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WCDMA Generation System

WCDMA transmission is spreading all over the world very fast. Here our objective is to discuss the basic operation of the physical layer of OSI model in the WCDMA (Wideband Code Division Multiple Access) transmission. WCDMA is an advanced version of CDMA, and is one of the 3^rd generation technologies with a little bit of extra facilities in the technology. Before describing the maim topic we have to know about the basic wireless technologies which lead to the WCDMA.

Generations of Communication Technologies:-

Up till now the technologies are categorized in three generations which are commonly available in the modern world. Now a days a new generation of telecommunication technologies is also being introduced which is known as 4^th generation (or simply 4G) technology. An overview of 1^st three generations is given below.

1^st Generation Systems:-

1^st generation (or 1G) systems were introduced in 1980. These systems were not efficient spectrally and were much insecure. Facility of roaming was not present in 1G system. [1]

2^nd Generation Systems:-

In 2^nd generation systems (2G systems), digital signals were used. These systems were more efficient than previous systems. Messaging service was introduced in addition to voice services in these systems. These systems also offered encrypted transmission. GSM (Global Systems for Mobile) is an example of 2G systems. [1]

3^rd Generation systems:-

3G systems were introduced in late 90s or start 2000s. These systems can expectedly handle broadband data and can also transmit videos over a channel. These systems use a spectrum of almost 2GHz. [1]

3G systems differ from others by their air interface and the spectrum used in it. Initially these systems were classified by UMTS and CDMA 2000. Both of these systems were tightly integrated and can define the whole system. Although the air interface of both systems was different but the services provided were same. [1]

3G systems can be classified into three types.

The air interface

Radio Access Network

Core network

The air interface is basically radio-hop from terminal to base station and core network links the router and extends the gateway. Whereas, Radio Access Network works as glue between the core network and base station and sometimes it allows the terminal mobility. [1]

History of 3G systems:-

Initially, 3G was conceived as a single standard available world wide, and called as FLMTS (Future Land Mobile Telecommunication System) by ITU. Later it consists of five standards which were collectively called IMT-2000 family of standards. After the ending phase of ITU in 1998.3GPP and 3GPP2 were two standards of 3G that are being deployed currently, which are also named as UMTS and CDMA-2000 respectively.3G can be divided into three phases.[1]

The Research Trimester

The IMT-2000 Trimester

The Standardization Trimester

Multiple Access Schemes:-

UMTS WCDMA FDD is a direct sequence CDMA system having a bandwidth of 5 MHz. UMTS WCDMA TDD also has a bandwidth of 5MHz, but now the frequency band is shared in both directions (WCDMA TDD and WCDMA FDD). In one half of the time it is used for forward direction transmission and for reverse direction transmission in other half. CDMA 2000 is an example of direct sequence CDMA FDD systems. In this system although its each carrier uses a bandwidth of 1.25 MHz but the total bandwidth is still 5MHz. Another system that is supported by IMT-2000 is based upon TDMA scheme, in which each physical channel is divided into a number of fixed, synchronized time slots. Here each user can transmit its data in its given time slot only. These multiple access techniques are very old in communication systems. Even the code division multiple access has found its applications in military systems for a long time. Some other techniques were also used in conventional and wireless local area networks [2] - [9]

Some multiple access techniques are given below.

Frequency Division Multiple Accesses (FDMA):-

In this scheme the available spectrum is divided into smaller bands of equal bandwidth. Each band is assigned to a different user for a particular duration. To have error free environment a guard band is present between two successive bands, so that the information of one user may not be mixed up with the other one. [2]

Time Division Multiple Access {TDMA):-

In TDMA the information of each user is sent in fixed time slot. Any given user is assigned one or more slots from them. In TDMA a user can transmit its data in its given time slot only. To avoid mixing up information of one user from other, a guard period is present between each time slot. [2]

Spread Spectrum Multiple Access:-

As we know that each user in TDMA and FDMA is assigned a particular slot on the spectrum. All users can transmit their data simultaneously on the entire bandwidth by using a PN Code. These codes repeat after a finite but long interval of time. The receiver separates the data of each use by correlating the received signal and these codes. [2]

CDMA Technology:-

In CDMA, each user is assigned a different code and then its data is being modulated and sent. At receiver's end, same process is applicable. First the data is being modulated and then it is extracted with the help of those codes assigned to it.

Direct-Spread CDMA Principles:-

PN codes have some unique properties. One of them is that any channel or application, when spread by a PN code at transmitter's end can be identified at receiver by multiplying the baseband signal with a coherent copy of that PN Code. Suppose that a user wants to detect a data from transmitter T_x. At receiver the signal is 1^st demodulated by a demodulator and then it is multiplied by that PN code, which is assigned to transmitter T_x. After that the signal is passed through an integrator and then a decoder that reads the output of integrator and decodes it into binary, following some rules. [2]

WCDMA:-

WCDMA and CDMA 2000 have many features in common, whereas many of them are different too. The largest difference is that the CDMA 2000 was developed by adding advancement in previous technology, while WCDMA was developed without having any previous concept in mind. [10]

History of WCDMA:-

In 1990s, wireless executives started to work in upgrading 2G networks and providing efficient services in it. In 1992 World Administrative Radio Conference (WARC) began its work to define new spectrum for this soul purpose. During the same time period, ITU's (International Telecommunication Union) scientists focused on standardization and defining the contents of the 3G systems. This group was to start IMT 2000(International Mobile Telecommunication Systems 2000) systems, which were also known as UMTS systems. These systems define wireless mobility systems allowing high speed data communication. A global committee was made to coordinate this purpose, knows as 3GPP(Third Generation Partnership Program). This program was setup to ensure that CDMA 2000 standard must meet IMT 2000 standard, alongside the original partnership that was working on what must become the WCDMA systems. Finally the first deployment of WCDMA systems has taken place in Japan. [10]

WCDMA Specifications:-

The WCDMA systems have some specifications which are different from 2G and other 3G systems too. These systems use a bandwidth of 5MHz which is the same as that of CDMA systems, but it uses two duplex modes which are TDD (Time Division Duplex) and FDD (Frequency Division Duplex). The spectrum used in WCDMA is a spread spectrum having a chip rate of 3.84 Mbps and its frame length is 10 ms. The channel encoding in this system is convolutional type, the handover type used in WCDMA are the soft handovers and interfrequency handovers. [11]

WCDMA Basics:-

The user data in DS-CDMA, is spread over a wider bandwidth through multiplication by a pseudo-random bits sequence which is called chip. It is spread over a signal having a higher bit rate. Transmitted signal has pseudo-random characteristics. The spread spectrum looks like noise when it is transmitted over a radio interface. According to a different spreading code, each user data stream is spread. At receiver's end, the data stream is recovered by dispreading the set of received signal with respect to appropriate spreading code. If the number of users keeps on increasing then the possibility of recovering a specific user's data will keep on decreasing. [12]

The ratio of the chip rate to the user's data is called spreading factor. Also the higher the spreading factor, the greater will be the capability to recover a user's data. [12]

Air Interface Technologies for Third Generation Systems:-

For most appropriate multiple access technology for 3^rd generation systems, a number of multiple access schemes were introduced. Some of which used by 3G systems are Wideband CDMA (WCDMA), EDGE and WCDMA TDD. OFDM is also included in the air interface of 3G systems. [11]

The WCDMA Air Interface:-

As WCDMA air interface uses FDD, so it is clear that it has a different carrier frequency for uplink and downlink. The frequency range is dependent upon local regulations, but it has a common spectral allocation of 1920-1982 MHz for uplink and 2110-2170 MHz for downlink. The uplink and the downlink band are spited into channels of 5 MHz. widths. Whereas, the frequency spacing between both the uplink and downlink is 190 MHz. Because of the difference between frequencies of uplink and downlink transmission, the short- term channel quality of uplink and downlink is not correlated. [13]

Both the uplink and downlink transmissions are organized in time frames having duration of 10ms. As the chip rate is 3.84 Mcps, a frame contains 38400 chips. Each frame is then further divided into 15 time slots, or power control periods, of length 2560 chips each. [13]

The spreading spectrum of WCDMA varies from 4 to 256, allowing transmitted symbol rates between 960 K symbols and 15 K symbols/s on a single code. [13]

As WCDMA uses a chip rate of 3.84 Mcps. We also know that it uses a specific code to spread data of a specific user on the spectrum. In reality, however, there are multiple simultaneous data streams from multiple users and also multiple data streams from a single base-station, so not only this is necessary to separate data streams of one user of base station, but also it is important to separate data streams which a single user can generate. [12]

The air interface basically works on physical layer.

Physical Layer Procedures:-

A part from basic operations of a channel, some physical layer procedures are given below.

The cell search procedure carried out by the mobile to synchronize with the network

The paging procedure applied by the network to locate a particular UE

The RACH procedure for transmitting UL data on the PRACH

The transmit diversity procedure

Several channel synchronization and reconfiguration procedure. [14]

WCDMA Air Interface: Physical Layer

Before examining the WCDMA air interface, we have to study the OSI protocol model of WCDMA air interface. Air Interface protocol model is depicted by the Figure 1. [15]

Session Management

GPRS Short Messaging Service Support

Call Control

Supplementary Services

Short Messaging Service

Layer 3

Layer 2

Layer 1

Mobility Management

GPRS Mobility Management

Primary Reference Clock

Packet Data Convergence Protocol

Medium Access Control

Radio Link Control

Broadcast Multicast Control

Physical Layer Device

Figure 1 Air Interface Protocol Model. [15]

In air interface, physical layer is the lower most layer of UMTS radio interface. This layer is responsible for the transmission of data over physical channels. Physical layer have to perform several operations which are inter-related to each other. [16] Some of the basic operations of Physical layer in WCDMA air interface are discussed afterwards.

Forward Error Correction Encoding/Decoding:-

Besides transmission of data, physical layer is also responsible for correcting errors, which occur during the transmission. For correction of these errors in transmission, physical layer uses FEC (Forward Error Correction) scheme. Three other schemes are also available which are Convolutional coding, Turbo coding and no FEC coding respectively. The main principle behind FEC scheme is to add redundancy in transmitted signal (bit stream), so that the receiver can correct occasional bit errors while transmission procedure is active. Normally, Convolutional coding is more suitable for low data rates, while turbo coding for higher rates. FEC coding is also referred as channel coding. [16]

In UTRAN, channel coding and CRC error detection function are combined together to form hybrid ARQ scheme. It means that channel coding fixes as many errors as possible and then error-detection function checks the correction of the result. A large number of packets are detected and indicated to high layers for retransmission. [15]

Error Detection on Transport Channel:-

Error detection is used to make sure whether the block of data is received correctly or not. Error detection is done on transport block by using a cyclic redundancy check (CRC). There are five CRC polynomial lengths in use (0, 8, 12, 16 and 24 bits), the higher layers choose that the polynomial of which length is to be selected. [15]

The transmitter calculates CRC checksum over the whole message and attaches it to the end of the message. At receiver's end, the receiver checks whether the received CRC is matched with the CRC of the received message. [15]

Multiplexing of Transport Channel and Demultiplexing of Coded Composite Transport Channels:-

Each UE (User Equipment) can use several transport channels simultaneously. Every 10 ms, one radio frame from each channel is multiplexed into a Coded Composite Transport Channel (CCTrCH). This is a serial multiplexing. [15]

There can be more than one CCTrCH per channel. In FDD mode, there is only one CCTrCH on the uplink, while there are more than one CCTrCH in TDD. On the downlink there may be several CCTrCH per channel. Each CCTrCH can have its own C/I requirements to provide different quality of services (QoS) on mapped transport channel. [15]

Modulation and Demodulation/Spreading of physical Channels:-

Modulation:-

“Modulation is a process of conditioning a transmitted waveform in order to convey the information message to the receiver.”[17]

Demodulation:-

“Demodulation is a process of extracting the message from conditional signal.”[17]

A physical channel can be considered of a combination of frequency, channelization code and scrambling code, relative phase is also significant in uplink. [12]

A number of different channels are used in uplink. A channel is selected according to the requirement of UE as to what it should do. For example, to send a data bit by bit or to send a stream of data. [12]

Normally a terminal uses at least two physical channels which are given below.

Dedicated Physical Data Channel (DPDCH)

Dedicated Physical Control Channel (DPCCH)

The DPDCH uses the user data. Depending on amount of data, a user may use a single DPDCH and as much as six DPDCH. A single DPDCH can support up to 480 Kbps, while six DPDCH can support a data of 2.3 Mbps. A DPDCH can have a variable spreading factor. DPDCH can be 4, 8,16,32,64,128 or 256 corresponding to bit rate of 15 Kbps (3.84 *10⁶/256 = 15 * 10³) up to 960 Kbps (3.84 * 10⁶/5 = 960 * 10³). [12]

The DPCCH controls the user information and uses channelization code. Each DPCCH is spread to the chip rate by a channelization code. A user may use DPCCH according to the DPCCH used in the physical channel. [12] DPCCH and DPDCH are shown in Figure 2

Chip Rate

Gain ß_d

C_d6

C_d1

Channelization Code

DPDCH₆

DPDCH₁

∑

DPDCH₄

DPDCH₂

DPDCH₅

DPDCH₃

C_d4

C_d2

C_d5

C_d3

Gain ß_d

Chip Rate

Scrambling Code

S_DPDCHn

Figure 2. Uplink Channelization and Demodulation/Scrambling. [12]

The modulation and demodulation is performed both on uplink and downlink. So we have to examine it accordingly.

Uplink Spreading/Demodulation and Modulation:-

Uplink Demodulation:-

After the spreading of different channels by channelization codes, they are combined by the appropriate scrambling code. According to the type of physical channel, there are two types of scrambling codes used with 2²⁴ possibilities of each type. The long or short scrambling code is used according to the higher layers. The scrambling codes are pseudo-random and hence they should satisfy all properties of pseudo random sequence. [12]

A pseudo-random sequence is generated by a linear shift of feedback register. The long scrambling codes are also known as the Gold Codes, and are generated by the modulo 2 addition of portions of two binary sequence. [12]

Uplink Modulation:-

In uplink, WCDMA uses QPSK (Quadrature Phase Shift Keying) modulation. The stream of spread and scrambled signals forms complex values stream of chips. The real and imaginary parts are separated in it. The real part forms the in-phase branch and the imaginary part forms the quadrature phase branch. [12]. Uplink modulation is shown in figure 3.

Real Part

Split Real and Imaginary parts of Spread and Scrambled Signal

Complex valued spread and Scrambled Signal

(S)

Pulse Shaping

I Part

90^o

-sin(wt)

cos(wt)

∑

Figure 3. Uplink Modulation (QPSK). [12]

Downlink Demodulation:-

The downlink scrambling code separates the transmission of one cell from another. These codes are similar to Gold Codes used in uplink demodulation. A total of 2¹⁸ scrambling codes are used but all of them are not used simultaneously. The problem in such kind of usage is that sometimes a receiver may check the message against all codes. Therefore in downlink, the scrambling codes are divided into 512 groups. A group contains one primary and 15 secondary code. [12]

Downlink Modulation:-

The modulation in downlink is same as in uplink modulation. Here each complex valued chip is split into its constituent parts. The real part is sent on the I branch of modulator and the imaginary part is sent on the Q branch of the modulator. [12]

Frequency and Time Synchronization:-

This procedure takes place after the switching on of UE. The synchronization procedure starts with downlink SCH. The UE knows the SCH primary synchronization code, which is common to all cells. The slot timing is obtained by receiving primary synchronization channel (P-SCH) and detection of peaks in output of a filter, which is matched to the universal synchronization code. Afterwards the UE correlates the received signal with Secondary SCH with all Secondary Synchronization Codes (SCS). The S-SCH is sent in 1^st 256 chips only and one SSC is sent in every time slot. A sequence consists of 15 SSCs, and the sequences are arranged in such a way that in any nonzero cyclic shift, less than 15 of any 64 sequences are not equal to any other sequence. [15]

RF Processing:-

The RF characteristics in the UTRAN are defined in 3G TS 25.1XX series of specifications. Some sections of RF processing are given below.

UE Power Classes:-

There are four power classes; the maximum output of these four classes is given below

+33dBm

+27dBm

+24dBm

+21dBm [15]

Frequency Bands:-

The total bandwidth of 3G is divided between the FDD and TDD modes. The paired band is used for FDD mode whereas, the smaller unpaired band is used for TDD mode. [15]

Modulation Techniques:-

Some techniques used in WCDMA are

DS-CDMA

Frequency-Hopping CDMA

Time-Hopping CDMA

Multicarrier CDMA[15]

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