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PERFORMANCE ANALYSIS ON MODULATION TECHNIQUES IN

WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS

WCDMA SYSTEM

2.1     Introduction

The development of wireless mobile systems is subject to rapid changes throughout the world nowadays. However, it is essential to understand the development of wireless mobile systems before further discussing the current technology available in the market.

2.2     Evolution of Communication Systems

First Generation (1G) system was introduced in mid 1980s using analog transmission. It uses frequency modulation (FM) and frequency division duplex (FDD). A typical example of first generation cellular system is Advanced Mobile Phone Systems (AMPS) used in North America. The radio interface used was simple.

Thus it was very insecure that allowed snoopers to listen ongoing calls even with a simple radio tuner. The system also suffered from low spectrum efficiency and lower user capacity.

Second Generation (2G) system was introduced in the early 1990s using digital voice coding and digital modulation. The most popular 2G system is Global System Mobile (GSM) but several others are used around the world. The GSM was developed as a common mobile standard for European countries. It was supported by the European Technical Standards Institute (ETSI) and the system also has been deployed in non- European countries such as Asia, Australia and South America. In contrast to 1G system which primary design for voice, the 2G system has been designed to provide paging and other services which only support low data rate service such as short messaging service (SMS), voice mail and caller ID.

Due to high demand in data communication, most operators in the world upgraded their network to 2.5G which promise higher data speeds. The term 2.5G normally refers to data orientated technology such as WAP (Wireless Application Protocol) and GPRS (General Packet Radio Service). But, the offered speed still is not enough for some high-speed applications.

Third generation (3G) systems are an extension of the 2.5G system and was introduced in 2000’s. It results from the growing demands on high speed mobile services that can provide high traffic volumes as well as flexibility in communication bandwidth or services [8]. Thus this new standard was developed by International Mobile Telecommunications 2000 (IMT2000) and was designed to support real time data communication while maintaining compatibility with second generation systems. The standard are developed around 2GHz frequency band which will provides data rates up to 2 Mbps.

2.3     International Mobile Telecommunication 2000 (IMT2000)

The International Telecommunications Union-Radio communications (ITU-R) developed the third generation (3G) specifications to facilitate a global wireless infrastructure for terrestrial and satellite systems and fixed and mobile access for public and private networks. The vision of 3G system is based on integration of various wireless services that include multimedia, packet switching and wideband radio access. The following service requirements were established by the ITU for IMT-2000: [9], [10]

Multi-rate services delivering the following minimum bit rates: 2 Mbps to fixed locations, 384 kbps to pedestrian users, and 144 kbps for mobile users (automobile speeds).

Seamless coverage across pico, micro, and macro cells supporting different user densities.

Worldwide roaming capabilities.

High spectrum efficiency (to maximize capacity).

Flexible quality of service (bit rate errors and transmission delay can vary for different applications).

High communication security (to protect content and access).

Compatibility of services within IMT2000 and with fixed network.

High degree of commonality of design worldwide.

IMT-2000 was created to facilitate the developments of standards for global wireless system. In order to harmonize the new global standard, regional standard bodies have submitted the 3G proposal to ITU for review and discussion. In the proposal the backward compatibility of the existing 2G network was considered for the ease of the evolution to 3G system. Among the proposal submitted includes the UMTS (Universal Mobile Telecommunications System) European standard from ETSI (European Telecommunications Standards Institute), the US derived CDMA 2000 leads by TIA (Telecommunications Industry Association), the Japanese WCDMA (Wideband Code Division Multiple Access) system proposed by ARIB (Association of Radio Industries and Business), the Telecommunications Technologies Association (TTA) from Korea has prepared two proposals which one close to WCDMA scheme and other is similar to cdma2000 approach and many others proposal from several standard bodies.

Series of discussions have been conducted in order to achieve consensus and harmonization on standard between regional standard bodies. A harmonization would lead to a quasi-worldwide standard, which allow economic advantages for customers, network operators and manufacturers. Thus two international bodies have been established: [10]

·         The Third Generation Partnership Project (3GPP) to harmonize and standardize in detail the similar ETSI, ARIB and related TDD proposal which based on WCDMA scheme.

·         3GPP2 for the cdma2000 based proposal from TIA and TTA which originated from cdmaOne technology.

2.3.1  Comparisons between WCDMA and Cdma2000

2.3.1.1        WCDMA

Wideband CDMA (WCDMA) was defined in 1999 and is the wideband transmission method for UMTS, a mode of the third generation of mobile technology (3G). In conjunction with GPRS and EDGE which are evolutionary developments of the GSM technology. WCDMA system can be applied for both cost-effectively and with little need for additional resources to the 2G network infrastructures of the GSM standard. Thus mobile telephones and other devices used in WCDMA mode will be able to use GSM, GPRS or EDGE, which ensures a seamless transition within the existing networks.

WCDMA attains its high performance by transmitting signals from the various services which require variable data rates by assigning bandwidth flexibly (bandwidth on demand). Each signal is coded, and then modulated and distributed ("spread") across a 5 MHz transmission bandwidth. The frequency band is available to all subscribers simultaneously. The coding identifies the signals destined for the each individual subscriber. The codes act as a filter or encryption system which only extract the data destined for a particular subscriber from this enormous volume of data. All other users that do not have the appropriate codes will only receive the sum total of all signals in the form of undefined noise. This extreme form of spreading across a wide frequency band helps to prevent disruption and intermittent operation as a result of overlapping frequency harmonics (or fading).

2.3.1.2        Cdma2000

Cdma2000 covers a family of mobile communication technologies that further develop the 2G mode cdmaOne, whose use is restricted to the USA, South America, Korea and Japan. From a political point of view within the industry, Cdma2000 competes with the technologies that follow on from GSM (i.e. GPRS, EDGE, UMTS/WCDMA). Cdma2000 is designed to use transmission bandwidth of 1.25 MHz. Cdma2000 is a family of mobile technologies that are based on a narrow band (1.25 MHz channel bandwidth) version of CDMA and are derived from the Interim Standard No. 95 (IS-95) which was published in 1993 by the North American trading organization, the TIA. Although a multi-carrier CDMA that is able to handle multiple sequences IS-95 carriers was originally suggested within the framework of IMT-2000, only the single carrier solution of the Cdma2000 family has remained (hence the name 1x). The first step takes the form of 1xRTT (also called Cdma2000 1x) a slightly improved variant of IS-95 including the integration of a packet-switching core network which delivers similar performance to GPRS. Since the data throughput did not meet the 3G guidelines, the HDR (High Data Rate) system proposed by Qualcomm in 1998 was introduced as an evolution phase and was accepted as a standard known as Cdma2000 1xEV-DO in August 2001. 1xEV-DO is designed especially for data services that are not runtime critical, and requires a separate frequency band. This means that transmission capacities are reserved exclusively for data, even if there is no need, which can be a waste of radio resources. In order to eliminate this problem, the new version, Cdma2000 1xEV-DV, is designed to handle both voice traffic and data on a single frequency bandwidth. Only this second stage of evolution can be compared to WCDMA.

2.3.1.3        Technical Comparison of WCDMA and Cdma2000

WCDMA and Cdma2000 are both based on the spread spectrum technology, Code Division Multiple Access. Spread spectrum technology was first utilized in military applications because it is very difficult to jam, difficult to interfere with, and difficult to identify (it looks like noise). Both WCDMA and cdma2000 are direct sequence (DS) CDMA technology where many users transmit over the same wideband frequency, each transmitter is assigned a distinct code and the intended receiver is able to utilize that code and descramble the information from the other conversations which just appear as noise to the receiver.

Both of the proposed standards meet the overall technical requirements set forth by the IMT-2000 including the support of high bit rate multimedia services, packet data, and IP access. Many industry members contend that from a technical standpoint, “WCDMA and Cdma2000 are nearly identical and will provide little or no basis for competitive differentiation.”[11] The key technical differences between the two standards are as follows [9]:

Table 2.1 :      System comparison (Air Interface for 3GPPs’ Release99)

PARAMETERS

3GPP2 (Cdma2000)

3GPP(WCDMA)

Multiple access Technique and duplexing scheme

Multiple access:

DS CDMA (UL);

MC-CDMA (DL)

Duplexing: FDD

Multiple access:

DS-CDMA

Duplexing: FDD

Chip rate

N·1.2288    Mcps

(N=1,3,6,9,12)

3.84 Mcps

Frame length

5,10,20,40,80 ms

10 ms with 15 slots

Modulation and

detection

Data: UL-BPSK; DL-QPSK

Spreading: UL-HPSK, DL-QPSK

Detection: Pilot aided coherent detection

Data: UL- dual-channel

QPSK; DL - QPSK

Spreading: QPSK

Detection: Pilot aided coherent detection

Channelization

code

Walsh codes (UL)

Walsh codes or quasi-orthogonal

codes (DL)

Orthogonal Variable Spreading

Factor (OVSF) codes

Pilot structure

Code divided continuous dedicated pilot (UL)

Code divided continuous common pilot (DL)

Code divided continuous common or dedicated auxiliary pilot (DL)

Dedicated pilots (UL)

Common and/or dedicated

pilots (DL)

Scrambling code

Long code (with a period of 2 42 -1 chips for N=1)

Short PN code (with a period of 215 chips for N=1) with N ~spreading rate number

UL - Short code (256 chips)

or long code (38400 chips,

Gold code based)

DL: Gold code based

Access Scheme

Flexible random access scheme allowing 3 modes of access:

- Basic Access

- Power controlled Access

- Reserved Access

Designated Access scheme- access scheme initiated by BS message

Acquisition indication based random access mechanism with power ramping on preamble followed by message

Inter-base station;

operation

Synchronous Asynchronous

Synchronous (optional)

2.4     DSSS WCDMA

As the name given, Wideband Code Division Multiple Access (WCDMA) is wideband transmission that supports variable data rates by assigning bandwidth flexibly. It is based on CDMA system, which means that the available frequency channel is broken down by different code sequences that are multiplied by the user signals of the individual subscribers. All subscribers transmit on the same frequency and at the same time as depicted in Figure 2.1 below.

Figure 2.1:      Frequency sharing in CDMA

More specifically, the technique use for WCDMA is known as direct sequence spread spectrum technology (DS-SS). With this technique, the carrier frequency is modulated by a periodic spreading sequence. There are several possible spreading sequences. They can be complex valued or just real valued, quite long or very short and there are codes with various statistical properties to choose from. One of the most common is pseudo-random binary sequence also often denoted as PN-code (pseudo noise). This is a unique digital data sequence that repeats itself periodically, say, every 1023 bits (or even (242-1) bits in the case of long codes). A symbol of the spreading code (here PN-code) is called a chip. Figure 2.2 below shows how spreading is done.

The data stream, with symbol duration  is ''multiplied'' with a spreading sequence with  denoting the chip duration. The spreading factor is defined as



                                                                                                             (2.1)

which is corresponding with the number of chips within a symbol duration. The period of the spreading sequence used is greater or equals the spreading factor. This multiplication with a sequence of higher frequency also yields a resulting signal of higher rate, therefore the spectrum of the original signal is spread by the factor .

This signal is broadcast and can be seen on the right hand side of the upper figure. One of the main advantages of this scheme is its robustness against narrow band noise or interference. At receiver, the incoming signal is multiplies with the (conjugate complex) spreading sequence again to obtain the original signal.

Figure 2.2:      Spreading technique

The data rate used by a user depends on spreading factor assigned to that particular user. If several users use the same spreading factor, the signals are distinguished through different code sequence.

2.5     DSSS CDMA Bit Error Probability Calculations

There are two approaches to calculate BER for DSSS CDMA operating under AWGN channel [12]-[14]. The first approach uses accurate BER approximations because it is presumed that BER evaluation is numerically cumbersome. There are many researches on this approach and most widely used approximation is the so called Standard Gaussian Approximation (SGA) [12]-[14]. In the SGA, a central limit theorem (CLT) is employed to approximate the sum of the multiple-access interference (MAI) signals as an AWGN process additional to the background Gaussian noise process. To detect desired user signal, the receiver design consists of a conventional single-user matched filter (correlation receiver). The average variance of the MAI over all possible operating conditions is used to compute the SNR at the filter (correlator) output. SGA is widely used because it is easy to apply. However, it is known based on performance analysis that SGA often overestimate system performance especially for small number of users. Thus, Improved Gaussian Approximation (IGA) is created to overcome the limitations in SGA. IGA is more accurate that SGA especially for small number of users but with exploiting numerical integration and multiple numerical convolutions.

Simplified IGA (SIGA) is created where neither the knowledge of the conditional variance distribution, nor numerical integration nor convolution is necessary to achieve acceptable BER estimation. This approach is chosen in this project to calculate BER in the channel of WCDMA system.

The second approach is to perform the evaluation of the DS-CDMA system BER without knowledge of or assumptions about the MAI distribution. This approach is based on previous study on ISI. There are a number of ways to achieve this method. They include moment space technique, characteristic function method, method of moments, and an approximate Fourier series method [15], [16]. Generally, these techniques can achieve more accurate BER estimate than CLT-based approximations at the expense of much higher computational complexity. For BER of DSSS-CDMA systems operating in Rayleigh fading channels, an accurate method has been proposed by [8]. It gives in depth treatment on a generic DSSS-CDMA system with Rayleigh-distributed users under both synchronous and asynchronous operations for random sequences where the IGA and SIGA methods are extended to a Rayleigh fading channel system.

2.6     Theoretical DSSS CDMA System

2.6.1  Transmitter Model

If BPSK modulation scheme is used in the W-CDMA system model, the transmitted signal of kth user in reverse link (mobile to base station) can be represented as [14].



                                                                      (2.2)

where P_k represents transmitted signal power, b_k(t) is data signal, a_k(t)  is spreading signal, w_c is carrier frequency and_k is carrier phase. The kth user’s data signal is a random process that is a rectangular waveform, taking values from with service rate, and is expressed as



                                                                                (2.3)

where P_T(t) = 1, for 0  t  T , and P_T = 0, otherwise. The jth data bit of kth user is denoted as . Data source are assumed uniform, i.e.



                                                                      (2.4)

The spreading signal a_k₍t) can be expressed as



                                                                   (2.5)

where is an arbitrary chip waveform that is time-limited to [0,Tc] and is chip duration. Chip waveform is assumed to be normalized according to



                                                                                                      (2.6)

The lth chip of the kth user is denoted , which assumes values from {-1,+1}. All signature sequences {}are assumed to be random in the following sense. Every chip polarity is determined by flipping an unbiased coin. Further justification for the random chip sequence assumption is provided in. There are N chips for one data symbol and the period of the signature sequence is N. We normalize the chip duration so that T_c=1 and thus, T=N. Note that if the chip waveform is rectangular, i.e.



                                                                             (2.7)

the transmitted signal becomes the well known phased coded SS model [12].

For QPSK modulation scheme, the transmitted signal of  kth  user in the subsystem i is



            +             (2.8)

where are the In-phase and Quadrature-phase signal.

2.6.2  Receiver Model

The received signal r(t) at the input of the matched filter receiver is given by



                                                                                                                                  (2.9)



where * denotes convolution and is assumed a uniform random variable over [0, 2. The average received power of the kth signal is E[Pr] = E[]P_k_.

2.7     Channel Model

2.7.1  AWGN

The transmitted signal for BPSK modulation is subjected to AWGN process n(t), that has two-sided power spectral density N₀/2  and   = 1, k=1, ….,K. A_k is independent, Rayleigh distributed and account for the fading channel attenuation of all signal. The first order of probability density function (pdf) is given by

                                                                                      (2.10)

Due to the fact that SGA considers an average variance value for Multi Access Interference (MAI) or in other words, the first moment of the IGA exploits knowledge of all moments of .It was shown in [17] that the BER for an AWGN channel obtained from IGA is significantly more accurate than the BER obtained from the SGA especially for small number of user, k. Thus by applying SIGA, overall BER can be represented as [18].



                                                                                                                                (2.11)

where and are given by

                                                                                  (2.12)

and

                                         (2.13)

where this Holtzman’s method is extended by applying first and second moment for the received power.

2.7.2  Rayleigh Fading

The output of a low pass filter (LPF) of a synchronous system i.e. for user 1 can be represented as

                                                    (2.14)

where n₁ is a zero-mean Gaussian random variable with variance is the signal component =, and the interference term I₁is given by

                                                      (2.15)

Since a sum of independent Gaussian random variable has Gaussian distribution, it follows that I1 is a Gaussian random variable with zero-mean and variance

                                                                                                   (2.16)



By symmetry and using the independence I₁ and n₁, one has

                                               =Q                                  (2.17)

and averaging over the pdf of A1, BER for a Rayleigh-faded user is

                                                                             (2.18)

From the equation above, one sees that the interferers act like additional independent Gaussian background noise. This is because the MAI on the flat Rayleigh fading channel has a Gaussian first-order distribution assuming synchronous transmission. This implies that the optimum receiver that does not perform user-interference cancellation is a correlator detector. However, this is not the case of asynchronous transmission. For uniformity, uniform random signature sequences   and

                                                                                    (2.19)

In asynchronous transmission subjected to flat Rayleigh fading, average BER is computed by using characteristic function, . The proof for the following characteristic function can be found in [14]. Average characteristic function of MAI I_k, given B, is

(w)=                                                                                                                                                                                                                        (2.20)

Using the fact that the I_k's given B are independent, the characteristic function for total interference term I, given B, is

                                                                                          (2.21)

The conditional BER for target user, after averaging over pdf of A1, can be expressed by symmetry as

=

                                                                                                                                (2.22)

When the effect of background noise is negligible, and 1 , thus this equation becomes

                                                                                                                                (2.23)

Equation (2.20), (2.21), and (2.19) [or (2.18), (2.20) and (2.19) for noiseless case] give the average BER experience by a target user with a signature sequence that has a given value of B. The average BER for all users of for one target user averaged over all signature sequences randomly assigned by a base station for each request is

                                                                                   (2.24)

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Evolution of Communication Systems IN WCDMA SYSTEM WITH DIFFERENT CHANNEL CONDITIONS

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PARAMETERS	3GPP2 (Cdma2000)	3GPP(WCDMA)
Multiple access Technique and duplexing scheme	Multiple access: DS CDMA (UL); MC-CDMA (DL) Duplexing: FDD	Multiple access: DS-CDMA Duplexing: FDD
Chip rate	N·1.2288 Mcps (N=1,3,6,9,12)	3.84 Mcps
Frame length	5,10,20,40,80 ms	10 ms with 15 slots
Modulation and detection	Data: UL-BPSK; DL-QPSK Spreading: UL-HPSK, DL-QPSK Detection: Pilot aided coherent detection	Data: UL- dual-channel QPSK; DL - QPSK Spreading: QPSK Detection: Pilot aided coherent detection
Channelization code	Walsh codes (UL) Walsh codes or quasi-orthogonal codes (DL)	Orthogonal Variable Spreading Factor (OVSF) codes
Pilot structure	Code divided continuous dedicated pilot (UL) Code divided continuous common pilot (DL) Code divided continuous common or dedicated auxiliary pilot (DL)	Dedicated pilots (UL) Common and/or dedicated pilots (DL)
Scrambling code	Long code (with a period of 2 42 -1 chips for N=1) Short PN code (with a period of 215 chips for N=1) with N ~spreading rate number	UL - Short code (256 chips) or long code (38400 chips, Gold code based) DL: Gold code based
Access Scheme	Flexible random access scheme allowing 3 modes of access: - Basic Access - Power controlled Access - Reserved Access Designated Access scheme- access scheme initiated by BS message	Acquisition indication based random access mechanism with power ramping on preamble followed by message
Inter-base station; operation	Synchronous Asynchronous	Synchronous (optional)