Adv. Wireless Comm. Systems - Cellular Networks - Objectives Understand the advantage of a cellular network topology Acquire basic methods for capacity planning Outline The cellular network concept Frequency planning in cellular networks – uniform traffic Methods to increase network capacity Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Adv. Wireless Comm. Systems - Cellular Networks - Frequency Reuse Bandwidth (spectrum) is scarce Tradeoff between transmission-capacity and reception- quality that best utilize a given spectrum A single base station accommodates channels Hz total spectrum Hz per channel Fig. 3.1 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Transmission power attenuates with distance Reuse channel frequency is sufficiently apart Sufficiently apart distance Fig. 3.2 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Partition the service region into cells A cell comprises the BTS coverage area Mobiles at each cell are served by cell’s BTS Fig. 3.3 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Hexagon is a geometrical shape that is most close to a circle that covers a region w/o overlapping Commonly accepted abstraction for resource and capacity planning in cellular networks Characterized by its radius R We address the questions: For a given traffic (calls/unit area), how to select R ? How to select transmitter power as a function of R ? How to allocate frequencies (channels) among cells ? R Fig. 3.4 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Reuse Principle for Uniform Traffic Uniformly distribute all channels to a cell cluster of size N Example: A cluster with N=7 1/N is called “frequency reuse factor” Keep co-channel cells as far apart B G C A F D The feasible N are determined from: E Fig. 3.5 (3.1) integers j Examples: N=3,4,7,9,12 i Fig. 3.6 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Reuse Principle (cont.) B Replicate cluster keeping co-channel cells as far apart Transmission capacity: G C A F D E B (3.2) B G C G C A S – Total no. of channels N – Cluster size A F D F D E Lowering N Increases capacity C Increases co-channel interference E Fig. 3.7 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Trading off Capacity and Interference Measurements show that the average received power approximately follows the “exponent decay law” (3.3) (Power in dB) - Power received at reference point Fig. 3.8 - Path loss exponent (typically between 2 – 4) Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Trading off Capacity and Interference (cont.) A common measure for co-channel interference is the Signal Interference Ratio (SIR), namely The ratio between the received signal power and the co-channel interference power Exercise: Verify that the worst case is given by: (3.4) No. of cells in the 1st tier of co-channel interfering cells Fig. 3.9 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Trading off Capacity and Interference (cont.) If all cells have the same size and base stations transmit in the same power, then is independent of the transmitter power Exercise: Verify that the “co-channel reuse ratio” satisfies (3.5) Hence, the “capacity” and the worst SIR become ; (3.6) Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Trading off Capacity and Interference (cont.) A sensible tradeoff rule is to maximize Subject to: (3.7) Thus, we take the smallest N that satisfies Eq. (3.7) N 3 4 7 9 12 SIR (dB) 11.3 13.8 18.66 20.85 23.34 n = 4 Example: US AMPS cellular system requires SIR of at least 18 dB Exercise: Refine the calculation ([Rappaport, Ch. 3.5.1] ) Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Cell Radius Selection From Eq. (3.6), the selectable N above is independent of the cell radius R The radius R is determined by other considerations Base station cost drives to large R Mobile battery power however, limits R by the constraint (3.8) or - Minimum received power at base station Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Micro-cell Concept A smaller R generally implies Lower transmitter power Better area coverage Higher capacity These are the basic principles of micro-cells discussed below. No. of cells per unit area however, is Thus, halving R results in 4 times more cells Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Micro-cell Concept (cont.) Traffic is not uniform in general and some spots need more allocated channels than others One way to resolve it is to: Add base stations at hot spots which transmit at a lower power Without changing the frequency assignment to the macro-cells N =3 3 micro-cells Fig. 3.10 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Micro-cell Concept (cont.) The transmit power at the micro-cells is determined by its radius and through the equation (3.9) , for some c - transmission power in the macro-cell - transmission power in the micro-cell For k=2 and n=4 we have Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Micro-cell Concept (cont.) Frequency planning with micro-cells is not obvious In a ”3-cluster cell plan” we may embed ”6 micro-cell clusters” N =3 Fig. 3.11: An oversimplified example 6 micro-cells with radii R/2 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) The Micro-cell Concept (cont.) In a “6-cluster” cell plan” we may embed “6 micro-cell clusters” N = 6 Fig. 3.12: An oversimplified example 6 micro-cells with radii R/2 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Sectoring Cell splitting into micro-cells increases capacity by rescaling It decreases R while keeping D/R constant. Hence packing more channels per unit area “Sectoring” decreases D/R while keeping R constant Base station uses several directional rather one omni-directional antenna, which increases the receiver’s SIR 1 1 6 2 2 3 5 3 4 Fig. 3.13: 3 sectors per cell Fig. 3.14: 6 sectors per cell Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -

Frequency Reuse (cont.) Sectoring With 3 sectors per cell and a cluster size of 7, the number of 1st tier interfering co-channel cells reduces from 6 to 2, and with 6 sectors, to 1. Fig. 3.14 Lecture 3: Frequency reuse for uniform traffic Adv. Wireless Comm. Systems - Cellular Networks -