ECE/CSC 575 – Section 1 Introduction to Wireless Networking

ECE/CSC 575 – Section 1 Introduction to Wireless Networking Lecture 9Dr. Xinbing Wang

Current Wireless Systems: Cellular Systems--UMTS • Fundamentals of cellular communications • System capacity • frequency reuse • Cell splitting • Admission control • handoff • Universal mobile telecommunication system (UMTS) • Network architecture • Functional units • Quality of service • Mobility management Dr. Xinbing Wang

Handoff Management • Initial Phase: make decision based on the measures of received signal strength • Without hysteresis: a handoff is initiated as soon as the average signal level from the new base station exceeds that from the current BS. • With hysteresis: a handoff is initiated when the average signal level from the new BS exceeds that from the current BS by a threshold amount specified by the hystersis level. • Execution phase • Channel assignment • Exchange of control messages Dr. Xinbing Wang

Handoff Strategies • Mobile controlled handoff (MCHO): the MT initiates and controls the procedure of handoff. • Network controlled handoff (NCHO): the network (e.g., MSC) makes decision and initiates handoff based on the measurements informed by BSs/APs. • MT assisted handoff (MAHO): the MT measures the network (e.g., MSC) makes decision and initiates handoff based on the measurements collected by the MT. Dr. Xinbing Wang

Types of Handoff • Hard handoff is characterized by an MT having a radio link with only one BS/AP at any time. • This mode of operation is referred to as break (old connection) before make (new connection). • Soft handoff in which an MT can simultaneously communicate with more than one BS/AP during the handoff. • This mode of operation is referred to as make (new connection) before break (old connection). • Backward handoff: The handoff is predicted ahead of time and initiated via the existing radio link. • May be affected by a sudden loss or rapid deterioration of radio links • Forward handoff: The handoff is initiated via the new radio link.It may result in large delay when combined with hard handoff. Dr. Xinbing Wang

Handoff Performance • Call dropping probability – probability that a call is terminated due to a handoff • Handoff blocking probability – probability that a handoff cannot be successfully completed • Handoff probability – probability that a handoff occurs before call termination • Rate of handoff – number of handoffs per unit time • Interruption duration – duration of time during a handoff in which a mobile is not connected to either base station • Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur Dr. Xinbing Wang

Introduction to UMTS • UMTS (Universal Mobile Telecommunications System) is the European version of a 3rd Generation (3G) mobile communication system. • It is proposed by 3GPP (3rd generation partnership project). • It includes two parts: UTRAN (Universal Terrestrial Radio Access Network) and the Core network inherited from GSM (Global System for Mobile Communications). • UMTS is a wideband, circuit- and packet-based transmission systems of text, digitized voice, video, and multimedia with data rates up to 2 Mbps (possibly higher). Dr. Xinbing Wang

UMTS Services and Their Relationship to the Internets Dr. Xinbing Wang

Data rate and Spectrum • Maximum data rate and maximum speed for different hierarchical layer • Macrolayer: 144 kbps with max. speed of 500km/h. • Microlayer: 384 kbps with max speed of 120km/h • Picolayer: 2Mbps with 10km/h • Bit Error Rate (BER) • Real-time applications: 10-3 to 10-7 with maximum constant delay: 20ms to 300 ms • No real-time applications: 10-5 to 10-8 with maximum delay >= 150ms. • Spectrum: 1900 MHz-2025 MHz, and 2110 -2200 MHz • FDD (macro- and micro- cells: uplink is from 1920 MHz to 1980 MHz, downlink is from 2110 MHz to 2170 MHz • TDD (pico- cells: not divided by use of different frequency carriers (not suitable for large prop delays). Dr. Xinbing Wang

Network Architecture CN UTRAN VLR PSTN Node B MSC Radio Network controller GMSC UMTS Subscriber Identity module ISDN Node B HLR User equipment Radio Network controller Node B Mobile equipment SGSN GGSN Internet Dr. Xinbing Wang

WCDMA Air Interface • In UMTS, the UTRAN (UMTS Terrestrial Radio Access Network) is used to keep the mobility management (MM) and connection management (CM) layers independent of the air interface radio technology • This idea is realized as the concepts of access stratum (AS) and non-access stratum (NAS) • AS: functional entity that includes radio access protocols between the user equipment (UE) and the UTRAN (terminate here). • NAS: includes core network (CN) protocols between the UE and the CN itself. • The NAS protocols can be kept the same, thus, the GSM’s MM and CM resources are used almost unchanged in 3G NAS. Dr. Xinbing Wang

UMTS Architecture UE UTRAN CN Non-access Stratum Core network protocols Core network protocols Access Stratum Radio Protocols lu Protocols Radio Protocols lu Protocols Iu-interface Uu-interface Dr. Xinbing Wang

Node B • Node B is the UMTS equivalent of a base station transceiver. It may support one or more cells, although in general only one cell one Node B. • It is a logical terminal and the base station is often used for physical entity. • Functions • Mapping of Node B logical resources onto hardware resources • Uplink power control • Reporting of uplink interference measurements and downlink power information • Contains the air interface physical layer, it has to perform many functions such as RF processing, modulations, coding, and so on. Dr. Xinbing Wang

Radio Network Controller (RNC) • One RNC controls one or more Node Bs. • It may be connected via Iu interface to an MSC (IuCS), or to an SGSN via Iu (IuPS). • The interface between RNCs (Iur) is logical interface, and a direct physical connection does not necessarily exist. • An RNC is comparable to a base station controller (BSC) in GSM networks. Dr. Xinbing Wang

RNC Functions • Iub (Node B and RNC) transport resources management • Control of Node B logical O&M resources • System information management and scheduling • Traffic management of common channels • Soft handover • Power control for uplink and downlink • Admission control • Traffic management of shared channels • Macro diversity combining/splitting of data streams transferred over several Node Bs. Dr. Xinbing Wang

Admission Control • In 2G TDMA systems • The capacity in a TDMA cell is typically not interference limited, but it depends on the number of channel elements in the BS. • It is easy to determine the amount of unused resources such as the number of free time slots • Admission control is threshold-based • In 3G CDMA systems • A new call increases the overall interference level in the cells, and thus it has a direct effect on the QoS the other users in the cell • Uplink: most of the interference is intracell, but intercell interference has to be considered as well. • Downlink: orthogonal codes are used, and thus the capacity-limiting factor is mostly intercell interference Dr. Xinbing Wang

Admission Control (2) • The admission-control algorithm must reside in the RNC, which has sole access to all the required information to run an efficient admission control policy. • General idea • Estimate the increase in the usage of resources the new user would cause • If the estimation reveals that the new status would exceed the preset threshold, then the request for the resource has to be rejected • When the total level in a cell is low, an increase to the load level increases the interference level only by a small amount, otherwise, it will cause a large increase in the interference level. Dr. Xinbing Wang

AC Procedure • Determine the amount of available channels, i.e., the number of channels for accepting new and handoff requests. • When the N-th request arrives, i.e., there are (N-1) ongoing services. If there are enough resources to admit the N-th request, then the new request is admitted. Otherwise, it will be denied. • In order to maintain the continuity of a handoff call, handoff calls are given higher priority than new call requests. • The prioritized call admission is implemented by reserving channels for handoff calls. This method is referred to as guard channels. • Fixed reservation and dynamic reservation. Dr. Xinbing Wang

Fixed Channel Assignment (FCA) • Each cell is allocated a predetermined set of voice channels. • The BS is the entity that allocates channels to the requests. If all channels are used in one cell, it may borrow a channel from its neighbors through MSC. • Fast allocation, but may result high call blocking probabilities. Dr. Xinbing Wang

Dynamic Channel Assignment (DCA) • Voice channels are not allocated to each cell permanently. • When a request is received at the BS, this BS request a channel from MSC. • DCA can reduce the call blocking probability, but it needs real-time data collection and signaling transmission between BS and MSC. Dr. Xinbing Wang

Examples of AC • Admission control schemes are not specified in 3GPP technical specifications. • Signaling procedures and measurements are needed to support admission control. • Admission control algorithms are the internal functions of RNC, defined by the operator or by the equipment manufacturer. • It is very difficult to discuss because some services have never been used before. • The packet-level admission control is a new experience for mobile systems, which needs to consider the delay as a requirement whereas not equally evident for the wired networks. Dr. Xinbing Wang

Ex1: Interference-based AC Estimation of the increased interference level in uplink Interference Maximum allowed Interference level interference 2 load 2 load 1 load interference 1 Dr. Xinbing Wang

Ex. 2: Received-power admission control (RPAC) • Assume the service-dependent threshold for a particular service i is Zit , and the total received power before the new user (at a BS) is Zk, then a new bearer service i will be admitted if Zk < Zit • Service-dependent threshold is used to describe the preset parameters for a specific type of services, e.g., real-time voice service. Dr. Xinbing Wang

Layered Architecture • There are three protocol layers in the AS • Physical layer (L1) • Data link layer (L2) • Medium access control (MAC) • Radio link control (RLC) • Broadcast/multicast control (BMC) • Packet data convergence protocol (PDCP) • Network layer (L3) • Radio resource control (RRC) • There is one layer (L3) in the NAS • Mobility management • Call management Dr. Xinbing Wang

RLC Services • Segmentation and reassembly of higher-layer PDUs (Protocol Data Unit) into/from smaller RLC payload units • Padding • Transfer of user data • Error corrections • In-sequence delivery of higher-layer PDUs • Ciphering • Sequence number check These functions are provided to upper layers: Dr. Xinbing Wang

RLC Functions • Segmentation and reassembly of higher-layer PDUs (Protocol Data Unit) into/from smaller RLC payload units • Padding • Transfer of user data • Error corrections • In-sequence delivery of higher-layer PDUs • Flow control • Ciphering • Sequence number check These functions (for itself) are supported by the RLC: Dr. Xinbing Wang

After Class • Reading materials • Chapter 10.5 • Reading materials for UMTS • Exercises • What is handoff? • Describe the architecture of UMTS. • What is the main difference between UMTS and 2G systems? • List 3 functions of RNC and Node B, respectively • What is admission control? In which entity AC is implemented? • What is the difference between FCA and DCA? • Explain two schemes in the lecture notes. • Can you think of two other schemes? Dr. Xinbing Wang

ECE/CSC 575 – Section 1 Introduction to Wireless Networking