1 Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Vocational Training Council

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Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile NetworksDepartment of Information & Communications TechnologyVocational Training Council - IVE (Tsing Yi) TN3431 Mobile NetworksDepartment of Information & Communications Technology

4.3 CDMA Air Interface Design

The starting point for the air interface design are the system requirements. Understanding these is critical for a good end result of the design process. Basic system requirements determine data rates, bit error rate (BER), and delay, as well as channel models, which define the radio environment.

CDMA radio interface design involves several areas, as shown in Figure 4.4.1. Each area itself is very board, and different parameters can be optimized alone.However, a good design is always a trade-off between several and often contradicting requirements. Good radio performance, for example, requires more advanced receivers resulting in higher costs and complexity, while another system requirement might be precisely the minimization of equipment cost.

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System requirement

• data rates, BER, delay

• radio environment

• available frequency bands

• synchronization requirements

• signaling requirement

• complexity aspects

Radio interface design:

Joint Optimization of:-

• physical channels

• spreading codes

• modulation

• error control schemes

• multirate scheme

• packet data transmission

• random access

• receiver and transmitter

• handover

• power control

• admission and load control

Figure 4.3.1 CDMA air interface design process

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4.3.1 Layered Air Interface Structure

Network Layer:

Call control,

Mobility management

Radio resource management

Link access layer

Medium access control

Physical layerLayer 1

Layer 2

Layer 3

Figure 4.3.2 The layered structure of the air interface


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As shown in Figure 4.3.2, the air interface functions are structured into protocol layers. The physical channel performs coding, modulation, and spreading for the physical channels. As shown in Figure 4.3.2, the link layer is further divided into the two sublayers: mcdium access control (MAC) and link access control (LAC). The medium access layer coordinates the resources offered by the physical layer. The link access control performs the functions essential to set up, maintain, and release a logical link connection. The network layer contains call control, mobility management, and radio resource management functions. (i.e. to those that function in different protocol layers that depend on the underlying multiple access method, in this case, wideband CDMA) These layers include the physical layer, the MAC layer, and radio resource management in the network layer.

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• ETSI’s Special Mobile Group is responsible for UMTS standardization. It has selected the basic technology for UTRA (UMTS Terrestrial Radio Access)– 1920 - 1980 MHz, 2110 - 2170 MHz paired band will use

WCDMA in FDD operation– 35 MHz unpaired band will use TD-CDMA in TDD operation

• ARIB (Association of Radio Industry and Business) and ETSI are working towards a common proposal

• Requirement of IMT-2000 is 384 kb/s for full area and 2 Mb/s for local area access

• GSM evolution to IMT-2000– EDGE use high level modulation for a 200 kHz TDMA migration

from existing bands in small spectrum chunks– UMTS is based on 5 MHz WCDMA– Reuse GSM’s mobility support, authentication, circuit switching


4.3.1.1 Introduction

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4.3.1.2 Physical layer (Basic Radio Parameters)

• UTRA/FDD is based on 4.096 Mchips/s in 5 MHz band. 8.192 and 16.384 Mchips/s are also specified for future evolution to >2 Mb/s data rate.

• 10 ms radio frame for low delay speech and fast control message

• Carriers are on a 200 kHz carrier grid with spacing 4.2 - 5 MHz. Larger spacing is needed in different cell layers

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4.3.1.3 Physical Layer (Transport Channels)

• Transport channels are PHY services offered to higher layers. • Always unidirectional, either common or dedicated• Broadcast control channel (BCCH) -- downlink channel for broadcasti

ng system and cell specific control information• Forward Access Channel (FACH) -- downlink common channel to ent

ire cell or part of cell using adaptive antenna array• Paging Channel (PCH) -- downlink common channel to mobiles whos

e locations are not known• Random Access Channel (RACH) -- uplink common• Dedicated Channel (DCH) -- down or up link dedicated channel to and

from a mobile station• Data are organized into transport blocks. Different channels have diff

erent transmission time interval ranging from 10 to 80 ms

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Table 1. Transport-channel formats for some typical service cases

• Service //Transmission-time interval// blocks per tran. Interval//block size

• Variable-rate speech 10 or 20 ms Fixed (=1) Variable

• Packet data 10 80 ms Variable Fixed (~300 bits)

• Circuit-switched data 10 80 ms Fixed (>=1) Fixed

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4.3.2 Logical Channels

The logical channels can be divided into control channels and traffic channels. A control channel is either common or dedicated. A common control channel is a point-to-multi-point control channel that carries connectionless messages, which are primarily intended to carry signaling information necessary for access management functions broadcast information (access request, access grant, paging messages,and user packet data). A dedicated control channel is a point-to-point bidirectional control channel, intended to carry signaling information such as handover measurements, service adaption information, and power control information. Traffic channels carry a wide variety of user information streams. The procedures described lead to the following logical channel structure:

• Synchronization channel;

• Random access channel;

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• Broadcast channel;

• Paging channel;

• Dedicated control channel;

• Traffic channel.

4.3.3 Physically Channels

In general, the functions provided by logical channels need to be mapped into physical channels. The mapping depends on several aspects such as frame design, modulation method, and code design.

A synchronization channel is used to provide the receiver with chip, bit and frame synchronization. A pilot signal can be used as a reference signal for chip level synchronization and coherent detection, as discussed below. The mobile station uses the downlink synchronization channel for handover measurements and synchronization.

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The broadcast channels system-specific information. The system designer needs to determine the type and rate of information to be transmitted so that the broadcast channel data rate can be set. For the paging channel, the number of paging channels and the data rate need to be determined.

The random access channel structure depends on how fast the synchronization needs to be established and on the selected access strategy.

A dedicated control channel is bundled together with a traffic channel. Traffic and dedicated control channels can be multiplexed, code multiplexed or I&Q multiplexed.

4.3.3.1 Measurement Signaling

Signaling requirements depend on the measurement needs (i.e. how often measurements for power control, handover, and load control are performed and how they need to be transmitted between mobile stations and the base station). When the rough transmission requirements for signaling have been defined, appropriate channel structures can be designed. The signaling traffic can either

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be transmitted on a dedicated control channel (outband signaling) or on a traffic channel by puncturing the user data (inband signaling). The selected method depends on the quality requirements for user data and mobile station complexity.

4.3.3.2 Pilot Signals

Because of fading channels, it is hard to obtain a phase reference for the coherent detection of data modulated signal. Therefore, it is beneficial to have a separate pilot channel. Typically, a channel estimate for coherent detection is obtained from a common pilot channel. Typically, a channel estimate for coherent detection is obtained from a common pilot channel.

The user dedicated pilot symbols can either be time or code multiplexed. Figure 4.3.3 and 4.3.4 depict block diagram of a transmitter and receiver for time multiplexed pilot symbols and, a code multiplexed, parallel pilot channel, respectively. A drawback of a pilot channel transmitted on a separate code is that it requires extra correlators for despreading. The performance of both

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approaches is the same if the same amount of power is used for the pilot signals, assuming that optimal least mean squares channel recovery techniques are employed.

For time multiplexed pilot symbols, the ratio between the number of data symbols (Ld) and the number of pilot symbols (Lp) needs to be determined. For the parallel pilot channel, the power ratio of pilot and data channels needs to be determined. In the receiver, pilot symbols are averaged over a certain time period.

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LpLpLp Data (Ld) Data (Ld)

MUXSpreading

and modulation

Coded data symbols

Pilot symbols

Correlator

BankDEMUX

Detection

and RAKE

combiner

Channel Decoding

Figure 4.3.3 Time multiplexed pilot symbols.

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Spreading

and modulation

Coded data symbols

Pilot symbols

Spreading

Detection

and RAKE

combiner

Channel Decording

Correlator

bank for data

Figure 4.3.4 Code multiplexed, parallel pilot channel.


Spreading

Spreading

Correlator

bank for pilot

Data Channel

Unmodulated PilotRelative Power

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4.3.4 Modulation

Figure 3.4.5 illustrates DS-CDMA transmission functions related to modulation. The data modulator maps the incoming coded data bits into one of M(=2m) possible real or complex valued transmitted data symbols. Typical data modulation schemes are BPSK and QPSK. Typical spreading modulation schemes are BPSK used with spreading circuit, QPSK, and O-QPSK used with a quaternary or complex spreading circuit.

4.3.4.1 Data Modulation

Binary data modulation maps incoming data bits into transmitted data symbols with a mapping rule (1 = +1), (0 = -1). Quaternary data modulation converts two consecutive data bits into one complex data symbol. Typically, gray encoding is used, which has the following mapping rule:

00 = -1+j ; 01 = -1+j ; 11 = -1+j ; 10 = +1-j

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4.3.4.2 Spreading Modulation

Typically, linear modulation methods (BPSK, QPSK and offset QPSK) have been proposed for wideband CDMA because they offer good modulation efficiency. Band-limited linear modulation methods with nonlinear power amplifiers result in spectrum regrowth (i.e. spectrum leakage to adjacent carriers). Thus, especially in the uplink, the modulated signal should tolerate nonlinear power amplifier effects as much as possible, since a nonlinear power amplifier is more power efficient than a linear one. In practice, after the modulation method has been fixed, pulse shaping determines the final spectrum properties of the modulation scheme.

BPSK spreading modulation can be used with the binary spreading circuit. Non-filtered BPSK has a constant envelope and an infinite spectrum.

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QPSK and offset QPSK are quadrature types of modulation methods. The offset QPSK has a one chip delay in the Q channel, which prevents phase transitions via zero, as can be observed in Figure 3.4.5. Therefore, filtered O-QPSK has fewer envelope variations than QPSK, thus reducing the linearity requirements on the power amplifier. This is especially important in the uplink. However, if complex spreading is used, then QPSK should be used instead of offset QPSK.

The scattering diagrams of BPSK, QPSK and offset QPSK modulation methods are shown in Figure 4.3.5.

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(a)

Q

I

Q

I

(b)

Q

I

(c)

Figure 4.3.5: Scattering diagrams: (a) BPSK, (b) QPSK, and O-QPSK

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Comparison:

• GMSK has constant envelope; so nonlinear amplifier can be used to achieve high battery efficiency; good channel performance, and self-synchronization capability.

• / 4 – QPSK is linear modulation, hence high frequency efficiency and power efficiency. Drawback is: it needs linear power amplifier.

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4.3.5 Error Control Schemes

Error control schemes can be classified in two basic categories: forward error control (FEC) and automatic repeat request (ARQ) schemes. In addition, a combination of FEC and ARQ, a hybrid scheme, can be used. In FEC, an error correcting code, sometimes referred to as the channel code, is used to combat transmission errors in a fading radio channel. In ARQ, an error detecting code together with a retransmission protocol is used. In the hybrid scheme, FEC reduces the need for retransmission by improving the error rate before ARQ.

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4.3.5.1 Selection of Error Control Scheme

The choice of the error control scheme depends on the required quality of service (data rate, delay and BER) and the radio channel. Raw channel error rates for transmission in a fading channel are typically on the order of 10-1 to 10-2. For speech, the resulting error rate after demodulation must be on the order of 10-2 to 10-3 or better. This can be achieved, for example, with convolutional codes, which is the standard coding for all cellular systems. For data transmission, a bit error rate of 10-6 or better is required.

4.4.5.2 Convolutional Codes

For convolutional codes, constraint length and code rate need to be selected. The constraint length should be as large as possible to obtain good performance. However, the complexity of the decoder increases with increasing constraint length. State-of-the-art VLSI (very large scale integration) implementations have been obtained for constant length 9 convolutional codes. The code rate depends on the interleaving depth and the coherence time of the channel.

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4.3.5.2 Concatenated Reed-Solomon / Convolutional Coding

This coding scheme employs an outer Reed-Solomon (RS) code and aninner convolutional code in conjunction with outer and inner interleavers. The use of an inner bit interleaver / deinterleaver pair renders the fading channel memoryless and allow the convolutional code to mitigate multiuser interference more effectively.

4.3.5.3 Turbo Codes

Recently, turbo coding has received attention as a potential FEC coding scheme for cellular applications. Turbo codes are parallel or serial concatenated recursive convolutional codes, whose decoding is carried out iteratively. They have been demonstrated to closely approach the Shannon capacity limit on both AWGN and Rayleigh Fading channels.

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4.3.5.4 Hyhrid ARQ Schemes

The type-I hybrid ARQ scheme uses a code designed for error detection and correction. Thus, it requires more parity check bits (overhead) than a code used for error detection only. Type-II hybrid ARQ codes the first transmission of a message with parity check bits used solely for error detection. If an error is detected, the erroneous word is saved in the receiver buffer and a retransmission is required. The retransmission is a block of parity check bits formed from the original message and an error-correcting code. These parity check bits are then applied to the stored code word in the receivcr. In case there is still an error, the next retransmission might contain either the original code word or an additional block of parity check bits.

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4.3.5.5 Interleaving Schemes

A radio channel produces bursty errors. However, since convolutional codes are most effective against random errors, interleaving is used to randomize the bursty errors. The interleaving scheme can be either block interleaving or convolutional interleaving. Typically, block interleaving is used in cellular applications.

The performance improvement due to interleaving depends on the diversity order of the channel and the average fade duration of the channel. The interleaving length is determined by the delay requirements of the service. Speech service typically requires a shorter delay than data services. Thus, it should be possible to match the interleaving depth to different services.

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4.3.5.6 Multirate Schemes

Mulyirate design means that different services with different quality of service requirements are multiplexed together in a flexible and spectrum-efficient way. The provision of variable data rate with different quality of service requirements can be divided into three sub-problem:

• How to map different bit rates into the allocated bandwidth;

• How to provide the desired quality of service;

• How to inform the receiver about the characteristics of the received signal.

The first problem concerns issues like multicode transmission and variable spreading. The second spread problem concerns coding schemes, and the third problem concerns control channel multiplexing and coding.

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4.3.6 Packet Data

Since non real-time packet data services are not delay sensitive, they are the retransmission principle implemented with ARQ protocol to improve the error rate. The retransmission protocol can be either implemented in layer 2 as part of MAC and RLP or in the physical layer (layer 1). If packet data retransmission is implemented as part of layer, the transmission of packet data in the physical layer does not differ from the transmission of circuit switched data. So the multirate aspects discussed above also apply to the transmission of packet data. If the physical layer ARQ is used, then physical layer is modified depending on the ARQ scheme used. In both cases, the access procedure and handover for packet data services have certain special implication, which are discussed in the next subsection.

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4.3.6.1 Packet Access Procedure

The packet access procedure in CDMA should minimize the interference to other users. Since there is no connection between the base station and the mobile station before the access procedure, initial access is not power controlled and thus the information transmitted during this period should be minimized. There are three scenarios for packet access:

• Infrequent transmission of short packets containing little information;

• Transmission of long packets;

• Frequent transmission of short packets.

Since the establishment of a traffic channel itself requires signalling and thus consumes radio resources, it is better to transmit small packets within the random access message without power control. For long and frequent short packets, a dedicated traffic channel should be allocated.

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4.3.6.2 MAC Protocol

The task of the medium access protocol is to share the transmission medium with different users in a fair and efficient way. Sometimes, multiple access protocols such as FDMA, CDMA, and TDMA are also classified as medium access protocols. The medium access protocol is part of the link layer, while the multiple access scheme is part of the physical layer. The medium access protocol has to resolve contention between users accessing the same physical resource. Thus, it also manages the packet access procedure. Since the third generation systems offer a multitude of services to customers at widely varying quality of service requirements, the MAC needs to offer capabilities to manage the access demands of different users and different service classes. This can be performed using reservation and priority schemes. Services with delay constraints can use a reservation scheme to reserve capacity to guarantee the quality of service. Priority schemes can be used to prioritize the requests from different services.

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4.3.6.3 Packet Data Handover

Since CDMA operates with the reuse factor of one, it needs efficient and fast handover in order to avoid excessive interference with the other cells. This has been realized with soft handover in the case of circuit switched connection. Soft handover also improves performance through increased diversity. For packet connection, and especially for short packets, there may be no need to establish soft handover even if the user is at the cell edge. However, there is still the need to route packets via the base station that provides the best connection. This is more important with frequent packet transmissions than with infrequent transmission.This can be implemented with frequent rescheduling of packet data transmission (i.e. if the transmission time exceeds a predefined timer, rescheduling is performed and the base station offering the best connection is selected).

If soft handover is used, then the implementation of ARQ depends on where it is terminated (i.e., where in the network the retransmission decision is made).

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If the ARQ is terminated after the soft handover combining device typically situated in the BSC, then ARQ is implemented as without soft handover. The only drawback of this solution is that it increases the delay for retransmission because of the fixed network transmission between the BTS and the BSC.

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4.3.7 Multiuser Detection

Multiuser detection (MUD) and interference cancellation (IC) seek to improve performance by canceling the intra-cell interference and thus increasing the system capacity. The actual capacity increase depends on the efficiency of the algorithm, radio environment, and the system load. In addition to capacity improvement, MUD and IC alleviate the near/far problem typical to DS-CDMA systems.

Figure 4.3.6 depicts a system using multiuser detection / interference cancellation. Each user is transmitting data bits, which are spread by the spreading codes. The signals are transmitted over a Gaussian multiple access channel. In the receiver, the received signal is correlated with replicas of the user spreading codes. The correlator consists of a multiplier and an integrate and dump function. A matched filter can also be used. Multiuser detection processes the signal from the correlators jointly to remove the unwanted multiple access interference from the desired signal. The output of a multiuser detection block are the estimated data bits.

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Figure 4.3.6 System model of multiuser detection

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4.3.7.1 Capacity and Coverage Improvement

The performance of multiuser detection depends on the captured energy and impact of phase and code tracking errors. The fraction of captured energy is a function of chip duration multiplied by the number of RAKE branches divided by the delay spread.Furthermore, since typically the interference from a single cell (i.e., intracell interference) is cancelled, the intercell interference limits the achievable capacity. With a propagation power law of 4, intercell interference is 55% of intracell interference; therefore, ideally, MUD can improve the capacity of the system by a factor of 2.8, compared to a system without MUD. However, in practice, the MUD efficiency is not 100% but depends on the detection scheme, channel estimation, delay estimation, and power control error.

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Typically, MUD is studied mainly for the uplink. One of the main motivations, in addition to complexity considerations, to justify the study of multiuser detection for the uplink has been the claim that the capacity of DS-CDMA system is uplink limited. However, this claim comes from the studies for IS-95. The noncoherent uplink of IS-95 with antenna diversity has normally worse performance than the coherent downlink with orthogonal codes. Even for IS-95, however, the downlink is sometimes the limiting detection. All third generation wideband CDMA system use coherent detection also is the uplink and thus, together with antenna diversity, the uplink has better performance than the downlink. One way of improving downlink performance would be canceling the worse interferers from other cells.

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1 Vocational Training Council - IVE (Tsing Yi) TN3431 Mobile Networks Department of Information & Communications Technology Vocational Training Council