Chapter 1. Problem Delimitation. 1.1 Background

Transcription

1 Chapter 1 Problem Delimitation 1.1 Background The wireless communication has been possible due to the electromagnetic wave propagation through the air interface and its fast development achieves a global communication available from one person to another at any place and any time. The impressive growth of the cellular mobile telephony as well as the number of the Internet users poses an exciting potential for market that combines both innovations: cellular wireless data services. It is predicted that, there will be higher demand for wireless data services. In particular, high-performance wireless Internet access. The overview of mobile communication system starts with several mobile radio networks with low capacity, quality and small mobility range. These limitations were not solved until the appearance of cellular concept used in mobile communication systems. The main idea of cellular concept is to divide a large area into small cells in order to reuse the frequency in the distant cell without interference. Basic on this purpose, the first analog cellular communication system came into being. Although the analog cellular communication system invokes the revolution of reusing frequency for the aim of saving limited spectrum resources, it still exists a series of problems, such as only basic voice telephony, limited coverage and non-compatible among different networks. Instead of analog, the second generation of digital cellular communication system has been applied. Global System for Mobile Communications(GSM) is a typical representative among this stage and has been very stable and widely accepted standard for mobile communication. GSM uses circuit switched technology 1

2 to transmit both voice and data. It inherently support other technologies at its branches. GSM provides mobile services based on digital data interchange at up to 9.6kbps, in addition to voice communication. Each GSM carrier band is 200KHz wide carrier into eight timeslot. To provide a single voice channel, one timeslot is used. One timeslot is also used to provide a single 9.6kbps data circuit. However, all eight slots are used to provide one 64kbps full rate-circuit voice user using time division multiple access (TDMA) bearer slots. The cellular data services do not fulfill the needs of user and providers. From the user point of view, these data rates are too slow and the connection setup takes too long and is rather complicated. Moreover, the service is too expensive for most users Therefore, more applications has been added, such as Small Message Services(SMS). However, the fast growth of Internet requires a wireless data access which GSM is inefficient to support because the fixed data service.the General Packet Radio Service(GPRS) is the extension of GSM. It is implemented to provide packet data service over the GSM infrastructure. The channel capacity is highly increased and the amount of users is enlarged. The major new third generation, called the Universal Mobile Telecommunications System, UMTS, is definitely designed to achieve universal speech services and local multimedia systems and in process of development worldwide.. When considering the wireless communication system, the channel effects should not be ignored. The performance of a wireless system is strongly affected by its environment. In the characterization of fading channels, different components are distinguished, such as a large-scale path loss,a medium-scale slow-varying with lognormal distribution, and a small-scale fast varying component modelled with a Rician or Rayleigh distribution accordingly to the presence or absence of Line Of Sight(LOS). In this way phenomenons as multipath fading, shadowing, near-far effects are taken into account. 1.2 Problem shaping The importance of GPRS lies in the increasingly traffic, mainly data over voice, a GSM network without GPRS could not support. In GPRS and UMTS, the GSM is said to be the background under both previous technologies lay. By the way, they provide a packet switched extension for the access to IP orientated services. Even though GPRS can supply many new data services as well as higher efficient in utilizing the capability of transmission network, it has to share 2

3 the same radio resources with GSM (voice) services. That is, the air interface becomes a bottleneck and the GSM/GPRS operators have to balance the quality of existing voice service and that of newly introduced data services. As far as radio resource allocation is concerned, European Telecommunications Standards Institute (ETSI) proposes the fixed and on-demand channel allocation mechanisms. Fixed Resource Allocation (FRA) Dynamic Resource Allocation (DRA) Fixed Resource Allocation with Queue capability (FRAQ) Dynamic Resource Allocation with Queue capability (DRAQ) The purpose of this project is to develop a method/algorithm to optimally assign radio resources to GPRS services and avoid decreasing the quality of voice services. Some key issues in this project include: (1) Set criterion for evaluating the service quality of GPRS,i.e, blocking rate and/or data transfer speed. GSM voice service quality is also in consideration though its criterion has already been set in industry standard (blocking rate). (2) A certain GPRS data traffic model should be built. (3) An optimal algorithm will be designed to assign radio resources for packet-switched data to achieve the criterion in (1) under the traffic model. (4) All above will be implemented in a simplified simulation model to evaluate the optimal RRM algorithm. Since this project is designed to handle with the problem in real engineering world, and considering the time and human resource and facility limitations the design group has, the working scope of this project will be delimited as follow, Possible algorithm input and analysis model will be built on the basis of some current existing GPRS architectures and radio resource management strategies. That is, the developing methodology is not allowed to change the network structure. Input data or traffic model will be based on the network of an cooperation GSM/GPRS operator. The radio resources in this project refer to all the GSM slot and frequency band inside one cell cluster. 3

4 1.3 Solution steps and Time Schedule Firstly, a background of the GSM/GPRS systems is analyzed, and concretely the GPRS air interface of this infrastructure. Next to this introduction, the quality of the voice and data in GSM/ GPRS operators is matter of study. In this way the assignment of radio resources to packet switched services is evaluated from different operator strategies, defining scenarios, input parameters and algorithms, and finding out optimality criterions by means simulation methods. In order to get efficient radio resources utilization, some strategies to the dynamic channel allocation for data packets as the reservation of some time-slots in each TDMA to GPRS traffic will be study. The analytical results will be contrasted with those obtained from the simulations. Finally, in order to apply it to a real situation, a particular operator strategy and its actual traffic data is included as an example. 1. Phase 0 - Problem Delimitation: Feb 9 - Mar 4, Week 7 - Week 10 Task: Problem shaping. Understand the basic concepts of GSM and GPRS technique and analyze the work scope of the project. Milestone: Introduction chapter. 2. Phase 1 - GPRS Technical issues: Mar 5 - Mar 17, Week 10 - Week 12 Task: Understand and describe the GPRS air interface and the existing radio resource management (RRM) strategies, especially those currently operating on the network of a local operator, Sonofon. Milestone: Technique background chapter. 3. Phase 2 - Detailed Requirements: Mar 11th - 24th, Week 11 - Week 13 Task: Define scenarios, input parameters and evaluation criterions, for which an optimal algorithm for radio resource allocation in GPRS can be developed. These definitions maybe modified a couple of times according to the discussion with the GSM/GPRS operator which the design team co-operate with. 4. Phase 3 - Solution Development: Mar 25th - May 19th, Week 13 - Week 21 Task: Develop an optimal RRM solution and evaluate it in simplified simulation models or with the help of analytic models. Milestone: 1) Design and simulation chapter. 2) Conclusion chapter. 4

5 5. Phase 4 - Report: May 20th - Jun 3rd, Week 21 - Week 23 Task: Finalize the project report. Milestone: Report hand in before Jun 3nd, 12:00 AM. Figure 1.1: Time Table 5

6 Chapter 2 Introduction of GSM and GPRS 2.1 Cellular Concept Principal to any understanding of cellular communication system is the cellular concept. The main object of the cellular concept is to divide a large area into small cells, reusing the frequency of distant cells without interference in order to improve the channel capacity[1]. This strategy comes out the traditional problem between coverage area and system capacity, but before using it some limitations must be solved such as use of higher carrier frequencies with technological and legal regulations and development of new methods of addressing and monitoring the position of all the mobile stations within the system. The coverage area of cellular is divided into cells which are theoretically hexagonal shape. Each cell is assigned a set of channels and the group of cells which covers all the ranges of channel frequencies is named as a cluster. The cluster creates a certain distance for co-channel cells to avoid interference with a transmitter cell working with the same set of frequencies. Generally, a 7-cell cluster is widely used because it has good performance in presence of adjacent interference and good enough capacity per cell. Once a cluster pattern has been chosen,the other cells will be organized in the same way, so the work will be concentrate around the reference one. 6

7 Figure 2.1: Cellular Concept 2.2 Effects of Multipath Propagation The transmitted signal is transmitted between MS and BSS through the air interface, which causes the multipath propagation. This phenomenon is a result of disturbances on the receiving signals. multipath fading is introduced by the reflection of stationary obstacles, i.e. buildings, trees and moving vehicles, i.e. cars. Thus the signals with different amplitudes and phases will arrive at the receiver. This statistical distribution is as Rayleigh distribution. Shadowing is another effect due to the large structures between the transmitter and receiver. It follows as a log-normal distribution. The multipath propagation affects the quality of the voice/data information and have the probability of error or even get lost during the propagation. GPRS has the Automatic Retransmission Request(ARQ) mechanism, which is applied to inform the sender to retransmit the erroneous block; while it does not exist in GSM. However, the time delay is introduced due to the process of retransmission. In order to guarantee the packet quality, four coding schemes has been used in GPRS, which has the data rate from 9.05kbit/s for CS1 to 21.4kbit/s for CS4. Each coding scheme takes 1/2, 2/3, 3/4 and 1 convolution coding method respectively, which means the redundancy bit added for each coding scheme is 1 for CS1 to CS3, and 0 for CS4. Actually, CS4 is a scheme without coding. i.e. if the coded bits are 1000, the original bit should be 500, 666,750 and 1000 for CS1, CS2, CS3 and CS4, separately. The more redundancy bits added to the original code, the more protection for 7

8 the bits to be damaged. Therefore, the fewer bits should be retransmitted, which introduces fewer delay. As a result, higher quality can be achieved. Although GSM and GPRS have the same radio communication structure in the physic layer, the effects of GSM and GPRS affected by the multipath propagation behave in the different way, due to GPRS is packet switched traffic and GSM is circuit-oriented. Unlike the GSM situation with fixed channel allocation, GPRS dynamically allocates the channel to users, which means the channel is randomly occupied by the users during the uplink and downlink data transmission. Therefore, the time slot user used can be different in the uplink and downlink data transmission, which improves the channel capacity and spare less time, compared with the fixed channel mode. However, this method of allocation has a relatively high burstiness and causes the random co-channel interference on the data transmission in GPRS. 2.3 GSM 1. Definition Global System for Mobile Communications(GSM) is the second generation of the cellular telecommunication system. Compared to the first generation system with analog cellular, GSM is for digital cellular telephone and based on circuit switched. 2. Services GSM, being a Circuit Switched (CS) network was mainly developed for voice usage, but it also includes the possibility of sending Short Messages (SMS) in the control time slots to provide a pager-like service even during a phone call. It also provides the possibility of having slow rate data services (9.6Kbps), but the constant use of a time-slot is a waste of resources from the point of view of both the operator and the user, as it is occupied even when no data is transferred. This data service was mainly used for Fax. 3. Network Architecture The Network Architecture of GSM system consists of the Mobile Station(MS), the Base Station Subsystem(BSS) and the Network and Switching Subsystem(NSS). Mobile Station(MS) The MS contains the Mobile Equipment and Subscriber Identity Module(SIM). 8

9 Mobile Equipment The mobility refers to user mobility and device portability, so the mobile equipment can be a portable handset or a vehicle mounted unit.(1) SIM A SIM card is tesselated in the mobile equipment. It provides the subscriber information called International Mobile Subscriber Identity(IMSI). The information is obtained by the Mobile Switching Center(MSC) in order to distinguish the different users. Base Station Subsystem(BSS) BSS is divided into the Base Transceiver Station(BTS) and the Base Station Controller(BSC). BTS realizes the connection between MS and BSC and handles the radio link protocols with the MS. The BSC supports the control function and manages the radio resources, such as radio channel setup and handovers. The BSS is connected to MSC with the Transcoder and Adaptation Unit(TRAU). The TRAU multiplexes four of the 13kbps speech or data and converts them into 64kbps channel. It can be considered as a part of BSS. Network and Switching Subsystem(NSS) The central component of the Network Subsystem is the Mobile Switching Center(MSC). It achieves the control of the mobile subscriber, such as registration, authentication, location updating, handovers and etc. MSC is also the interface between MS and other networks. Besides MSC, NSS also contains four database registers -the Home Location Register(HLR), the Visitor Location Register(VLR), the Authentication Center(AuC) and the Equipment Identity Register(EIR). -HLR stores all the subscriber s information and current mobile location for controlling calls. -VLR includes some subscriber s information which is selected from HLR for the purpose of controlling calls that are currently in the geographical area. -AUC works as a protected file. The individual secret information is saved in it to keep security. -EIR is a database contains all the valid mobile equipment on the network. 9

10 Figure 2.2: GSM network 4. Drawbacks of GSM From the technical point of view, the drawback results from the fact that current wireless data services are based on circuit switched radio transmission. At the air interface, a complete traffic channel is allocated for a single for the entire call period. In case of a bursty traffic (Internet), this results in highly inefficient resources utilization. It is obvious that for bursty traffic, packet switched bearer services result in a much better utilization of the traffic channels. This is because a channel will only be allocated when needed and will be released immediately after the transmission of the packets. With this principle, multiple users can share one physical channel. Therefore, some disadvantages of GSM are listed as follows: GMS is based on circuit-switched connection oriented technology, where end systems are dedicated for the entire call session. This causes inaccuracy in the usages of bandwidth and resources. 10

11 GSM enabled system do not support high data rate (Data rate for voice: 13.6kbps and data: 9.6kbps). They are unable to handle complex data such as audio, video, etc. These devices have small hardware configurations with less powerful CPUs, memory and display units, and support simple functionality. Only basic messaging services such as SMS can supported. Also these services depend upon the service provider and the network characteristics. The GSM networks are incompatible with the current TCP/IP and other common network because of differences in network hardware, software and protocol. At present, there is no efficient way of handling frequent data traffic within GSM: In User point of view: Restricted user data rate (9.6kbps). Calls routed over the PSTN/ISDN to the data networks. Higher price (PSTN access as transmit network). Subscriber will be charged for connection time, not for usages. Long call setup time ca. 20s in case of modem usages. Restricted length of SMS For operator: No efficient resource management possible. Restricted number of user. SMS is not ideal match for many applications. 2.4 GPRS 1. Definition The General Packet Radio Service(GPRS) is established on the platform of GSM and it is the enhancement of GSM system. It is also known as the 2.5 Generation and a step toward the 3rd Generation 11

12 (3G). Furthermore interworking specifications have been developed between ANSI/ISA-136 and GSM platforms to get a logical extension of the overall scheme. Unlike GSM that was designed for voice services and requires a circuit switching transmission mode, GPRS provides a packet switching transmission mode. This feature allows an easy adoption to the bursty traffic generated by Internet application like , WWW and FTP. In comparison with 9.6kbps data transmission rate of GSM, GPRS offers a maximum theoretical data transmission rate of 172.4kbps.Another important goal of the technology is to make it possible for GSM license holders is to share physical resources on a dynamic, flexible basis between packet data services and other GSM services. Besides, different coding schemes can be used and it allows the user always online. Consequently, GPRS shares GSM frequency bands with telephone and circuit-switched data traffic, and makes use of many properties of the physical layer of the original GSM system most importantly the TDMA frame structure, modulation technique and structure of GSM time slots. GPRS provides synchronous and asynchronous interworking with X.25 networks, IP networks and other GPRS networks. The different types of bearer services described within GPRS are Point- To-Point (PTP) and Point-To-Multipoint (PTM). The example of Point-To-Point is the access to the Internet Whereas, Point-To-Multipoint caters Traffic Information. GPRS extends the packet data capabilities of the GSM networks to higher data rates and longer messages, therefore an amount of data services are available on mobile phones with internet access. 2. Services The increase of speed provided by Packet Switched (PS) networks as GPRS enlarged the amount of data services on mobile phones. The dynamic use of multiple time slots provided a way of not wasting resources and the Packet architecture allowed users to connect to IP services such as the web ( , ftp and http protocols...). The possibility of having a Point to Multipoint connection also allowed more services such as multiuser video-conferences or chatting. 3. Network Architecture 12

13 GPRS attempts to reuse the existing GSM platform as much as possible, but in order to build a packet based mobile network, some network elements, interfaces and protocols that handle packet data are required. GPRS involves two parts: circuit-based voice service and packet-based data service. In this case, the GPRS network architecture should include the function of the radio air interface transmission, besides the GSM network architecture.the way to implement it is to add new packet data nodes in GSM/TDMA networks and upgrade existing nodes (including software to some elements)to provide a routing path for packet data between the mobile terminal and a gateway node. In order to realize GPRS based on GSM, two node modules should be added: the Serving GPRS Support Node(SGSN) and the Gateway GPRS Support Node(GGSN).Additionally, enhancements will be made to the EIR and AuC databases to control the security and authentication of the mobile station. Also in the Base Subsystem it is required to add some Packet Control Unit Support Nodes(PCUSNs) to support the packets orientation to the SGSN. Concretely in the BSC a packet control unit(pcu) is installed and connect to the GPRS network. The new mobile terminals should be able to handle the enhanced air interface and packetize traffic directly in a compatible way with GSM for making voice calls using GSM.Also it follows a three state configuration, that is active, idle and standby. SGSN It can be viewed as a MSC as in GSM network. The SGSN will be connected to the BSC through PCU, when packets are delivered to MS. Additionally, SGSN is also linked with HLR to support the functions of authorization, authentication and location detecting of the mobile subscribers.it also acts as the VLH. GGSN GGSN works as a gateway realizing the connection between GPRS network and public data networks (PDN). It communicates with SGSN through the GPRS backbone network. In order to enable the packet data transferred, a Packet Data Protocol(PDP) is required to associate the SGSN and the GGSN. GGSN also works together with the HLR in order to update the location directory of the subscribers. 13

14 Communications between the GPRS Serving Nodes (GSNs) makes use of a technique known as tunneling.the GPRS network encapsulates all data network protocols into its own encapsulation protocol, called the GPRS Tunnelling Protocol (GPT), to ensure security in the backbone network and simplify the routing mechanism and delivery of data over the GPRS network. In mobile IP, packet are only tunnelled from the fixed network to the mobile station (MS). Packets flowing from the MS to fixed nodes use normal routing. GPRS, by contrast, uses tunnelling in both directions. A new set of interfaces has been developed for GPRS labelled with Gx, where x identifies the different kind of these (Gb, Gn, Gi,...). GSN/GPRS networks must track and monitor the monitor, so data packet routing and the mobility management represents main functions to be achieved by the NSS. Figure 2.3: GPRS network 14

15 4. Advantages in GPRS GPRS offers a number of benefits to the operator and end user. The operator benefits of GPRS are: Optimal support for packet switched traffic. The operator can join the Internet boom with true IP connectivity. The possibility to offer new innovative services. New user segments such as telemetry of e.g. electric meters will be become accessible to the operator. The ability to profit with idle capacity that would otherwise be used only to cover peak-hour traffic. Many users can use one time-slot simultaneously. The network can be expanded smoothly to meet growing demand and maximize investment. The end user benefits: GPRS has a potential to offer global mass-market wireless access to the Internet and other packet-based networks. Applications will be user friendly with a seamless on line network connection independent of time and place. Very fast set-up time and staying online for long periods. Easier to use existing application and also enables new applications. High bit rates in peak-hour, and uncompressed data rate of over 100kbps. End users are charged only according to the amount of data transferred. 2.5 Circuit versus Packet Circuit switched users occupy the same traffic channel for a relatively long time. In such scenario, the nulls placed to decrease the produced interference are reasonable. But users on a packet switched traffic channel may change from uplink to downlink. The spatial channel can be estimated for the desired users as well as the active interferers from the received uplink data. As a result of that, the received signal can be increased for the desired user and decrease the power received by the interferers. It is only possible in a circuit switched, when the interferers from both the uplink and downlink are same. Whereas, not possible in a packet switched transmission which relies on the dynamic allocation of channel. In a circuit switched, users are always active 15

16 in the uplink and downlink whereas in a packet switched, the active users during the uplink can be inactive during the downlink. As the other packet switched users can be active in the corresponding downlink traffic channel and thus the situation changes. Since no null can be placed in an unknown direction the produced downlink interference for these packet switched user can be more than intended. Additionally the degrees of freedom used to place the nulls for their corresponding uplink counterparts are wasted, and they decrease the beamforming gain for the served user within the cell. 2.6 Access Methods In the mobile communication field, radio frequency is limit. In order to make full advantage of it for all the users in an efficient way. Some methods are applied, such as Frequency Division Multiple Access(FDMA), Time Division Multiple Access(TDMA) and Code Division Multiple Access(CDMA). In GSM cellular system, the combination of FDMA and TDMA is employed. The uplink and downlink communications are assigned 25MHz frequency band respectively. The 25MHz is divided into 124 carrier frequencies and each frequency band is 200kHz. On the TDMA side, the channel is divided into different time and every carrier frequency consists of 8 time slots. Each slot occupies 0.577ms, so a TDMA frame which is made up of 8 time slots, is 4.615ms. TDMA realizes several users can share the same frequency in different time duration. Due to the data rate in TDMA is from 64Kbps to 120Mbps, it is preferred in the digital cellular system. 16

17 Figure 2.4: FDMA 2.7 Technic Roadmap What is the difference between GSM and GPRS? GSM(as for voice service) - Circuit switched transmission. - Radio frequency transmission: 9.6kbps/slot (FDMA/TDMA). - Duration-based charging. GPRS - Packet switched transmission on NSS side. - Multiple time-slot can be integrated. - Different coding schemes. - Volume-based charging(always On mode) GSM upgrade roadmap The new extensions to GSM Phase II are: 17

18 High-speed circuit-switched data (HSCSD), by using several circuit channels GPRS,as it was mentioned before to provide packet radio access to external packet data networks such as IP and X.25 Enhanced Data rate for GSM Evolution, (EDGE), applying a new modulation scheme Universal Mobile Telecommunication System, (UMTS)(3nd Generation), a new wireless technology using new infrastructure deployment All these technologies enable higher data throughput, better spectral efficiency and lower call setup times. 2.8 GPRS protocols In the GPRS Protocol Stack, the focus will be in the air interface and lower layers, which can be observed in the picture. LLC (Logical Link Control) : Allows a high reliability of the MS-SGSN link. This layer controls and codes data between those two independently from the lower layers RLC (Radio Link Control) : Provides a reliable link between the MS and BSS through the radio interface. This layer deals with the error detection and correction, and the data retransmission. It is responsible for segmenting and reassembling data sent over the air interface (LLC frames are too big to be sent over the air interface so RLC breaks LLC frames into blocks and encapsulates each block forming a RLC block) MAC (Medium Access Control) : Controls the radio channel access. This layer gives the resource to the MS only when it has data to transmit. It controls multiple MS sharing a common resource on the GPRS interface. GSM-RF : Describes the physical layer of the radio interface Um : TDMA frame, modulation/demodulation... It provides channel-coding functions, interleaving, radio channel measurement functions (received quality and 18

19 signal levels, timing advance measurement, physical link congestion detection) and radio management procedures (cell selection/reselection, power control,...). BSSGP (BSS GPRS Protocol) : Transports routing and QoS information between PCU and SGSN. This layer provides a connectionless link with unconfirmed data transfer between BSS and SGSN, it acts as an interface between LLC frames and the RLC/MAC blocks in the BSS, and as an interface between the RLC/MAC derived information and the LLC frames in the SGSN. Frame Relay/Network Service : Provides a packet type commutation between SGSN and BSS. Figure 2.5: GPRS Protocols Stack 2.9 GSM/GPRS Logical Channels Traffic Channels 1. GSM Traffic Channels 19

20 The Full rate traffic channel (TCH/F) in GSM are used to convey voice or data information in a circuit switched manner. A TCH/F is mapped on a time slot every frame, thus allowing to transfert 114 bits of coded information every time slot. The datarate for the voice is about 13Kbps, as only the most important data is highly coded, but circuit switched data information has a strong channel coding which allows a datarate of only 9.6Kbps. The Half rate traffic channel (TCH/H) can be used to tranfert data information. It is mapped on a time slot every other frame, and has half the datarate of TCH/F (4.8Kbps). 2. GPRS Traffic Channel In GPRS, the traffic channel is called PDTCH (Packet Data Transfert CHannel), and carry packet switched data. It can be dynamically mapped in any time slot, and uses four different coding schemes to protect the carried data, and to adjust the throughtput depending on the channel conditions Control Channels 1. Introduction Control channels deal with network management messages and channel maintenance task. Any non-traffic communication between the BS and the MS use these channels. Three type of control channels exist which are either one or two-way communication channels. 20

21 Type GSM GPRS Frequency correction Broadcast FCCH Synchronisation SCH System information BCCH PBCCH Paging Common PCH PPCH Random Access RACH PRACH Ressource Assignement AGCH PAGCH PTM Ressource Assignement PNCH Signalling procedure Dedicated SDCCH PACCH Measurements Timing Control SACCH PTCCH Time Critical msgs FACCH GSM/GPRS Control Channels 2. Broadcast Control Channels The Broadcast Control Channels are used in one-way communications from the BS to the MS to send general informations about the network. In GSM, we distinguish three types of broadcast control channels. FCCH (Frequency Correction CHannel) is listened by the MS to adjust it s frequency corresponding to the BS. It transmits 148 0s, which in GMSK corresponds to the carrier sine wave. It occupies one time slot in the physical layout. SCH (Synchronisation CHannel) uses one time slot to transmit 64 training bits and a 25 bits message containing the BS identification code, and the present frame number for the MS to synchronise both spacially and temporally. The BS uses the BCCH (Broadcast Control CHannel) to transmit all information needed by the MS to establish a call, such as control channels configuration, or access protocols. This channel takes four time slots as its uncoded message is 184 bits long. 21

22 PBCCH (Packet Broadcast Control CHannel) is an additional type of channel used in GPRS to transmit broadcast information as the BCCH, but for packet switched users. 3. Common Control Channels Common Control Channels are used in GSM for paging purposes, and to allocate dedicated control channels to the MS. Three types of common control channels are used. The Paging Channel (PCH) is a one-way control channel used to alert the MS of an incoming phone call. Paging groups are defined for the MS to listen only to a specific time slot, and an MS identification is sent in that specific channel when a call arrives to initiate communication to the good MS in the group. They occupie 36 frames of time slot 0 in the 52 multiframe structure. The RACH (Random Access CHannel) and the AGCH (Access Grant CHannel) are used to initiate a communication with the MS. The MS sends messages on the RACH to initiate a call, update its location, respond to a paging request, or initiate an SMS transmission. When an RACH message is received by the BS, it sends back a message on the AGCH to allocate a dedicated control channel to the MS, and initiate a two way communication. GPRS has equivalent common control channels for packet switched users (PCCCH). PRACH (Packet RACH) and PAGCH (Packet AGCH) are used to initiate a packet switched communication, and PPCH (Packet PCH) is used to page an MS prior to downlink packet transfert. Specific to GPRS is the PNCH (Packet Notification CHannel), a ressource assignment channel used by BS to initiate PTM (Point To Multipoint) packet transfer with multiple MSs. 4. Dedicated Control Channels Dedicated Control Channels are used to exchange control information between the BS and a dedicated MS. They are two-way channels and are mainly used in GSM for call management. Three different dedicated control channels exist. The Stand alone Dedicated Control Channel (SDCCH) is a low datarate control channel mainly used for call set-up issues such as authentication, mobility management, and ciphering negociations. This control channel is also used to perform location updates. Once a call is in progress, two control channels are associated with the traffic channel. The SACCH (Slow Access Control CHannel) transmits measurements (power, timing advance) to the BS in order to manage 22

23 the MS every 26 frames. The FACCH (Fast Access Control CHannel) has a higher datarate, and is used for time-critical messages, such as handoff orders, confirmations for connection or release of a traffic channel, or alerting (phone ring). It steals time slots from the traffic channel to perform its operations. In GPRS, only two types of dedicated control channels exist. The PACCH (Packet Associated Control CHannel) convey signaling information to a MS such as packet acknowledgment (ACK) and power control. It also carries assignement messages for either traffic channels or further PACCH. The PTCCH (Packet Timing Control CHannel) is used to adjust timing delays of packets, and is either uplink (PTCCH/U) or downlink (PTCCH/D) Mapping traffic and control channels on physical channels In both uplink and downlink, a multiframe structure of 51 frames is used to allocate physical resources to almost all control channels. Time slots 0 and 1 of every frame are dedicated to control channels which show a regular pattern every multiframe. Traffic channels are mapped into a 26 multiframe structure ( idle frame) and time slots 2 to 7 are used for traffic during 24 frames. Only the SACCH and FACCH can be mapped in the traffic multiframe. The SACCH occupy time slots 2 to 7 during one frame of each traffic multiframe, while the FACCH steals time slots from the current connection to operate. Fig 1 and 2 show how the different GSM channels can be mapped into the multiframe structure. Capacity on Demand - In order to allow GPRS service in cells where there are few (or no) GPRS users without the need for any permanently allocated resources, the concept of capacity on demand has been introduced. The operator can decide whether to dedicate some PDCHs for GPRS traffic. Load supervision is done in the MAC layer to monitor the load on the PDCH(s), and the number of allocated PDCHs in a cell can be increased or decreased according to demand. Unused channels can be allocated as PDCHs to increase the overall QoS for GPRS. If other services with higher priority request resources, deallocation of PDCHs is possible. However the existence of PDCH(s) does not imply the existence of packet control channels. When no control channels is allocated in a cell, all GPRS-attached MSs automatically camp on the existing GSM control channels. When a control channel is allocated in a cell, all GPRS-attached MSs camp on it. The control channels can be allocated either as the result of the increased demand for packet data 23

24 transfer or whenever there are enough available physical channels in a cell. If the network releases the control channels, the MSs return to the regular GSM control channels. Figure 2.6: Channel allocation in physical channels (Downlink) 24

25 Figure 2.7: Channel allocation in physical channels (Uplink) 2.10 Limitations of GPRS GPRS is an important new enabling mobile data service, which offers a major improvement in spectrum efficiency, capability and functionality compared with current non-voice mobile services. However, there are some limitation of GPRS and can be summarized as Time Delay, Speed much lower in reality and limited mode of operation. GPRS packets are sent in all different direction to reach the same destination. This opens the potential for one or some of those packets to be lost or corrupted during the data transmission over the radio link. The GPRS standards recognize this inherent feature 25

26 and that potentially generate transit delays. Because of this, application needed broadcast quality video such as news reports can be implemented using High Speed Circuit Switched Data (HSCSD).To achieve maximum theoretical GPRS transmission speed of 171.2kbps, all eight time slots from a single base station must be allocated for use by a single GPRS terminal. Since, GPRS handsets are configured to support a maximum of three or possible four time slots. Therefore, the bandwidth available to the GPRS user would be much less than the theoretical values QoS and RRM The quality of service QoS requirements of typical mobile packet data applications are very diverse (e.g., like real time multimedia, web browsing, and transfer). Support of different QoS classes, which can be specified for each individual session, is therefore an important feature. GPRS allows defining QoS profiles using the parameters service precedence, reliability, delay and throughput. The service precedence is the priority of a service in relation to another service. There exist three levels of priority: High, Normal and Low. The reliability indicates the transition characteristics required by an application. Three reliability classes are defined, which indicates the maximum values for the probability of loss, duplication, mis-sequencing, and corruption (an undetected error) of packets. The delay parameter defines the maximum value of the mean delay. The delay can be defined as the end-to-end transfer time between two communicating mobile stations or between a mobile station and the Gi interface to an external network. This includes all delay within the GPRS network e.g., the delay for request and assignment of radio resources and the transit delay in the GPRS backbone network. The throughput specifies the maximum/peak mean bit rate. Using these QoS classes, QoS profiles can be negotiated between the mobile user and the network for each session, depending on the QoS demand and the current available resources. In order to get the QoS requirements, some radio resources planning is developed. It can play with items such as the time slots inside a channel, the frequency, coding schemes, antenna characteristics, transmitted power,... 26

27 Radio network inherently suffer from relatively high bit error rate, which can only be improved for delay sensitive traffic through the use of forward error correction (FEC) techniques; retransmission technique must not be used because they add unacceptable delay. GPRS has four FEC channel coding strategies (CS1 to CS4) for real-time traffic, and an ARQ mechanism for data transmission. As per the simulation result discussed in technical paper based on two environments- urban pedestrian and vehicular and calculated Frame error rate. The result showed that interference was the main source of errors, with problem being greatest at cell boundaries. Frequency hopping (FH) can be used in GSM to improve the multipath error performance, but the evidence suggest that benefits decrease as the code rate of the coding scheme (CS) increases, in CS4, Frequency Hopping increases the block error rate and decrease throughput. The baseband bit rate of the GPRS coding scheme vary from 9.05kbps for CS1 to 21.4kbps for CS4, per allocated time slot. The evolution of second-generation mobile communication systems to wireless networks supporting packet data transmission (e.g.gprs of GSM) brings an additional challenge for the operation of adaptive antenna at the base station. Uplink reception gives current user and interference situation. That can be dealt by positioning the antennas main beam into the direction of the desired user and that can suppress interference by steering null towards the interference. Whereas the downlink scenario is totally different: the user served on the same timeslot and frequency in neighboring cell may change from uplink to downlink in packet switched data transmission. Thus, null steering for these interferers is impossible because the serving BS is unaware of the dynamic packet channel allocation in the distant cells using the same frequency. As a consequence, no interference rejection improvement due to null steering is possible compared to single beam point. Moreover, the beam pointing gain for the desired user can be slightly reduced. As situation worsens with increasing angular spread (AS) of the mobile radio channel caused by rich multipath propagation. In such propagation conditions broad null become necessary to fully exploit the interference rejection capabilities of the adaptive antenna array. But these broad nulls also reduce the beam forming gain significantly. In the next chapter, only some of these radio resources will be studied in detail to elaborate and implement the strategies in the scenarios to simulate. 27

28 Chapter 3 Radio Resource Management 3.1 Radio Resource Management: Open Issues A list of RRM schemes open issues is given below: Currently, the RRM scheme does not take QoS parameters into account. Commercial versions of the GPRS offer the best effort option only. The relation between user/service QoS parameters and networkrelated ones has to be investigated. Session Admission control (SAC) Algorithm In addition, the current versions of the GPRS do not employ a SAC algorithm that takes into account the loading conditions of the radio air interface. PDCH selection to be preempted by incoming circuit-switched call during congestions. Criteria must be defined for the selection of the proper PDCH to be preempted, such as the number of assigned USFs per PDCH.e mechanism that select that selects the proper PDCHs during the activity periods of a data session so as to satisfy the QoS Negotiated parameters. An efficient RRM scheme will be capable of prioritizing service requests and assigning the proper number of PDCHs as well as those that can provide the maximum throughput so as to respect the QoS Negotiated parameter value. Allocation of the GSM an GPRS slots/pdchs onto the TRXs. One of the main RRM issues is a allocation of timeslots/pdchs to circuit- and packet-switched traffic. In contemporary GSM, slot allocation is based 28

29 on the low interference criterion, that is, an incoming circuit-switched call is assigned to the slot exhibiting the lowest interference level. For the combined GSM/GPRS case, the following two alternatives can be envisaged: Apply the low interference criterion for GPRS traffic too. The GPRS data sessions reserve slot/pdchs as GSM calls, based on low interference. However, in this case, the scheme shall take into account that a GPRS session may need to reserve more than one PDCHs ( in the same TRX). Unavailability of PDCHs in a single carrier may lead to performance degradation- due to assignment of lower number of PDCHs than requested/needed for a single session although there may be available resources in the system. Consecutive slot allocation for both GSM and GPRS. According to this scheme, the system assign consecutive slots to the GSM and GPRS requests starting from different carrier. The major advantage of this scheme is that the GPRS performance is not compromised. On the contrary, it may lead to increase in the intracell handovers (Under high loading conditions). 3.2 GPRS Coding Schemes An important feature of GPRS is the presence of four coding schemes (CS) with different levels of resistance to transmission problems. So the throughput will be determined by the choice among the four coding schemes accordingly to the changes in the channel conditions.the most important variables that affects this choice are the channel quality as measured by carrier to interference ratio (C/I), the packet size and channel fading characteristics. Packets from higher protocol layers are segmented and passed down to the LLC layer as the payload of LLC frames. The maximum frame length is 1520 Bytes. The LLC frames are then segmented and passed down to RLC layer as RLC block payload with variable length depending on the coding scheme. Every coded RLC block has a fixed length of 456 bits. Each block is segmented into four bursts of length 114 bits and transmitted in 4 consecutive frames in the physical layer. The payload of each RLC block is added with RLC/MAC header and a block check sequence. For CS1, CS2 andcs3, the radio block is convolutional coded with code rate equal to 1/2, then punctured to the desired code rate. 29

30 CS4 does not contain forward error correction. The service data rate is calculated from the 240 ms duration of a GPRS superframe. The superframe consists of 52 GSM frames including 48 traffic frames and 4 control frames. The 48 traffic frames carry 12 RLC blocks. Therefore the data rate is P ayload bytes/block 12blocks/superframe 8bits/byte = 0.4 P ayload bytes(kbps) 240ms/superframe Among the 4 coding schemes, CS1 has the lowest coding rate, it is also the most robust coding scheme. Therefore, CS1 is used for all control messages. CS2 and CS3 have lower code rate and more information bits. CS4 is the most data efficient coding scheme and is the most vulnerable to channel impairment. The Base Station Subsystem can choose any of the coding schemes to optimize throughput and delay performance. The characteristics of each type are in the following table: CHANNEL CODING CS-1 CS-2 CS-3 CS-4 SCHEMES Pre-cod.USF Info bits without USF Parity bits BC Tail bits Output Conv Encoder Punctured bits Code Rate 1/2 2/3 3/4 1 Data Rate (kb/s) Maximum Data Speed with 8 TS 72.4(kb/s) 107.2(kb/s) 124.8(kb/s) 171.2(kb/s) 3.3 GSM Coding Speech Encoding: RPE-LPC GSM uses a method called RPE-LPC (Regular Pulse Excited - Linear Predictive Coder with a Long Term Predictor Loop) to turn our analog voice into a compressed digital equivalent. The electrical variations induced into the microphone are sampled and each sample is then converted into a digital code. The voice waveform is then sampled at a rate of 8 khz. Each sample is then converted into an 8 bit binary number representing 256 distinct values. Since we sample 8000 times 30

31 per second and each sample is 8 binary bits, we have a bitrate of 8kHz X 8 bits = 64kbps. This bitrate is unrealistic to transmit across a radio network since interference will likely ruin the transmitted waveform. GSM speech encoding works to compress the speech waveform into a sample that results in a lower bitrate using RPE-LPC. A LPC encoder fits a given speech signal against a set of vocal characteristics. The best-fit parameters are transmitted and used by the decoder to generate synthetic speech that is similar to the original. Information from previous samples is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps Channel Encoding Once we have a compressed digital signal, we must add a number of bits for error control to protect the signal from interference. These bits are called redundancy bits. The GSM system uses convolutional encoding to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below. 1. Bit Composition of the Speech Signal Recall that the RPE-LPC Encoder produces a block of 260 bits every 20 ms. It was found (though testing) that some of the 260 bits were more important when compared to others. Below is the composition of these 260 bits. Class Ia - 50 bits (most sensitive to bit errors) Class Ib bits (moderately sensitive to bit errors) Class II - 78 bits (least sensitive to error) 2. Channel coding As a result of some bits being more important than others, GSM adds redundancy bits to each of the three Classes differently. The 50 Class IA bits are encoded in a cyclic encoder (that adds 3 parity bits). The 132 Class Ib bits (together with the 53 encoded Class IA bits and 4 trailing zero bits) are encoded using convolutional encoding with rate 1/2 (that produce a new sequence of 378 bits). Finally, the Class II bits are merely added to the result of the convolutional encoder. The channel encoded bit sequence is now 456 bits long. Therefore, each 20 ms burst produces 456 bits at a bit rate of 22.8 kbps. 31