EEE 441 : Wireless And Mobile Communications Lecture 03 EEE 441 : Wireless And Mobile Communications

Quiz1 Today

The Cellular Concept Major breakthrough in solving the problem of spectral congestion due to finite radio spectrum Replaces single high power transmitter (large cell) with many low power transmitters (small cells)‏

Frequency Reuse Base station in each cell allocated group of channels A group of cells make a cluster Total number of channels replicated in next cluster Adjacent cells always assigned different channel groups from neighboring cells

Frequency Reuse (cont’d)‏ Cells with same letter use the same set of frequencies Cluster outlined in bold, replicated over coverage area Example: cluster size N = 7 frequency reuse factor = 1/7, as each cell contains 1/7 of total available channels. B G C A F D E B B G C G C A A F D F D E E

Choice of Shape of a Cell Factors: Equal area No overlap between cells Distance from center to farthest point in cell Possibilities: d d d C1 C 2 C3

For a given d: Area(C3) > Area(C1), Area(C2)‏ Hence, C3 provides maximum coverage area for a given value of d. Signal attenuation governed by d By using hexagonal geometry, the fewest number of cells are required to cover a given geographic region Actual cellular footprint determined by the contour of a given transmitting antenna

Channel Capacity Let a cellular system have total of S duplex channels The S channels are divided into the N cells in a cluster Number of channels available to each cell is: k = S/N

Channel Capacity (cont’d)‏ Let the cluster be replicated M times within the system Total number of channels, or the system capacity, is: C = MS = MkN i.e. The initial capacity, S, has been increased M-fold by forming M number of clusters

Design of cluster size N Each cell is assigned fraction 1/N of the total number of channels available to the system (S)‏ 1/N is the frequency reuse factor In order to connect without gaps between adjacent cells, N must follow: N = i2 + ij + j2 where i and j are positive integers Example: i = 2, j = 1 implies N = 7

To Find the Nearest Co-channel Neighbor of Particular Cell Algorithm: Move i cells along any chain of hexagons. Then turn 60 degrees counterclockwise and move j cells.

How to Locate Co-channel Cells in a Cellular System In this example N = 19 ( i.e. i = 3, j = 2 ) A A A A A A A

Problem If a particular FDD cellular telephone system has a total assigned bandwidth of 33 MHz, and if the phone system uses two 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels per cell if N = 7

Solution Total available system bandwidth = 33 MHz Channel bandwidth = 25 KHz x 2 = 50 KHz Number of channels, S = 33 MHz / 50 KHz = 660 Cluster size, N = 7 Channels per cell, k = S/N = 660/7 = 95

Channel Assignment Strategies Fixed Channel Assignments Each cell allocated a pre-determined set of voice channels If all channels in a cell are occupied, then a new call is blocked Variation includes a borrowing strategy: - a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied - borrowing is supervised by the MSC

Channel Assignment Strategies (cont’d) Dynamic Channel Assignments Voice channels not permanently allocated to each cell Cell base station requests a channel from MSC every time a new call request is made MSC then allocates a channel to the requesting call, based on a decision algorithm MSC needs real-time, system-wide info on channel occupancy, traffic distribution, roaming possibilities, signal strength …

Handoff Required when a subscriber travels between cells during a conversation interference occurs on current channel, hence call must be assigned to new channel MSC must handoff call/control from old channel in base station in old cell to new channel in base station in new cell MSC must identify the new cell determine available channels in the new cell perform handoff seamlessly Unnecessary handoffs need to be avoided

Handoff Scenario (a) Improper Handoff Situation PA Level at point A Handoff threshold PA Received signal level Minimum acceptable signal to maintain the call PB Level at point B (call is terminated)‏ Time A B PA– PB = ∆ BS1 BS2

Handoff Timing PA – PB = ∆ ∆ should not be too large or too small ∆ too large: too many handoffs ∆ too small: chance of call being lost

Handoff Scenario (cont’d)‏ (b) Proper Handoff Situation Level at point B Received signal level Level at which handoff is made Time A B BS1 BS2

Hard and Soft Handoffs Hard handoff Soft handoff MSC breaks mobile’s connection to base station in old cell makes makes new connection using new channel in new cell seamless transfer that goes unnoticed by user Soft handoff Mobile always retains same channel across the system MSC compares signals from neighbouring base stations to determine the strongest MSC selects between instantaneous signals received from a number of base stations This method is used in CDMA Spread Spectrum systems

Other Types of Handoff Mobile-Assisted Handoff (MAHO): Every mobile measures the received power from surrounding base stations, and continuously reports value to serving base station Faster hand-off rate than with the MSC-controlled type Particularly suited for micro-cell environments Inter-system Handoff: One cellular system to a different cellular system

The Umbrella Cell Approach Small microcells for low-speed mobiles Large “umbrella” cell with high-powered tower for high-speed mobiles Rappaport, Fig. 3.4

Notices Readings Rappaport, Ch 3.1 – 3.4