Chapter 3: Wireless WANs and MANs

The Cellular Concept To utilize space division multiplexing the area covered by a cellular network is divided into cells. An idealized model of the cellular radio system consists of an array of hexagonal cells with a base station (BS) located at the center of each cell and a number of mobile terminals (MTs) communicate with each other through the base station. Uplink channels for MTs to communication with the base station Downlink channels for BS to communicate with MTs. The cells are designed for frequency reuse. Same frequency can be reused in non-nearby cells. A cluster is a group of cells which uses the entire radio spectrum. The cluster size N is the number of cells in each cluster. Each cell within a cluster is allocated a distinct set of frequencies (channels) and cells labeled with a given number – i.e. co-channels reuse the same channel set. As the cell size decreases, traffic carrier capacity increases, and thus cells start big and split as system grows.

The Cellular Concept Can you tell which the real tree is? MT BS

The Cellular Concept With the shift parameters i and j defined in the figure, we see that the number of cells in a cluster is given by N = i2 + ij + j2 and the frequency reuse distance is given by D = R where R is the radius of a cell. Examples: Two types of interference in cellular systems: Co-channel interference results from the use of same frequencies in different clusters. Adjacent channel interference results due to usage of adjacent frequencies within a cluster. N {i, j} Reuse Distance 4 {2, 0} 3.46 R 7 {2, 1} 4.58 R 12 {2, 2} 6.00 R

The Cellular Concept Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Capacity Enhancement Cell-Splitting: To accommodate a very high density of mobile subscribers, a cell can divided into a smaller coverage area. This smaller cell is called microcell. Microcells are traditionally used in convention centers, airports and similar areas. A microcell can be further splitted into picocells. The number of handoffs is increased. Sectorization: To further reduce inter-cluster interference, each cell is quite often sectored – i.e. directional antennas are used at the mobile base stations. A cell is normally partitioned into three 120-degree sectors or six 60-degree sectors. Power Control: To avoid the near-far problem, the BS must issue power control orders to the MTs to receive a fairly constant, equal power from all MTs, irrespective of their distance form the BS.

Frequency planning f4 f5 f1 f3 f2 f6 f7 Frequency reuse only with a certain distance between the base stations. Standard model using 7 sets of frequencies. Fixed channel allocation (FCA) – Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells Dynamic channel allocation (DCA) – Dynamic frequency assignment : base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurement The allocation scheme is more complex. Hybrid channel allocation: Give two sets of channels to each cell: a set of local channels and a set of borrowable channels. Allow to dynamic allocate channels but with intermediate complexity.

Handoffs When a user moves from the coverage area of one BS to the adjacent one, a handoff (handover) has to be executed to continue the call. A handoff contains two main parts: Find an uplink-downlink channel pair from the new cell to carry on the call Drop the link form the original BS. Issues involved in Handoffs: Optimal BS selection Ping-pong effect: The call gets bounced back and forth in the boundaries between different cells. This should be avoided. Data loss Detection of handoff requirement: Three handoff schemes: Mobile-initiated: An MT monitors the signal strength and requests a handoff when the strength drops below a threshold. Network-initiated handoff: The BS forces a handoff if the signals from an MT weaken. Mobile-assisted handoff: An MT evaluates the signal strength and the BS decides the handoff.

Handoffs Handoff quality is measured by the following parameters (make sure the carrier to interference ratio (C/I) doesn’t fall below the minimum): Handoff delay: The signaling during a handoff causes a delay in the transfer of an on-going call from the current cell to the new call. Duration of interruption: In hard handoff, the channel pair is switched from the current cell to the new cell. Handoff success: The handoff strategies should maximize the handoff success rate. Probability of unnecessary handoff: Unnecessary handoffs increase the signaling overhead on the network. Improved handoff strategies: Prioritization: handoffs are given priority over new call requests. Relative signal strength: The signal strength in the new cell is stronger than the current one. Soft handoffs: A short period of time when more than one BS handles a call can be allowed. Predictive handoffs: the mobility pattern can be predicted. Adaptive handoffs: Users may have to be shifted from micro-cell to macro-cell.

Cellular Architecture SS7 Network STP HLR VLR GMSC PSTN EIR MSC VLR MSC MT Mobile Terminal BS Base Station HLR Home Location Register VLR Visitor Location Register EIR Equipment Identity Register AuC Authentication Center MSC Mobile Switching Center STP Signal Transfer Point PSTN Public Switched Telephone Network BSC Base Station Controller AuC BSC BSC