ETSI WIDEBAND CDMA STANDARD FOR THE UTRA FDD AIR INTERFACE.pdf

ETSI WIDEBAND CDMA STANDARD FOR THE UTRA FDD AIR INTERFACE

ETSI WIDEBAND CDMA STANDARD FOR THE UTRA FDD AIR

ETSI WIDEBAND CDMA STANDARD FOR THE UTRA FDD AIR

Nicky Jee-Ngai Yuen The University of British Columbia

ABSTRACT

“ETSI Wideband CDMA Standard for the UTRA FDD Air Interface” by Nicky Yuen

The core requirements for the IMT-2000 air interface technology to be used in Third Generation (3G) wireless systems are: 1) up to 2 Mbps data rate for local area coverage, 2) up to 384 kbps data rate for wide area coverage, 3) highly efficient utilization of the spectrum in contrast to current 1G and 2G systems, and 4) capability to support various multimedia information sources on an ongoing basis The radio access technology that was finally proposed in January of 1998 to meet these criteria was W-CDMA W-CDMA is based on Direct Sequence CDMA technology, with a chip rate of 4.096 Mcps It is designed to be flexible to accommodate third generation services as well as to be adaptable to current GSM systems This paper presents a background to 3G systems, some key features and technologies of the W-CDMA FDD air interface, and a description of the physical channel and frame structure

LIST OF ILLUSTRATIONS

Table 1 Key Parameters of W-CDMA 4

Table 2 Physical Channel Format 9

Figure 1 ITU Spectrum Allocation 3

Figure 2 Hierarchical Cell Structure for Smooth Handovers 6

Figure 3 Physical Channel Structure 9

Figure 4 Uplink DPDCH/DPCCH Structures 10

Figure 5 Uplink DPCH Spreading/Modulation 11

Figure 6 Random Access Scheme 12

Figure 7 Uplink PRACH Structure 12

Figure 8 Data Part of PRACH 13

Figure 9 Downlink Spreading/Modulation 14

Figure 10 Downlink DPCH 15

Figure 11 Primary and Secondary CCPCHs 16

Figure 12 Synchronization Channel 17

Figure 13 Test Route 18

Figure 14 Average BER Performance with Variable Chip Rate 20

LIST OF ABBREVIATIONS

BCCH Broadcast Control Channel BER Bit Error Rate BPSK Binary Phase Shift Keying BTS Base Transceiver Station CDMA Code Division Multiple Access CCPCH Common Control Physical Channel DLPCH Downlink Physical Channel DPCCH Dedicated Physical Control Channel DPCH Dedicated Physical Channel DPDCH Dedicated Physical Data Channel DS-CDMA Direct Sequence CDMA ETSI European Telecommunications Standards Institute FACH Forward Access Channel FDD Frequency Division Duplex FPLMTS Future Public Land Mobile Telecommunications System HCS Hierarchical Cell Structure IMT-2000 International Mobile Telecommunications – 2000 ITU International Telecommunications Union PRACH Physical Random Access Channel PSC Primary Synchronization Code SCH Synchronization Channel SSC Secondary Synchronization Code TD-CDMA Time Division CDMA TDD Time Division Duplex TFI Transfer Format Indicator TPC Transmit Power Control ULPCH Uplink Physical Channel UMTS Universal Mobile Telecommunications System UTRA UMTS Terrestrial Radio Access W-CDMA Wideband Code Division Multiple Access

2.0 UMTS TERRESTRIAL RADIO ACCESS 3

2.1 Significant Features of W-CDMA 3

2.2.5 Adaptive Antenna Arrays 7

2.2.6 Spreading and Scrambling Codes 7

2.2.7 Packet Access 8

3.0 PHYSICAL CHANNEL STRUCTURES 9

3.1 Physical Channel Format 9

1.0 INTRODUCTION

The capability to access vast amounts of data, as well as the growing need for user mobility, has been fueled by advances in wireless personal telecommunications The need to efficiently maximize the traffic capability of the available bandwidth and to support high-speed data for multimedia services has been the driving forces for developments in third generation, or 3G, infrastructures Perhaps a more subtle reason for the need for 3G is the growing necessity to be able to communicate and access information “Anywhere – Anyplace”

International Mobile Telecommunications in the year 2000 (IMT-2000) is the International Telecommunications Union’s (ITU’s) vision of global wireless access in the 21st century IMT-2000 is the new name for 3G mobile systems, which replaced the former name of “Future Public Land Mobile Telecommunications Systems” (FPLMTS) FPLMTS was targeted at developing the mobile telecommunications system including the air interface and infrastructure

The main requirements of the IMT-2000 air interface are:

1 full coverage and mobility at 144 kbps in a macrocell (i.e in large areas such as a city); mobile (vehicular),

2 moderate coverage at 384 kbps in a microcell (i.e a few square kilometers); portable (pedestrian),

3 limited coverage at up to 2 Mbps in a picocell; fixed, 4 high spectrum efficiency compared to existing systems, and

5 high flexibility to introduce and multiplex new services at different bit rates and Eb/N0 requirements

One of the more popular technologies being developed for IMT-2000 is known as wideband CDMA It has two versions: cdma2000 and W-CDMA Both versions differ in chip rate, downlink channel structure, and network synchronization, but they both promise wireless voice at much higher capacity and lower cost then current 2G and 2.5G systems The latter version, which is the focus of this paper, was proposed in January 1998 by ETSI, the European Telecommunications Standards Institute, as their proposal to the ITU for the Frequency Division Duplex (FDD) spectrum of IMT-2000 ETSI’s proposal is identified as UTRA, UMTS Terrestrial Radio Access

2.0 UMTS TERRESTRIAL RADIO ACCESS

The ETSI UTRA is based on a wideband, 5 MHz, 4.096 Mcps Direct Sequence (DS) CDMA technology The UTRA Network, or UTRAN, is connected to an evolved GSM core network to provide both circuit switched and packet switched services The chip rate is extendible to higher rates of 8.192 Mcps and 16.384 Mcps W-CDMA was selected for FDD (paired UMTS frequency band) operation, and time-division CDMA (TD/CDMA) was selected for TDD (unpaired UMTS frequency band) operation The selection makes UTRA efficiently cover all operation scenarios and makes full utilization of the UMTS spectrum allocation

For paired bands, the spectrum will be in the 1920 – 1980 and 2110 – 2170 MHz ranges For the unpaired band, a total of 35 MHz will be allocated A spectrum identification has been made to identify the parts of the 2 GHz band for IMT-2000 operation

Figure 1 ITU Spectrum Allocation

2.1 Significant Features of W-CDMA

The main features which make W-CDMA a promising air interface for 3G systems include: 1) improved performance over second-generation systems; improved capacity and

coverage,

2) flexible service implementation and support of multiplexing parallel services on a single physical connection,

3) support of a wide range of services at up to 2 Mbps,

4) two packet data operation modes: dedicated and common channel packet operation, 5) fast, efficient packet access,

6) support of intra-frequency, inter-frequency, GSM-UTRA handoffs,

7) support for enhanced capabilities such as adaptive antenna arrays, multiuser detection, and interference cancellation,

8) asynchronous base transceiver station (BTS) operation, and 9) fast transmit power control (TPC) in both directions

The following table lists the key parameters of W-CDMA Multiple-access scheme DS-CDMA

Chip rate 4.096 / 8.192 / 16.384 Mcps

Carrier spacing (4.096 Mcps) Flexible in the range 4.4 – 5.2 MHz (200 kHz carrier raster) Roll-off factor 0.22

Frame length 10 ms (20 ms optional)

Inter-cell synchronization Asynchronous; no synchronization needed (synchronous also

Coherent detection User-dedicated time-multiplexed pilot (uplink and downlink), Common pilot can also be used in downlink

Multirate Variable -spreading and multicode Spreading factor (ratio of chip

rate over information rate)

4 – 256 (4.096 Mcps)

Channel coding Convolutional coding (R = 1/3 or 1/2, K = 9), Turbo code Spreading Spreading code and Scrambling code

Spreading code (downlink) Variable -length orthogonal sequences for channel separation, Gold sequences for cell and user separation

Spreading code (uplink) Variable -length orthogonal sequences for channel separation, Gold sequence 241 for user separation

Packet access Dual mode (common and dedicated channel)

Interfrequency handover

Table 1 Key Parameters of W-CDMA

2.2 Performance Capabilities

Some of the technical benefits of W-CDMA which greatly enhance its performance include the following:

2.2.1 Capacity

The wide bandwidth of W-CDMA offers an improvement in the performance over previous cellular systems, since fading of the radio signal is reduced [9] W-CDMA RF transceivers can accommodate 8 times more voice users than narrowband transceivers Each RF carrier can handle 100 simultaneous voice calls, or 50 simultaneous data calls The capacity of W-CDMA is about double that of narrowband CDMA in urban and suburban environments [8] An operator will be able to accommodate at least 192 voice calls per sector, compared to about 100 voice calls per sector for GSM In addition, coherent demodulation on the uplink, which is a feature not previously implemented in earlier cellular systems, combined with fast power control on the downlink, hierarchical cell structuring, and adaptive antenna arrays all improve performance and reduce receiver threshold, especially in indoor and low-speed outdoor environments at low Doppler Cumulatively, these factors theoretically improve cell capacity by at least 3 dB (or a factor of 2)

2.2.2 Coverage and Link Budget

Coverage of W-CDMA is determined by the link performance through the link budget With W-CDMA, it is possible to reuse GSM1800 cell sites when moving from the GSM to W-CDMA infrastructure, since the latter uses a similar network protocol structure W-CDMA voice service will tolerate a few dB higher path loss than GSM This means that W-CDMA gives better voice coverage than GSM when reusing the same cell sites at the same frequency band

2.2.3 Asynchronous BTS Operation

W-CDMA does not require tight inter-BTS synchronization as in the case of narrowband CDMA This means that there is no requirement that each BTS should be capable of reliable GPS reception for external timing This reduces deployment efforts significantly, especially in indoor environments [9]

2.2.4 Handovers

Seamless interfrequency handovers through a downlink slotted mode is a remarkable feature of W-CDMA It is necessary for the support of hierarchical cell structures (HCS) which consists of overlaying macrocells on top of microcells and picocells to attain higher capacity Cells from different layers will be in different frequencies; therefore, interfrequency handovers are required

Figure 2 Hierarchical Cell Structure for Smooth Handovers

With the introduction of HCS, a cellular system can provide high system capacity through the microcell layer, at the same time offering full coverage and support of high mobility by the macrolayer [9] Interfrequency handover is then required for a handover between different cell layers

‘Hot-Spot’ scenarios also require the use of interfrequency handover In a hot-spot scenario, a certain cell that serves a high traffic area uses carriers in addition to those used by adjacent cells If deployment of extra carriers is to be confined to the actual hot spot area, then the possibility of interfrequency handover is necessary

2.2.5 Adaptive Antenna Arrays

In a W-CDMA system, packet and circuit switched services can be interleaved, with variable bandwidths, and delivered simultaneously to a single user, each with different qualities of service Each W-CDMA terminal can access several different services such as Internet, e-mail, or video at the same time

2.2.6 Spreading and Scrambling Codes

The spreading, or channelization, codes used in W-CDMA systems are Orthogonal Variable Spreading Factor (OVSF) codes In both the uplink and downlink, OVSF codes are used for channelization OVSF codes have the characteristic that preserves uplink and downlink transmit orthogonality between different physical channels (and hence different users) even if different spreading factors (ratio of chip rate over information rate, also known as the processing gain), with different rates, are used The use of OVSF codes greatly enhances service flexibility by being able to change bit rates in response to user demand Details of OVSF codes can be found from the literature listed in the References section

The downlink scrambling code is a pseudo-random noise (PN) sequence of length 40960 chips (10 ms) There are a total of 512 different variations of scrambling codes in the system For efficient cell search, the downlink scrambling codes are divided into 32 groups of 16 codes each On the uplink, the scrambling code is usually a PN sequence of length 40960 chips (10 ms) as on the downlink; however,

an alternative to this is a short (256 chips) Very-Large (VL) Kasami code to support low-complexity multi-user detection in the base station

2.2.7 Packet Access

In W-CDMA systems, high bit-rate services will mainly be packet-oriented, with efficient access to the Internet and IP-based services The two different types of packet data transmission modes are: common channel packet transmission, and dedicated channel packet transmission

In the former method, short data packets are appended directly to a random access burst 10 ms long Generally, this method is used for short, infrequent packets, where the overhead resulting from the link maintenance of a dedicated channel would be unacceptable In the latter method, larger and more frequent packets are transmitted using a single packet scheme where the dedicated channel is immediately released following a packet transmission The dedicated channel is maintained for the transmission duration by transmitting power control and synchronization information between subsequent packets

3.0 PHYSICAL CHANNEL STRUCTURES

The physical channel (PCH) in a W-CDMA system encompasses all aspects of transmission over the air and is defined by the frequency and code The physical channel can be classified into an uplink physical channel (ULPCH) and a downlink physical channel (DLPCH) Each of these can be further broken down into constituent channels The overall physical channel structure is shown in Figure 3

Figure 3 Physical Channel Structure 3.1 Physical Channel Format

The following table indicates the various parameters of the physical channel Spreading Code Chip Rate 4.096 Mcps (higher chip rates also available) Variable Spreading Factor 256, 128, 64, 32, 16, 8, 4

Data Rates 16, 32, 64, 128, 256, 512, 1024 kbps

Frame Duration 10 ms (16 slots) Number of Chips Per Slot 2560

Number of Symbols Per Slot 10, 20, 40, 80, 160, 320, 640

*Different channel-specific variations permitted, but only applies to primary CCPCH, SCH, and RACH

3.2 Uplink

The ULPCH can be further classified into a dedicated physical channel (DPCH) and a common control physical channel (CCPCH)

3.2.1 DPCH

In the uplink, the DPCH consists of a separate dedicated physical data channel (DPDCH) and a separate dedicated physical control channel (DPCCH) as shown in

Figure 4 Uplink DPDCH/DPCCH Structures

The uplink dedicated physical channel is divided into the DPDCH and DPCCH The DPDCH carries data generated at layer 2 (Data Link layer) and above The DPCCH carries the following control info:

1) known pilot bits for interference mitigation, channel estimation, and coherent detection,