Unit 2: Mobile Communication Systems Lecture 8, 9: Performance Improvement Techniques in Cellular Systems Today s Lecture: Outline Handover & Roaming Hard and Soft Handover Power Control Cell Splitting Sectorization 1
Learning Objectives Students will be able to answer: What is the difference between Handover & Roaming? How power control is helpful in cellular systems? How Cell splitting and Sectorization improves the cellular capacity of the system? Summary of Wireless Systems 2
Handover & Roaming Call handover (handoff in US) is the switching of on-going call to a different traffic channel (frequency/ time/code) Must be transparent, uninterrupted, infrequent and successful Handover inside cell 1 for performance or quality improvement Handover between cells 2 MS moves from one cell/bs within the same MSC (or operator) during an existing call. Handover & Roaming Roaming A MS is handed over to a service area (market) other than that from which the service has been subscribed Roaming MS moves from one cell to another cell with different MSC in the same 3 or different 4 network (?) Handover/Roaming: Involves identifying a new BS, allocate new channels for both voice/data and control signals associated with new BS. 3
Handover & Roaming Handover Scenario 4
Handover Scenario Types of Handover Hard Handover: Perform break before make make. Current channel with old BS is released before the new channel with new BS is setup (may cause call dropping). Remarks: Simple to implement. Cause a short interruption of communications. Used in TDMA/FDMA based systems (e.g., GSM, IS-136). 5
Types of Handover Soft Handover: Perform make before break break. New channel with new BS is setup in parallel with the current channel with old BS. Both channels (old and new) are used simultaneously for the communications. Remarks: Provide diversity to improve boundary conditions. No communication interruption during handover. Used in CDMA based systems (e.g., IS-95, WCDMA, CDMA2000). Methods for Reducing Handover Call Dropping Handover call dropping is less desirable compared to new call blocking. Handover calls are given higher priority than new calls in using network resources. Methods to reduce Handover call dropping: Handover queuing: If all channels are busy, handover calls will be placed in a buffer, unless the buffer is full. Handover reservation: A few channels, r, are reserved for handover calls. New calls will be blocked if number of free channels is <= r. 6
Power Control (PC) One of the limitation on cellular capacity is co- channel interference (CCI). To reduce CCI, power control could be used. Power Control: The concept is to increase/ decrease the transmitter power if it is lesser/more than enough. The idea is that no one will use enough power for transmission Power Control: Figure 2.8 7
Operation of Power Control (PC) MS/BS will estimate its received SIR on DL/UL transmission. If channel condition is good, then received SIR is better than required SIR. Thus, receiver (MS/ BS) will send a command to transmitter (BS/MS) to decrease the transmit power. If channel condition is bad, then received SIR is worse than required SIR. Thus, receiver (MS/ BS) will send a command to transmitter (BS/MS) to increase the transmit power. A single bit command is sent. Typically, 1/0 to decrease/ increase by about 1 db. Power Control (PC) How often PC command is sent is based on PC rate GSM is 2 Hz. IS-95 is 800 Hz. WCDMA is 1500 Hz. Advantages: PC not only helps extend the battery life of the device but also reduces the CCI to improve the cellular capacity. PC is specially important t to CDMA based cellular l networks because every user using the same frequency for transmission. May not be able to compensate the fast Rayleigh fading. 8
Cell Splitting When a cell reaches to its maximum capacity, one solution is to split large cell into several small cells to increase the capacity. Each Cell has its own BS and a corresponding reduction in Antenna Height and Transmit power. Cell Splitting 9
Cell Splitting Trasmitter Power Reduction 10
Sectorization Keep the cell radius but reduce the total co- channel interference by using directional antenna. Each cell is divided into 3 (6) sectors with 3 120 (6 60 ) directional antennas at the BS. For N>=4 The number of co-channel cells is reduced from 6 to 2 (1). Each sector is assigned a 1/3 (1/6) of the available channel per cell. Sectorization For N = 3 The number of co-channel channel cells is reduced from 6 to 3 (2) for 120 (60 ) directional antenna Each sector is assigned a 1/3 (1/6) of the available channel per cell. SIR is improved thus cluster size is reduced. Number of handovers required? 11
120 Directional Antenna Co-Channel Reduction in 120 Directional Antennas (N>=4) 12
Co-Channel Reduction in 60 Directional Antennas (N>=4) A simple analysis for Directional Antennas 13
SIR (worst case) analysis for 120 Directional Antennas SIR (worst case) analysis for 120 Directional Antennas 14
Example What are the SIR (in db) for N = 7, 4 and 3 using 120 directional antenna cellular system? Repeat it for 60 directional antenna? Assume the path-loss exponent 4. Home Work! Using 120 directional antenna SIR 7 =? SIR 4 =? SIR 3 =? Using 60 directional antenna SIR 7 =? SIR 4 =? SIR 3 =? Reason to use Sectorization 15
Unit 2: Mobile Communication Systems Lecture 10: Overview of Cellular Traffic Engineering Lecture 7: Outline Definition of Traffic in Cellular Systems Trunked System Traffic Load Calculations Trunking Efficiency 16
Learning Objectives Students will be able to answer/learn: What are basic definitions iti of Traffic in Cellular System? What are Erlang B formula and Erlang C formulas? How Traffic load and Trunking efficiency in Cellular System is computed? Definitions 17
Definitions Examples 18
Traffic Load? Examples 19
Trunked System Two types of Trunked Systems: Blocked Calls Cleared (BCC) No queuing for call requests. The requesting user is granted a channel immediately if it is available. Otherwise, the user is blocked and free to try again. The grade of Service (GoS) for BCC is to find the blocking probability, P b, obtained by Erlang B formula. Blocked Calls Delayed (BCD) Queue for blocked call requests. A queue is to hold blocked calls. If no channel is available, the call is delayed until a channel is free. The grade of Service (GoS) for BCC is to find the P(delay>t), that a call is blocked after waiting for t seconds, obtained by Erlang C formula. Erlang B Formula 20
Erlang B Formula Erlang C Formula 21
Erlang C Formula 22