Control channels –used to exchange information having to do with setting up and maintaining calls 2. Traffic channels –carry voice or data connection between users

Typical call Monitor for strongest signal Request for connection Paging Call accepted Ongoing call Handover (handoff) or termination Also: call blocking, call drop, call to fixed phone network

Mobile Radio Propagation Effects Take into consideration in design: 1. Signal strength 2. Must be strong enough (signal quality at the receiver) 3. Must not be too strong (co-channel interference) 4. Fading 5. Cell size 6. Carrier freq., antenna positions, … 7. Differ between urban, suburban and open areas

Handover Also called Handoff (mainly in US) Network initiated or mobile-assisted Performance metrics that may be used in decision: 1. Cell blocking probability (new call being blocked) 2. Call dropping probability (termination of call due to handoff) 3. Call completion probability (admitted call is not dropped before it terminates) 4. Probability of unsuccessful handover (handoff is executed while the reception conditions are inadequate)

5: Handover blocking probability (handoff cannot be successfully completed) 6: Handover probability (handoff occurs before call termination) 7: Rate of handover (number of handovers per unit time) 8: Interruption duration (the interruption of time during the handoff in which the mobile is not connected to either BS ) 9: Handover delay (the mobile moves from one point to another where the other handoff should occur but it delays)

Handover Strategies Used to Determine Instant of Handover Relative signal strength Because signal strength fluctuates due to the multipath effects, even with power averaging, this approach can lead to a ping pong effect in which a mobile unit is repeatedly passed back and forth between two base stations.

Relative signal strength with threshold Handoff occurs if 1. Signal at a current BS is sufficiently weak (less then a predefined threshold) 2. The other signal is stronger of the two. 3. If the signal at a current BS is sufficiently strong, handoff is unnecessary. 4. There are three threshold schemes (th1,th2 & th3) 5. For th1, no need for handoff, for th2 handoff will occur at L2, for th3, handoff will occur at L4. this reduces the quality of communication and probability of call dropping increased. 6. It is because of the interference between powers of to base stations.

Relative signal strength with hysteresis 1. The power of the BS 2 is sufficiently stronger, and the power of the BS 1 weakens gradually is the case of perfect handoff (by a margin H). 2. In this case handoff occurs at L3. 3. This scheme prevents ping pong effect, because when handoff occurs, the effect of margin H reversed. 4. Hysteresis (H) is known as relay hysteresis. 5. Handoff from BS 1 to BS 2 occurs when signal strength reaches of exceeds H and vice versa.

Relative signal strength with hysteresis and threshold Handoff occurs when, 1. Current signal level drops below the threshold 2. Target BS is stronger then the current one by a hysteresis margin H. 3. In our case it is at th3 Prediction techniques The handoff decision is based on the expected future value of the received signal strength.

Power Control Why is it desirable to include dynamic power control in a cellular system? 1. Received power must be sufficiently above the background noise for effective communication 2. Desirable to minimize power in the transmitted signal from the mobile Co-channel interference Health concerns Battery power In spread spectrum systems using CDMA it is crucial for the decoding to equalize the received power level from all mobile units at the BS

Types of Power Control Open-loop power control 1. Depends solely on mobile unit with no feed back from BS, and is used in some SS (spread spectrum) systems 2. Uses the pilot signal 3. Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength (important in CDMA) 4. Pilot allows the MS to acquire the timing of the forward (BS to MS)CDMA channel and provides the phase reference for demodulation. 5. MS monitors the received power level of pilot and sets the transmitted power in the reverse (Ms to BS) channel inversely proportional to it. 6. That’s why CDMA mobile sets get less battery power then GMS mobile sets.

Closed-loop power control 1. Adjusts signal strength in reverse channel (MS to BS) based on metric of performance 2. Which means BS makes the power adjustment decisions, received SNR, received BER, received signal power level. 3. BS makes power adjustment decision and communicates to mobile on control channel 4. Closed loop power control is used to adjust a power in the forward channel. 5. Which means that power for the GSM mobile sets are adjusted by the BS. So because of dual power control they take more battery consumption then CDMA mobile sets. Example: GSM uses closed-loop with 8 power classes in BS ( W) and 5 in mobile station ( W)

Traffic Engineering “To size the network to be able to handle some expected level of load” In practice, not feasible to have capacity handle all possible load For L subscribers and capacity of N simultaneous users: 1. L <N–non blocking system 2. L> N–blocking system

Blocking System Performance Questions (Assignment) Probability that call request is blocked? What capacity is needed to achieve a certain upper bound on probability of blocking? What is the average delay? What capacity is needed to achieve a certain average delay?

Traffic Intensity Load presented to a system: A=λh λ= mean rate of calls (requests) attempted per unit time h =mean holding time per successful call A= traffic intensity [erlang]

Factors that Determine the Nature of the Traffic Model Manner in which blocked calls are handled 1. Lost calls delayed (LCD) –blocked calls put in a queue awaiting a free channel 2. Blocked calls rejected and dropped 3. Lost calls cleared (LCC) –user waits a random time interval before another attempt 4. Lost calls held (LCH) –user repeatedly attempts calling Number of traffic sources, L 1. Whether number of users is assumed to be finite or infinite 2. Infinite L easier to analyze (reasonable assumption when L is at least 5 to 10 times the capacity)