WCDMA Link Budget

Link budget planning is part of the network planning process, which helps to dimension the required coverage, capacity and quality of service requirement in the network. UMTS WCDMA macro cell coverage is uplink limited, because mobiles power level is limited to (voice terminal 125mW). Downlink direction limits the available capacity of the cell, as BTS transmission power (typically 20-40W) has to be divided to all users. In a network environment both coverage and capacity are interlinked by interference. So by improving one side of the equation would decrease the other side. System is loosely balanced by design. The object of the link budget design is to calculate maximum cell size under given criteria:

Type of service (data type and speed) Type of environment (terrain, building penetration) Behavior and type of mobile (speed, max power level) System configuration (BTS antennas, BTS power, cable losses, handover gain) Required coverage probability Financial and economical factors (use of more expensive and better quality equipment or not the cheapest installation method) and to match all of those to the required system coverage, capacity and quality needs with each area and service. In an urban area, capacity will be the limiting factor, so inner city cells will be dimensioned by required Erlangs/km² for voice and data. Even using 25dB as inbuilding penetration loss into the building core area, link budget would typically allow about 300m cell range, which is a way too much for a capacity purposes. In a rural area uplink power budget will determine the maximum cell range, when typically cells are less congested. A typical cell range in rural areas will be several kilometers depending on a terrain. Below is an example of how WCDMA voice call link budget can be done. Some of the values can be debated, including the propagation model, but it gives an idea of the calculation methods. UMTS link budget

Overview UMTS

1. 3G Systems

3G Systems are intended to provide a global mobility with wide range of services including telephony, paging, messaging, Internet and broadband data. International Telecommunication Union (ITU) started the process of defining the standard for third generation systems, referred to as International Mobile Telecommunications 2000 (IMT-2000). In Europe European Telecommunications Standards Institute (ETSI) was responsible of UMTS standardisation process. In 1998 Third Generation Partnership Project (3GPP) was formed to continue the technical specification work. 3GPP has five main UMTS standardisation areas: Radio Access Network, Core Network, Terminals, Services and System Aspects and GERAN.

3GPP Radio Access group is responsible of:

Radio Layer 1, 2 and 3 RR specification Iub, Iur and Iu Interfaces UTRAN Operation and Maintenance requirements BTS radio performance specification Conformance test specification for testing of radio aspects of base stations Specifications for radio performance aspects from the system point of view 3GPP Core Network group is responsible of: Mobility management, call connection control signalling between the user equipment and the core network. Core network signalling between the core network nodes. Definition of interworking functions between the core network and external networks. Packet related issues. Core network aspects of the lu interface and Operation and Maintenance requirements 3GPP Terminal group is responsible of: Service capability protocols Messaging Services end-to-end interworking USIM to Mobile Terminal interface Model/framework for terminal interfaces and services (application) execution Conformance test specifications of terminals, including radio aspects 3GPP Services and System Aspects group is responsible of: Definition of services and feature requirements. Development of service capabilities and service architecture for cellular, fixed and cordless applications. Charging and Accounting Network Management and Security Aspects Definition, evolution, and maintenance of overall architecture. Third Generation Partnership Project 2 (3GPP) was formed for technical development of cdma2000 technology which is a member of IMT-2000 family. In February 1992 World Radio Conference allocated frequencies for UMTS use. Frequencies 1885 - 2025 and 2110 - 2200 MHz were identified for IMT-2000 use. See the UMTS Frequency page for more details. All 3G standards are still under constant development. In 1999 ETSI Standardisation finished for UMTS Phase 1 (Release '99, version 3) and next release is due December 2001. UMTS History page has a list of all major 3G and UMTS milestones. Most of the European countries and some countries round the world have already issued UMTS licenses 2. UMTS Services UMTS offers teleservices (like speech or SMS) and bearer services, which provide the capability for information transfer between access points. It is possible to negotiate and renegotiate the characteristics of a bearer service at session or connection establishment and during ongoing session or connection. Both connection oriented and connectionless services are offered for Point-to-Point and Point-to-Multipoint communication. Bearer services have different QoS parameters for maximum transfer delay, delay variation and bit error rate. Offered data rate targets are: either by beauty contest or auctions. 144 kbits/s satellite and rural outdoor 384 kbits/s urban outdoor 2048 kbits/s indoor and low range outdoor UMTS network services have different QoS classes for four types of traffic: Conversational class (voice, video telephony, video gaming) Streaming class (multimedia, video on demand, webcast) Interactive class (web browsing, network gaming, database access) Background class (email, SMS, downloading) UMTS will also have a Virtual Home Environment (VHE). It is a concept for personal service environment portability across network boundaries and between terminals. Personal service environment means that users are consistently presented with the same personalised features, User Interface customisation and services in whatever network or terminal, wherever the user may be located. UMTS also has improved network security and location based services. 3. UMTS Architecture A UMTS network consist of three interacting domains; Core Network (CN), UMTS Terrestrial Radio Access Network (UTRAN) and User Equipment (UE). The main function of the core network is to provide switching, routing and transit for user traffic. Core network also contains the databases and network management functions. The basic Core Network architecture for UMTS is based on GSM network with GPRS. All equipment has to be modified for UMTS operation and services. The UTRAN provides the air interface access method for User Equipment. Base Station is referred as Node-B and control equipment for Node-B's is called Radio Network Controller (RNC). UMTS system page has an example, how UMTS network could be build. It is necessary for a network to know the approximate location in order to be able to page user equipment. Here is the list of system areas from largest to smallest. UMTS systems (including satellite) Public Land Mobile Network (PLMN) MSC/VLR or SGSN Location Area Routing Area (PS domain) UTRAN Registration Area (PS domain) Cell Sub cell 4. Core Network The Core Network is divided in circuit switched and packet switched domains. Some of the circuit switched elements are Mobile services Switching Centre (MSC), Visitor location register (VLR) and Gateway MSC. Packet switched elements are Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). Some network elements, like EIR, HLR, VLR and AUC are shared by both domains. The Asynchronous Transfer Mode (ATM) is defined for UMTS core transmission. ATM Adaptation Layer type 2 (AAL2) handles circuit switched connection and packet connection protocol AAL5 is designed for data delivery. The architecture of the Core Network may change when new services and features are introduced. Number Portability DataBase (NPDB) will be used to enable user to change the network while keeping their old phone number. Gateway Location Register (GLR) may be used to optimise the subscriber handling between network boundaries. MSC, VLR and SGSN can merge to become a UMTS MSC. 5. Radio Access Wide band CDMA technology was selected to for UTRAN air interface. UMTS WCDMA is a Direct Sequence CDMA system where user data is multiplied with quasi-random bits derived from WCDMA Spreading codes. In UMTS, in addition to channelisation, Codes are used for synchronisation and scrambling. WCDMA has two basic modes of operation: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). UTRAN interfaces are shown on UMTS Network page. The functions of Node-B are: Air interface Transmission / Reception Modulation / Demodulation CDMA Physical Channel coding Micro Diversity Error Handing Closed loop power control The functions of RNC are: Radio Resource Control Admission Control Channel Allocation Power Control Settings Handover Control Macro Diversity Ciphering Segmentation / Reassembly Broadcast Signalling Open Loop Power Control 6. User Equipment The UMTS standard does not restrict the functionality of the User Equipment in any way. Terminals work as an air interface counter part for Node-B and have many different types of identities. Most of these UMTS identity types are taken directly from GSM specifications. International Mobile Subscriber Identity (IMSI) Temporary Mobile Subscriber Identity (TMSI) Packet Temporary Mobile Subscriber Identity (P-TMSI) Temporary Logical Link Identity (TLLI) Mobile station ISDN (MSISDN) International Mobile Station Equipment Identity (IMEI) International Mobile Station Equipment Identity and Software Number (IMEISV) UMTS mobile station can operate in one of three modes of operation: PS/CS mode of operation: The MS is attached to both the PS domain and CS domain, and the MS is capable of simultaneously operating PS services and CS services. PS mode of operation: The MS is attached to the PS domain only and may only operate services of the PS domain. However, this does not prevent CS-like services to be offered over the PS domain (like VoIP). CS mode of operation: The MS is attached to the CS domain only and may only operate services of the CS domain. UMTS IC card has same physical characteristics as GSM SIM card. It has several functions: Support of one User Service Identity Module (USIM) application (optionally more that one) Support of one or more user profile on the USIM Update USIM specific information over the air Security functions User authentication Optional inclusion of payment methods Optional secure downloading of new applications

CDMA Overview

ACCESS SCHEMES

For radio systems there are two resources, frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.

Multiple Access Schemes

CODING

CDMA uses unique spreading codes to spread the baseband data before transmission. The signal is transmitted in a channel, which is below noise level. The receiver then uses a correlator to despread the wanted signal, which is passed through a narrow bandpass filter. Unwanted signals will not be despread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the baseband data. The rate of a spreading code is referred to as chip rate rather than bit rate.
See coding process page for more details.

CDMA spreading

CODES

CDMA codes are not required to provide call security, but create a uniqueness to enable call identification. Codes should not correlate to other codes or time shifted version of itself. Spreading codes are noise like pseudo-random codes, channel codes are designed for maximum separation from each other and cell identification codes are balanced not to correlate to other codes of itself.
See codes page for more details.

Example OVSF codes, used in channel coding

THE SPREADING PROCESS

WCDMA uses Direct Sequence spreading, where spreading process is done by directly combining the baseband information to high chip rate binary code. The Spreading Factor is the ratio of the chips (UMTS = 3.84Mchips/s) to baseband information rate. Spreading factors vary from 4 to 512 in FDD UMTS. Spreading process gain can in expressed in dBs (Spreading factor 128 = 21dB gain).
See spreading page for more details.

CDMA spreading

POWER CONTROL

CDMA is interference limited multiple access system. Because all users transmit on the same frequency, internal interference generated by the system is the most significant factor in determining system capacity and call quality. The transmit power for each user must be reduced to limit interference, however, the power should be enough to maintain the required Eb/No (signal to noise ratio) for a satisfactory call quality. Maximum capacity is achieved when Eb/No of every user is at the minimum level needed for the acceptable channel performance. As the MS moves around, the RF environment continuously changes due to fast and slow fading, external interference, shadowing , and other factors. The aim of the dynamic power control is to limit transmitted power on both the links while maintaining link quality under all conditions. Additional advantages are longer mobile battery life and longer life span of BTS power amplifiers
See UMTS power control page for more details.


HANDOVER

Handover occurs when a call has to be passed from one cell to another as the user moves between cells. In a traditional "hard" handover, the connection to the current cell is broken, and then the connection to the new cell is made. This is known as a "break-before-make" handover. Since all cells in CDMA use the same frequency, it is possible to make the connection to the new cell before leaving the current cell. This is known as a "make-before-break" or "soft" handover. Soft handovers require less power, which reduces interference and increases capacity. Mobile can be connected to more that two BTS the handover. "Softer" handover is a special case of soft handover where the radio links that are added and removed belong to the same Node B.
See Handover page for more details.

CDMA soft handover

MULTIPATH AND RAKE RECEIVERS

One of the main advantages of CDMA systems is the capability of using signals that arrive in the receivers with different time delays. This phenomenon is called multipath. FDMA and TDMA, which are narrow band systems, cannot discriminate between the multipath arrivals, and resort to equalization to mitigate the negative effects of multipath. Due to its wide bandwidth and rake receivers, CDMA uses the multipath signals and combines them to make an even stronger signal at the receivers. CDMA subscriber units use rake receivers. This is essentially a set of several receivers. One of the receivers (fingers) constantly searches for different multipaths and feeds the information to the other three fingers. Each finger then demodulates the signal corresponding to a strong multipath. The results are then combined together to make the signal stronger.

What is DMA (Code-Division Multiple Access)?

CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes, which enhances privacy and makes cloning difficult.

The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide roaming.

The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.