KKKT 4033

COMMUNICATION RADIO & SATELLITE

SEMESTER I (2009/2010)

CDMA VS WCDMA

NAME: CHENG SOON WAI (A111224)

OOI WEN HUI (A113479)

LECTURER: EN. KAMARULZAMAN BIN MAT    

DUE DATE : 27 OCTOBER 2009

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Table of Contents

Table of Contents....................................................................................................................i

1 CDMA.................................................................................................................................1

1.1 Definition of CDMA 1

1.2 Spread Spectrum Characteristics of CDMA 2

1.3 Feature of CDMA 3

1.4 Technical specification of WCDMA 4

1.5 Power Control in CDMA 4

1.6 Handoff in CDMA 5

1.7 Rake receiver 5

1.8 Advantages of CDMA 5

1.9 Disadvantages of CDMA 5

2 WCDMA.............................................................................................................................6

2.1 Definition of WCDMA 6

2.2 Features of WCDMA 6

2.3 WCDMA Specifications 6

2.4 WCDMA Based System 7

2.5 Spreading and Modulation 8

2.6 Modulation Principle for WCDMA 8

2.7 Purpose of Power Control in WCDMA 9

2.8 Power Control Types inWCDMA 9

2.9 WCDMA Handover Types 10

2.10 Technology 11

2.11 Advantages of WCDMA 11

Conclusions..........................................................................................................................13

References............................................................................................................................13

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1 CDMA

1.1 Definition of CDMACode division multiple access (CDMA) is a channel access method utilized by various

radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), which use CDMA as an underlying channel access method.

In CDMA systems, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message. All users in a CDMA system use the same carrier frequency and may transmit simultaneously. Each user has its own pseudorandom codeword which is approximately orthogonal to all other code words.

Figure: CDMA in which each channel is assigned a unique PN code which is orthogonal to PN codes used by other user

CDMA is a form of Direct Sequence Spread Spectrum communications. In general,Spread Spectrum communications is distinguished by three key elements:

1. The signal occupies a bandwidth much greater than that which is necessary to send the information. This results in many benefits, such as immunity to interference and jamming and multi-user access, which we’ll discuss later on.

2. The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes in which the data modulation will always spread the spectrum somewhat.

3. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.

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Figure: Direct Sequence Spread Spectrum System

1.2 Spread Spectrum Characteristics of CDMAMost modulation schemes try to minimize the bandwidth of this signal since

bandwidth is a limited resource. However, spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudorandom code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudorandom sequence used to encode the data. CDMA is also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.

CDMA can also effectively reject narrowband interference. Since narrowband interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signals are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any given time. Like the narrowband interference this will result in only a small loss of data and can be overcome.

Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudorandom codes will have poor correlation with the original pseudorandom code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudorandom codes are such that this

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slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.

Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlators tuned to the path delay of the strongest signal. [5]

Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is an important consideration. The frequencies used in different cells need to be planned carefully in order to ensure that the signals from different cells do not interfere with each other. In a CDMA system the same frequency can be used in every cell because channelization is done using the pseudorandom codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudorandom sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.

Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communicate simultaneously with two or more cells. The best signal quality is selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.

Figure: CDMA Spread Spectrum illustration

1.3 Feature of CDMAMany users of a CDMA system share the same frequency. Either TDD or FDD may be used

1. Unlike TDMA or FDMA, CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises floor in a linear manner. Thus, there is no absolute limit on the number of users in CDMA. Rather, the system performance gradually degrades for all users as the number of users is increased, and improves as the number of users is decreased.

2. Multipath fading may be substantially reduced because the signal is spread over a large spectrum. If the spread spectrum bandwidth is greater than the coherence bandwidth of the channel, the inherent frequency diversity will mitigate the effects of small-scale fading.

3. Channel data rates are very high in CDMA systems. Consequently, the symbol(chip) duration is very short and usually much less than the channel delay

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spread. Since PN sequences have low autocorrelation, multipath which is delayed by more than a chip will appear as noise. A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signal.

4. Since CDMA uses co-channel cells, it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base station. The MSC may choose the best version of the signal at any time without switching frequencies.

5. Self-jamming is a problem in CDMA systems, Self-jamming arises from the fact that the spreading sequences of different users are not exactly orthogonal, hence in the dispreading sequences of a particular PN code, non-zero contributions to the receiver decision statistic for a desired user arise from the transmissions of other users in the system.

6. The near-far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user.

1.4 Technical specification of WCDMA1. Core network: ANSI-41 MAP2. Channel bandwidth: 1.25 MHz (1X), 3.75 MHz (3X)3. Channelization codes: 4-128 (1X), 4-256 (3X)4. Chip rate: 1.2288 Mbps (1X), 3.6864 Mbps (3X)5. Synchronized base station: Yes6. Frame length: 5 ms (signaling), 20, 40, 80 ms physical layer frames7. Multi-carrier spreading option: Yes, but in cdma2000 1X (direct spread)8. Modulation: QPSK (forward link), BPSK (reverse link)9. Modes of operation: FDD10. Source identification code for sector: One PN code (32,768 chips), 512 unique

offsets are generated using PN offsets11. Source identification code for mobile: One long PN code (242) chips, unique

offsets are generated based on ESN, not assigned by sector12. Channel coding: Convolutional and turbo code13. Power control: Both links (800 Hz)14. Circuit-switched core network: IS-41, MSC/HLR/ AC15. Packet-switched core network: IETF based, PDSN/ AAA/HA/FA16. Voice coder: EVRC17. Peak data rate: 614 kbps18. Multimedia services: Yes19. Overhead: Low (because of shared pilot code channel)

1.5 Power Control in CDMAThe power seen by the handset is directly proportional to the distance between the

transmitter and receiver. Thus, when a receiver is closer to the base station, the power with which it transmits is much higher than the power of a receiver at the edge of the cell. This is seen as interference to the receiver farther from the transmitter and is called the near-far effect. In order to avoid or reduce interference caused by this effect, the receiver design includes power control measures that reduce the power with which it transmits to the base station depending on the distance.

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1.6 Handoff in CDMASoft or Hard Handoff occurs when the mobile moving between cells is switching its cell station. At cell edges, the mobile tends to receive information from two or three neighboring base stations. The mobile performs continuous calculations of the SNR (signal to noise ratio) and decides to receive information from the base station that transmits with maximum SNR. When switching from one base station to the other, if the mobile disconnects with one base station before linking to the other, it is called hard handoff. In most practical situations, at some point of time during switching, the mobile maintains a connection with more than one base station. This is called soft handoff.

1.7 Rake receiver The receiver in CDMA is based on the rake principle where multiple fingers receive the transmitted output and appropriate combining techniques (Maximal Ratio Combining or Equal Gain Combining) are used to process these multipath components.

1.8 Advantages of CDMA1. Reduction of dropped calls thanks to Soft Handoff2. Capacity is CDMA's biggest asset. It can accommodate more users per MHz of

bandwidth3. than any other technology 4. CDMA has no built-in limit to the number of concurrent users5. CDMA uses precise clocks that do not limit the distance a tower can cover6. CDMA consumes less power and covers large areas so cell size in CDMA is large7. CDMA is able to produce a reasonable call with lower signal (cell phone

reception) levels8. CDMA's variable rate voice coders reduce the rate being transmitted when the

speaker is not talking, which allows the channel to operate more efficiently9. CDMA has a well-defined path to higher data rates

1.9 Disadvantages of CDMA1. Most CDMA-based technologies are patented and must be licensed from

Qualcomm.2. As the number of subscribers using a certain site goes up, the range of that site

goes down3. CDMA towers interfere with themselves so they are usually installed on short

towers and4. CDMA system may not perform well in hilly areas5. Currently CDMA covers a smaller portion of the world compared to GSM which

has more subscribers and is in more countries worldwide

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2 WCDMA

2.1 Definition of WCDMA

W-CDMA (Wideband Code Division Multiple Access) is a wide band spread-spectrum channel access method that utilizes the direct-sequence spread spectrum method of asynchronous code division multiple access to achieve higher speeds and support more users. WCDMA is a type of 3G cellular network. It is the technology behind the 3G UMTS standard and is allied with the 2G GSM standard.

2.2 Features of WCDMA

The key operational features of the WCDMA are listed below:

1. Support of high data rate transmission: 384 Kbps with wide area coverage, 2 Mbps with local coverage.

2. High service flexibility: support of multiple parallel variable rate services on each connection.

3. Both Frequency Division Duplex (FDD) and Time Division Duplex (TDD)

4. Built in support for future capacity and coverage enhancing technologies like adaptive antennas, advanced receiver structures and transmitter diversity.

5. Support of inter frequency hand over and hand over to other systems, including hand over to GSM.

6. Efficient packet access.

7. Employs coherent detection on uplink and downlink based on the use of pilot symbols.

8. Multiple types of handoffs between different cells including soft handoff, softer handoff and hard handoff

2.3 WCDMA Specifications

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Frequency utilization with W-CDMA

Parameters of WCDMA

2.4 WCDMA Based System

1. All users share the same frequency time domain.2. Users separated by the codes.3. Codes are orthogonal4. FDD frequency division duplex.

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– Uplink, downlink in separate frequency bands

5. TDD time division duplex.– Uplink, downlink in the same

2.5 Spreading and Modulation

WCDMA applies a two-layered code structure consisting of an orthogonal spreading codes and pseudo-random scrambling codes. Spreading is performed using channelization codes, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. Orthogonality between the different spreading factors can be achieved by the tree-structured orthogonal codes. Scrambling is used for cell separation in the downlink and user separation in the uplink.

2.6 Modulation Principle for WCDMA

The complex-valued chip sequence generated by the spreading process is QPSK modulated. Figure below illustrates the modulation principle used in the uplink and downlink. The pulse shaping is root-raised cosine with roll-off factor 0.22 and is the same for the mobile and base stations.

Modulation principle

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2.7 Purpose of Power Control in WCDMA

Power control in WCDMA can removes near far effect. It can also mitigates fading and compensates changes in propagation conditions. In the system level, it helps to decrease interference from other users and increase capacity of the system. Power control in uplink must make signal powers from different users nearly equal in order to maximize the total capacity in the cell. However, in downlink the power control must keep the signal at minimal required level in order to decrease the interference to users in other cells.

Power control in WCDMA

2.8 Power Control Types inWCDMA

There are three types of power control in WCDMA:

• Open Loop power control

The open loop power control is for initial power setting of mobile station across the air

interface.

• Fast closed loop power control

Fast closed loop power control can mitigate fast fading rate up to 1.5 kbps on uplink

and downlink. It uses a fixed quality target set in mobile station or base station.

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• Outer loop power control

Outer loop power control compensates changes in environment. It adjust the SIR target

to achieve the required FER/BER/BLER. It depends on MS speed available, multipath

diversity. In the soft handover comes after frame selection.

2.9 WCDMA Handover Types

WCDMA has several different types of handovers:

Soft, softer, and hard handover; Interfrequency handover; Handover between FDD and TDD modes; Handover between WCDMA and GSM.

The handover algorithm to make the handover decision needs different types of measurement information. Base stations in WCDMA need not be synchronized, and therefore, no external source of synchronization, like GPS, is needed for the base stations. Asynchronous base stations must be considered when designing soft handover algorithms and when implementing location services. Before entering soft handover, the mobile station measures observed timing differences of the downlink SCHs from the two base stations. The mobile station reports the timing differences back to the serving base station. The timing of a new downlink soft handover connection is adjusted with a resolution of one symbol (i.e., the dedicated downlink signals from the two base stations are synchronized with an accuracy of one symbol). That enables the mobile RAKE receiver to collect the macro diversity energy from the two base stations. Timing adjustments of dedicated downlink channels can be carried out with a resolution of one symbol without losing orthogonality of downlink codes.

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Measurement timing relation between WCDMA and GSM frame structures.

2.10 Technology

W-CDMA may use unpaired or paired spectrum, though the current implementations of W-CDMA (i.e. FOMA and UMTS) all use a pair of 5MHz spectrum, one for uplink and one for downlink. FOMA uses 16 slots per radio frame, where as UMTS uses 15 slots per radio frame.

2.11 Advantages of WCDMA

1. Service flexibility

WCDMA allows each carrier of 5MHz to process mixed service from 8Kbps to 2Mbps. In addition, circuit switched service and packet switched service can be carried out in the same channel, and a single terminal is used to carry out multiple circuit and packet switched services, so as to realize genuine multimedia service. WCDMA supports services with different requirements (such as voice and packet data) and ensures high quality and perfect coverage.

2. Spectrum efficiency

WCDMA can make highly efficient use of available radio spectrums. Because single cell multiplexing is adopted for WCDMA, no frequency planning is needed. Network capacity can be notably improved by using technologies of hierarchical cell structure, adaptive antenna array and coherent demodulation (bidirectional).

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3. Capacity and Coverage

The number of voice users which can be processed by WCDMA RF transceiver is 8 times as many as that of typical narrowband transceiver. Every RF carrier can handle 80 voice calling users at one time or every carrier can handle 50 internet data users at one time.

4. Every connection can provide multiple services

Packet and circuit switched services can be freely mixed in different band width and can be provided to the same user at the same time. Every WCDMA terminal can access up to 6 kinds of different services at the same time, which may be combinations of various data services such as voice, fax, e-mail, video, etc.

5. Network scale economics

When WCDMA wireless access is added to existing digital cellular network (such as European GSM) and the network is running in two systems, the same core network can be multiplexed and use the same base stations. The latest ATM mode i.e. micro-cell transmission procedure is used for the links between WCDMA access network and GSM core network. This method can process highly effective data packets, which can enhance the capacity of standard E1/T1 lines to 300 voice calls compared to 30 voice calls for that of existing network. It is estimated that about 50% transmission costs can be saved.

6. Outstanding voice capability

Although the main purpose for the next generation mobile access is to transmit high bit rate multimedia communication, with regard to voice communication, it can also fully make use of specs

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Conclusions

1. The term CDMA in the mobile world typically refers to the CDMA family of standards developed by Qualcomm. They are protocols, sets of defined specifications of mobile communications.

2. CDMA (the multiplexing technique) is used as the principle of the W-CDMA air interface protocol, as well as Qualcomm's CDMA protocols.

3. W-CDMA strictly refers to a mobile phone protocol with detailed specifications, as defined in IMT-2000.

4. The W-CDMA protocol was developed independently of the CDMA protocol developed by Qualcomm, although drawing on Qualcomm's research the CDMA family of standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of standards.

The overview of CDMA and WCDMA highlights the potential of increasing capacity in future cellular communications. This paper describes the mobile radio environment and its impact on narrow-band and wide-band propagation. The advantage of having CDMA and WCDMA in cellular systems is depicted. The concept of radio capacity in cellular is also introduced. The natural attributes of CDMA and WCDMA provide the reader with the reasons that cellular is considering using it.

References

[1] Garg, Vijay K.. 2007. Wireless Communications and Networking. San Francisco: Elsevier Inc.

[2] Lee, William C. Y.. 1991. Overview of Cellular CDMA. IEEE Transactions on Vehicular Technology. VOL. 40, NO. 2: 291-303

[3]Rappaport, T.S. 1992. Wireless Communication: Principles and Practice. 2nd Ed. New Jersey: Prentice Hall.