15.WCDMA Air Interface

Version 1 Revision 0 JTO PH II (3G) Ericsson

WCDMA AIR INTERFACE

History

In 1992, the World Administrative Conference (WARC) of the ITU (International Telecommunications Union) chose frequency bands around 2 GHz to be available for third generation mobile systems. These third generation systems are also called International Mobile Telephony 2000 (IMT-2000), by the ITU.IMT-2000 defines several different air interfaces for third generation systems based on either Wideband Code Division Multiple Access (WCDMA) or TDMA technology. The same air interface, WCDMA, is to be used in Europe and Asia, including Japan and Korea, using the frequency bands around 2 GHz.

WCDMA AIR INTERFACE

Beside WCDMA, other air interfaces such as EDGE and cdma2000 can provide 3G services. EDGE (Enhanced Data Rates for GSM Evolution) can provide bit rates up to 500 kbps using a GSM carrier with a bandwidth of 200 kHz.Cdma2000 can be used as an upgrade for the existing IS-95 operators. Spectrum allocation in Europe, Japan and Korea is 1920 – 1980 MHz uplink and 2110 – 2170 MHz downlink.

WCDMA MILESTONES

In January 1998, the European standardization body ETSI decided upon WCDMA as the third generation air interface. The pre- commercial testing phase took place in Europe at the beginning of 2002.The first commercial network was opened in Japan during 2001 for commercial use in key areas

EVOLUTION FROM 2G TO 3G

The second generation (2G) networks are designed and optimized for circuit switched services such as voice and low bit-rate circuit switched data. They are not optimized for packet data and can offer at best a maximum data throughput of 14.4 kbps (per timeslot). It should be noted that there are various enhancements becoming available such as GPRS and EDGE to improve the 2G network’s data handling capabilities, to increase its data transfer rate and allow packet data services.

BRBRAITT, Jabalpur. Issued in SEPT-2009 15-1


Third Generation (3G) networks, on the other hand, have been designed for data transmissions, and support not only circuit switched voice and circuit switched data but also high-speed packet switched data as well as multi services.

The demands on the 3G networks are going to be very different to the basic voice communication requirement of the 2G networks. The 3G networks will require a very flexible air interface that can meet the demands of both packet services and circuit switched voice or data, and handle these in the most efficient way.WCDMA RADIO ACCESS BEARERS (RABS)

The purpose of a Radio Access Bearer (RAB) is to provide a connection segment using the WCDMA RAN for support of a UMTS bearer service. The WCDMA RAN can provide Radio Access Bearer connections with different characteristics in order to match requirements for different UMTS bearers. The conversational speech RAB is tailored to 12.2 kbps Adaptive Multi Rate (AMR) speech and will also be used to carry emergency calls. Video telephony service may be offered across the Conversational 64 kbps Circuit Switched (CS) RAB. Streaming 57.6 kbps is used to support v.90 modem connections. The maximum data rate supported by the Interactive or Background Packet Switched (PS) RAB is 384 kbps in the downlink and 64 kbps in the uplink, making it ideal for email or web browsing. High Speed Downlink Packet Access (HSDPA) enables up to 4.32 Mbps (in P5 up to 14 Mbps) in the downlink and 384 kbps in the uplink. The Multi-RAB is used for both 12.2 kbps AMR and PS 64/64 kbps. In P5 several new RAB combinations are added. See the following figure 1 .



Figure 1. RAB Combinations



MULTIPLE ACCESS TECHNOLOGIES

There are three basic air interface multiple access techniques - frequency, time and code division multiple access Figure 2

Frequency Time Division Spread

Code

Division Multiple Spectrum

Division

Multiple Access Multiple

Multiple

Access

Access

Access

MultipleTransmitters

1 2 3 1 2 3 N and

Us

er

Us

er

Use

r

Use

r

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er

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er

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Multiple Data

Channels

Frequency Time Frequency

Each User has a unique Each User has a uniqueEach Transmitter has

a unique

frequency time slot Scrambling Code

(1 voice channel per user) Each Data Channel has a unique Each Data Channel has a unique

position within the time slot Channelization code

All users transmit at the same Several users share the

Many users share the same frequency

and timetime same frequency

AMPS, NMT, TACSIS-136, GSM, PDC

IS-95, cdma2000, WCDMA

Figure 2: Multiple Access Approaches.

Frequency Division Multiple Access (FDMA) is very common in the first generation of mobile communication systems. Examples of systems using this technique are NMT, TACS and AMPS. The available spectrum is divided into physical channels of equal bandwidth. One physical channel is allocated per subscriber. The physical channel allocated to the subscriber is used during the entire duration of the call and is unavailable for use by another subscriber during this time.



In Time Division Multiple Access (TDMA) the available spectrum for one carrier, is divided in time. The subscriber is allocated a set amount of time referred to as a time slot. Subscribers can only use the air interface for this amount of time. An example of a system that uses this principle is D-AMPS, which explains why D-AMPS is sometimes called TDMA. Other mobile telephony systems that use TDMA, for example GSM, also split the available frequency band into several distinct carriers, and are, in a sense, hybrids since they use both TDMA and FDMA.

Wideband Code Division Multiple Access (WCDMA) allows many subscribers to use the same frequency at the same time. In order to distinguish between the users, the information undergoes a process known as spreading that is, the information is multiplied by a channelization and scrambling code. Hence WCDMA is referred to as a spread spectrum technology. This technology was first developed by the military to avoid the possibility of their signals being jammed or listened to by the enemy.

TDMA TRANSMITTER

The TDMA transmitter is illustrated in Figure 3.

Sync.The Multiplexer allows various data channels

to share the same timeslot.Bits

The timeslot selector allows multiple

Control/transmitters to share the same carrier

Error frequency, by assigning a unique timeslot to

SignalingError

each transmitter.Protection

Data ProtectionTimeslotSelector

Data Filtering RFError FilteringData Out

Vocoder Error Multiplexer Transmit +Vocoder Protection Transmit +MultiplexerProtection Gating RF

Gating RFModulationModulation

User Data ErrorError

Channel 1 ProtectionProtection


Channel N ProtectionProtection

Figure 3: TDMA Transmitter



The voice channel is passed through a vocoder, which produces a digital representation of the input analogue signal. After error protection this representation is fed into a data multiplexer where it is multiplexed with synchronization bits, control/signaling data and user data channels. This combined signal is passed to the transmit gating device. This allows transmission during the specified timeslot for a particular user, in the same way as a ‘push-to-talk’ button is used in a two- way radio. This allows multiple transmitters to share the same frequency by assigning a unique time slot to each.Finally, filtering and RF modulation is performed and the signal is passed to an antenna system.

WCDMA TRANSMITTER

The WCDMA transmitter looks similar to the TDMA transmitter, with the synchronization, control/signaling and multiple user data channels. However, in the case of a WCDMA transmitter, neither time nor frequency is used to separate different users, but codes in an operation known as spreading.

Channelization Codes provide uniqueChannelization identification of each data channel

Scramblingcode 1

Code

Sync.Scrambling Codes (SC) provideBits

Channelization Scramblingunique identificationof each transmitter

code 2 Code

Control/ ErrorSignaling

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Channelization Scrambling

code 3 Code

Error Linear Filtering RFVocoder Error Linear Filtering Out

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Summation RFChannelization Scrambling

RFModulation

code 4 Code Modulation


Channel 1 ProtectionProtection

Channelization Scrambling User 1

code N Code User 2

User 3

User Data Error ...Error

Channel N Protection

ProtectionFrequency

Figure 4: The WCDMA transmitter



In the case of the TDMA transmitter these data channels were time multiplexed. However, the WCDMA transmitter simply multiplies each channel by a different binary code known as a channelization code. This process provides the necessary separation between the data channels, which can then simply be added together in a summation device. The output of this block is a digital data stream that contains different logical levels depending on the number of channels that were added together. If, for example, two data streams that contain levels between +1 and -1 are added together, they will contain a stream that contains levels between +2 and –2. Three data streams added produce levels between +3 and -3 and so on. In reality this varying level, depending on the number of channels, cannot be sent to the modulator so each channel is weighted to ensure that the combined result is a fixed level. This explains why power is the shared resource. The WCDMA transmitter now needs some method of providing separation between this signal and other transmitters, but cannot use time slots like the TDMA case. This separation is achieved by multiplying this composite signal by another binary code called a scrambling code. Filtering and RF modulation are then performed to produce an RF output that contains all the information from all the users at the same time and on the same frequency. It is important to note that this transmitter diagram is not accurate and is included merely to show some of the main points of the technology. The receiver needs to know the scrambling code to perform the reverse process and then use the same channelization codes to retrieve each data channel. Error detection and error protection of the data channels are performed using Cyclic Redundancy Check (CRC) coding, Forward Error Correction (FEC) and interleaving. It should be remembered that this user data could be voice from a vocoder, user data or control data. The next stage is to perform a 1:2 de-multiplexing of the stream (downlink only). This effectively doubles the data rate by taking all the even bits from the input stream and placing them on the I- branch, and by taking all the odd bits onto the Q-branch. This step is used to take advantage of an RF modulation scheme known as I/Q-modulation. The data is then converted from a binary signal ranging from 0 to 1 to an real value signal that ranges from –1 to +1.

The error-protected signal is then multiplied by a particular channelization code to provide the necessary channel separation. This is necessary since all the channels will be added together, which will produce a composite data stream. Scrambling of the signal is then performed using a complex multiplier, effectively using a separate scrambling code for the I- and Q- branches. This complex scrambling code is generated using a linear shift register. The channels are then summed together. After pulse shape filtering, the I- and Q-branch are passed to the I/Q-modulator, which will produce an RF output that can be fed to the antenna system. Each of these stages is explained in more detail in the rest of this chapter.

ADAPTIVE MULTI-RATE

The type of voice coding used for WCDMA is a combination of coding called Algebraic Code Excited Linear Predictive (ACELP), which uses codebook references to represent speech sounds and Adaptive Multi Rate (AMR) coding, which allows different speech rates to be used, depending on the environment or application. Another feature of this coder is that a sample of the background noise is periodically sent to the receiver. Since most voice conversations are made up of approximately 50% silence this sample can be used to recreate the background noise, thus reducing the amount of data to be sent and hence increasing system capacity, since no interference will be caused during the idle periods. The process uses a closed loop system that compares the sound sample of the voice with what is stored under a predicted code reference. The output from this process will represent the error between the two and is passed through a perceptual weighting device that will mimic the sensitivity of the human ear to gauge



how much distortion this error will produce. After error analysis, a new codebook reference may be chosen that should be a better match to the incoming speech. This closed loop should produce a very close codebook reference that can be used in the receiver to recreate the speech.

The receiver will simply contain the same codebook, a speech generator and a filter. The voice-tone activity detectors will handle the multiplexing of the background noise to be used in the receiver for idle periods. Discontinuous transmission bits indicate when to use this background noise. The two main advantages of using discontinuous transmission are:

1.Less power will be transmitted by the mobile and hence less interference, with the result that there will be an increase in capacity.

2.Longer mobile battery life.

The multi-rate speech coder is a single integrated speech codec with eight source rates: 12.2 (GSM), 10.2, 7.95, 7.40, 6.70 (PDC), 5.90, 5.15 and 4.75 kbps. The AMR rates can be controlled by the radio access network. To facilitate interoperability with existing cellular networks some of the modes are the same as in existing cellular networks. The AMR is capable of switching its bit rate every 20 ms speech frame upon command.

The bit rate of the AMR speech connection is controlled by the radio access network depending on the air interface loading and the quality of the speech connections. During high loading, such as during busy hours it is possible to use lower AMR bit rates to offer higher capacity while providing slightly lower speech quality. Also, if the mobile is running out of the cell coverage area and using its maximum transmission power, a lower AMR bit rate can be used to extend the cell coverage area.Adaptive multi-rate also contains error concealment. The purpose of frame substitution is to conceal the effect of lost speech frames. If several frames are lost, muting is used to prevent possibly annoying sounds as a result of the frame substitution.


15.WCDMA Air Interface

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