CMC UNIT2

CELLULAR AND MOBILE COMMUNICATION:

CELLULAR AND MOBILE COMMUNICATION ----------------ELEMENTS OF CELLULAR RADIO SYSTEM DESIGN

OVERVIEW::

OVERVIEW: General description of the problem Concept of Frequency Channels Co-Channel Interference Reduction Factor Desired C/I from Omni directional antenna system Cell Splitting Components of the cellular system

Cellular System:

Cellular System Basic Elements Mobile Switching Centre (MSC) Connected to PSTN Base Station Mobile Station Common air interface 4 different Channels: FCC, RCC, FVC, RVC (Forward/Reverse; Control/Voice Channels)

General description of problem:-:

General description of problem:- Serving many customers in busy hours Maintaining many subscribers into a system Requirement of Frequency channels Efficient spectrum utilization

(a)Maximum number of calls per hour per cell(Q):

(a)Maximum number of calls per hour per cell(Q) Q can be found from this scenario Need to know Size of the cell Traffic condition

(b)Maximum number of frequency channels per cell(N):

(b)Maximum number of frequency channels per cell(N) Is closely related to average calling time in the system If avg calling time T=1.76min Maximum calls per hour per cell ( Qi ) Offered load (A)=( Qi × T)⁄60 Ex:- Blocking Probability B=2%, Qi =28,000 ,T=1.76min, A=(28000×1.76)/60 =821 i.e., N=820 channels per cell

(a).1 Relations:

(a).1 Relations If large area covered by 28cells, Kt=28, total no. of customers Mt=∑ i=1->K Mi in the system increases Mi = ƒ(Qi,n c ) ; n c = % of car phones used in busy hours Qi =ƒ(B,T,N) K=7 Freq reuse pattern total no. of required channels Nt=7×N

2. Frequency Reuse Channels:

2. Frequency Reuse Channels A Radio channel consists of pair of freqs both for Tx and Rx (Full-Duplex operation) Cell C1 uses Freq F1in one area with radius R, same F1 is used in other cell separated by D distance Partition the service area into smaller cells One antenna (base station) serves each cell, transmitting lower power, using only a subset of the available channels Original channel subset used in a cell that is far away from the first cell

Real World:

Real World In the practice of cell planning , cells are not hexagonal as in the theoretical studies. Computer methods are being used for optimised planning of base station location and cell frequencies. Pathloss and link budgets are computed from the terrain features and antenna data. This determines to coverage of each base station and interference to other cells. Frequency reuse plan for C = 7 ( i =2, j =1).

Frequency Reuse:

Frequency Reuse An efficient way of managing the radio spectrum is by reusing the same frequency, within the service area, as often as possible This frequency reuse is possible thanks to the propagation properties of radio waves

Frequency Reuse:

Frequency Reuse For hexagonal cells, the number of cells in the cluster is given by

Cellular Design Reuse Pattern:

TI - 1011 12 Cellular Design Reuse Pattern Example: cell cluster size K = 7, frequency reuse factor = 1/7, assume T = 490 total channels, N = T/K = 70 channels per cell Assume T = 490 total channels, K = 7, N = 70 channels/cell Clusters are replicated M=3 times System capacity = 3x490 = 1470 total channels

Cluster Size:

TI - 1011 13 Cluster Size From geometry of grid of hexagons only certain values of K are possible if replicating cluster without gaps K = i 2 + ij + j 2 where i and j are non-negative integers

3. Cell Design Issues:

14 3. Cell Design Issues Frequency reuse = D/R D: the shortest distance between two cells that use the same frequency R: radius of cells N: reuse pattern = number of different frequencies in a cluster For 7-cell group that has 3-mile radius cells, D  13.74 miles For 7-cell group that has 2-mile radius cells, D  9.16 miles

3. Cell Design Issues:

15 3. Cell Design Issues Transmission interference Adjacent channel interference Several frequency bands are effective in a cell Interference between two adjacent frequency bands Co-channel interference Interference between signals of the same frequencies generated from or to the different base station Co-channel interference reduction factor (q) q = D / R

Handover decision:

Handover decision

Frequency Assignment:

TI - 1011 17 Frequency Assignment Aim: To increase the number of available channels without compromising the quality of service e.g. 1) Efficient Utilization of Spectrum 2) Increase Capacity 3) Minimize Interference Two Types Fixed Channel Allocation (FCA) The number of traffic channels is fixed. If all channels are busy a new call to or from a mobile will be blocked (rejected by BS) Dynamic Channel Allocation (DCA) The BS requests a channel for the MSC when needed The MSC allocates the channel taking into account

Frequency Assignment (cont):

TI - 1011 18 Frequency Assignment (cont) a) likelihood of future blocking within the cell b) the frequencies of use of the candidate channel c) the reuse distance of the channel The dynamic channel assignment reduces the probability of blocking (the number of available channels to a cell is increased) Increase in the complexity of the MSC which has to collect data on; Channel Occupancy Traffic distribution Radio signal strength of all channels Cell borrowing technique : a case of FCA in which a cell is allowed to borrow a channel from its neighbour under MSC’s supervision

Handoff Strategies:

TI - 1011 19 Handoff Strategies When a mobile moves from one cell to the next during a call the MSC automatically transfers the call to a new channel belonging to the next cell. This operation is called HANDOFF Handoff is similar to an initial call request The handoff has the priority over a new call to avoid call cut off in the mid conversation In reality, a fraction of total channels can be reserved for handoff requests in each cell The handoff must be successful, as infrequent as possible and unnoticeable to the user The minimum acceptable level is establised for the received signal to maintain the call. The handoff threshold is slightly above that level. The margin is Δ = P handoff - P min

Improper Handoff:

TI - 1011 20 Improper Handoff

Handoff mechanism :

Handoff mechanism There are two kinds of handoffs, hard and soft. The hard handoff is “brake before make”. The soft handoff is “make before brake”. The soft handoff is used in CDMA systems. Because in CDMA, K = 1, the soft handoff is carried out when the mobile enters the neighboring cell.

Handoff:

Handoff Two co-channel cells using the frequency F1 separated by a distance D are shown in Fig.a . The radius R and the distance D are governed by the value of q. Now we have to fill in with other frequency channels such as F2, F3, and F4 between two co-channel cells in order to provide a communication system in the whole area. The fill-in frequencies F2, F3, and F4 are also assigned to their corresponding cells C2, C3, and C4 Fig.b . according to the same value of q.

Proper Handoff:

TI - 1011 23 Proper Handoff

Handoff (cont):

TI - 1011 24 Handoff (cont) If the margin is too large there are too frequent and unnecessary handoffs which burden the MSC If the margin is too small, there may be not enough time to complete the handoff, particularly when the mobile moves fast The time a mobile spends in a cell without handoff is called dwell time For high speed mobiles, large umbrella cells with wide range are used For low speed mobile, microcells with small coverage area are used The speed is estimated by the BS or MSC from average signal strength

Interference and System Capacity:

TI - 1011 25 Interference and System Capacity Interference is a limiting factor in the performance of cellular systems Co-Channel interference (CCI) is caused by signals at the same frequency Adjacent channel interference (ACI) is caused by signals from neighbouring frequencies In traffic channels, interference causes crosstalk from undesired users In control channels, interference causes errors which result in wrong instructions To reduce co-channel interference, co-channel cells must be separated sufficiently

Co-Channel Interference (CCI):

TI - 1011 26 Co-Channel Interference (CCI) Let R be the radius of a cell and let D be the distance between the centers of co-channel cells The CCI is independent of the transmit power By increasing the ratio D/R we reduce CCI We define the co-channel frequency reuse ratio as Q=D/R, then for hexagonal cells, Q= √3K By reducing Q The cluster size K is reduced The systems traffic capacity is increased (the number of channels per cell is increased) CCI is increased

Avoiding Co-Channel Interference :

Avoiding Co-Channel Interference • “Honeycomb” alternates 3 channels for B/G • More channels to work with for 5 GHz band • Think in 3-D: signal can come from floor above or below – Can stagger the honeycombs on different floors – Avoid co-channel interference between floors

CCI (cont):

TI - 1011 28 CCI (cont) By increasing Q Cluster size K is increased The system capacity is decreased CCI is decreased Mathematically, CCI ratio Calculation Let Ni be the number of co-channel cells Signal-to-interference ratio (SIR) is; S/I = S / ( Σ Ni Ii) Where S is power from desired BS and li is the power from i-th interferer BSi Let P 0 be the received power at a distance d 0 from the transmitter. The received power of the mobile at the distance d from the transmitter is P r =P 0 (d/d 0 ) - n

Antenna Types :

Antenna Types Omni – Omni-directional Dipole – Omni-directional – Radiates in pattern like donut centered on the pole Directional, Patch, and Sector – May looks like a bulgy wall mount plate, most of signal goes into the room – Varying range of degrees of coverage Yagi – Mostly one direction, say 30-55 degree beam width Dish – Commonly used for pt-pt bridge applications – Beam width 10-15 degree: energy mostly in one direction

Cell Splitting:

TI - 1011 30 Cell Splitting Cell splitting is the process of splitting a mobile cell into several smaller cells. This is usually done to make more voice channels available to accommodate traffic growth in the area covered by the original cell If the radius of a cell is reduced from R to R/2, the area of the cell is reduced from Area to Area/4. The number of available channels is also increased. Cell splitting is usually done on demand; when in a certain cell there is too much traffic which causes too much blocking of calls. The cell is split into smaller microcells.

Cell splitting:

TI - 1011 31 Cell splitting

Cell Splitting Drawbacks:

TI - 1011 32 Cell Splitting Drawbacks In practice not all cells are split simultaneously, therefore we may have cells of different sizes. Also the handoff between the cells and microcells has to be taken care off so that high speed and low speed mobiles are equally served. Decreasing cell size results in more handoffs per call and higher processing load per subscriber. Thus, the handoff rate will increase exponentially

Solution: Topology of Different Areas:

TI - 1011 33 Solution: Topology of Different Areas 100 100 100 20 60 100 100 60 60 20 20 20 20 20 20 60 40 20 20 Town Rural Suburb Highway

CELL SPLITTING  :

CELL SPLITTING There are two ways of splitting. original cell site is not used, New cell radius = old cell radius 2 Then following equation is true. New cell area = old cell area 4 Let each new cell carry the same maximum traffic load of the old cell then in theory, New traffic load = 4 × traffic load Unit area    unit  area

Components of Cellular System::

Components of Cellular System: (1)Antennas Antenna pattern, antenna gain, antenna tilting, and antenna height all affect the cellular system design Different antenna patterns and antenna gains at the cell site and at the mobile units would affect the system performance and so must be considered in the system design. Antenna tilting can reduce the interference to the neighboring cells and enhance the weak spots in the cell

Components of Cellular System::

Components of Cellular System: (2)Switching Equipment switching equipment can link to other switching equipment so that a call can be carried from one system to another system without the call being dropped Data Links Each data link can carry multiple channel data (10 kbps data transmitted per channel) from the cell site to the MTSO. This fast-speed data transmission cannot be passed through a regular telephone line. Therefore, data bank devices are needed.