WCDMA / UMTS - Freematthieugautier.free.fr/China/UMTS_2009.pdf · WCDMA / UMTS UMTS (Universal Mobile Telecommunications System) is a new ... In theory, it can reach 2 Mbps, a transmission

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WCDMA / UMTS UMTS (Universal Mobile Telecommunications System) is a new mobile standard. We talk about the third generation of telephony or 3G. Purists prefer the term W-CDMA (Wideband Code Division Multiple Access) which takes the name of the technology deployed in Europe and some Asian operators. Its principle: use a frequency band wider in order to transmit more data and thus obtain a higher datarate. In theory, it can reach 2 Mbps, a transmission speed equivalent to that proposed for the Internet reached by ADSL or cable. Principle of Spectrum Spreading Frequency used

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FDD Technical summary Frequency band:1920 MHz -1980 MHz and 2110 MHz - 2170 MHz (Frequency Division Duplex) UL and DL Minimum frequency band required: ~ 2x5MHz Frequency re-use: 1 Carrier Spacing: 4.4MHz - 5.2 MHz Maximum number of (voice) channels on 2x5MHz: ~196 (spreading factor 256 UL, AMR 7.95kbps) / ~98 (spreading factor 128 UL, AMR 12.2kbps) Voice coding: AMR codecs (4.75 kHz - 12.2 kHz, GSM EFR=12.2 kHz) and SID (1.8 kHz) Channel coding: Convolutional coding, Turbo code for high rate data Duplexer needed (190MHz separation), Asymmetric connection supported Tx/Rx isolation: MS: 55dB, BS: 80dB Receiver: Rake Receiver sensitivity: Node B: -121dBm, Mobile -117dBm at BER of 10-3 Data type: Packet and circuit switch Modulation: QPSK Pulse shaping: Root raised cosine, roll-off = 0.22 Chip rate: 3.84 Mcps Channel raster: 200 kHz Maximum user data rate (Physical channel): ~ 2.3Mbps (spreading factor 4, parallel codes (3 DL / 6 UL), 1/2 rate coding), but interference limited. Maximum user data rate (Offered): 384 kbps (year 2002), higher rates ( ~ 2 Mbps) in the near future. HSPDA will offer data speeds up to 8-10 Mbps (and 20 Mbps for MIMO systems) Channel bit rate: 5.76Mbps Frame length: 10ms (38400 chips) Number of slots / frame: 15 Number of chips / slot: 2560 chips Handovers: Soft, Softer, (interfrequency: Hard) Power control period: Time slot = 1500 Hz rate Power control step size: 0.5, 1, 1.5 and 2 dB (Variable) Power control range: UL 80dB, DL 30dB Mobile peak power: Power class 1: +33 dBm (+1dB/-3dB) = 2W; class 2 +27 dBm, class 3 +24 dBm, class 4 +21 dBm Number of unique base station identification codes: 512 / frequency Physical layer spreading factors: 4 ... 256 UL, 4 ... 512 DL Scheme of WCDMA transmitter

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1. Spread spectrum and CDMA multiple access technique 1.1 Principles of spread spectrum The spread spectrum can be defined as “a technique used to transmit an information signal over a bandwidth many times higher than the minimum bandwidth that the signal demands”. For this reason, the spread spectrum is also considered as a modulation. In a spread spectrum system, the transmitted signal is "spread" with a code that is independent of the information message. After synchronization with the transmitter, the receiver must use the same code to "despread" the signal and to recover the information message. To understand why the spread spectrum has been so successful, the work of Claude Shannon should be reread, he was the first that formalized the concept. Starting from the famous expression that determines the capacity of a channel:

C = B*log2(1+(S/N)) where C is the channel capacity in bits per second, B the bandwidth of the transmitted signal in Hz, S is the signal power in Watt, N the noise power in Watt. By developing this function in series, we can easily show that:

C ≈ B/ln(2)*(S/N) → S/N ≈ C*ln(2)/B (S/N <<1)

The expression shows that there is an inverse link between the bandwidth B occupied by the transmitted signal and the signal to noise ratio S/N, which is measured at the receiver. Specifically, we observe that if B increases, a smaller signal to noise ratio is needed to have the same channel capacity C. Thus the virtues of spread spectrum are demonstrated: the spectral bandwidth is increased in order to reach good performances at the reception side, because the signal to noise ratio is reduced to a minimum. 1.2 CDMA and WCDMA techniques The CDMA (Code Division Multiple Access) is a multiple access technique that allows users to communicate simultaneously in the same frequency band. The distinction between different users is done with an dedicated code which is only known by the transmitter and receiver.

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Figure 1: CDMA system The CDMA is used by the second-generation American standard “CDMA One”. It is also the access mode chosen for the radio access network of “UMTS”. Depending on the bandwidth occupied by the spread signal, a CDMA system is "wideband" (wideband CDMA) or, conversely, "narrowband" (narrowband CDMA). The two radio access technologies of the UMTS UTRA-FDD and UTRA / TDD are wideband CDMA systems, while CDMA One is an example of narrowband CDMA system. CDMA One bandwidth is 1.25 MHz while UTRA-FDD and UTRA / TDD bandwidth is 5 MHz.

1.3 Direct Sequence Spread Spectrum technique: DS-CDMA The spread spectrum is considered as a kind of modulation, because the original message is transformed so that the spectral width after processing is several times larger than the original message. This transformation can be performed in several different ways (FH-CDMA, TD-CDMA, DS-CDMA). In a DS-CDMA (Direct Sequence CDMA) system, the information signal is directly modulated by a sequence or code that has particular statistics properties.

Figure 3: Direct Sequence Spread Spectrum

Channel Receiver Transceiver

Baseband

Baseband

Bandwidth

Channel noise

Spreading C1

RF frequency

Spreading C2

Despreading C1

Despreading C2

Baseband

Code with length L

Data

Spreading :

RF frequency

Rate Rate

Spreading signal

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The chips of the spreading code have a duration Tc, and are usually pulses +1 and -1. The chip rate Bc is equal to 1/Tc and expressed in chips per second (cps). The length of the information symbols is Ts and the symbol rate Bs = 1/Ts is expressed in symbols per second (sps). We define the "spreading factor" SF as the ratio between chip rate and symbol rate:

SF = Bc/Bs = Ts/Tc equals to the length L of the code used. It is important to note that the symbol rate Bs is variable and depends on the service (voice, video, data), while the chip rate Bc = 1/Tc is constant. Thus, a high datarate service will require fewer chips to encode a information symbol than a low-rate service. For FDD and TDD modes of UMTS, the chip rate is Bc = 3.84 Mcps. Once the code is generated, the spreading is done in a very simple manner: simply multiply the message with the chips that take +1 and -1 as value. A narrowband signal is transform in a wideband signal, because Bc> Bs or, equivalently, Ts> Tc (see Figure 3). Finally, the spread signal is transposed at the carrier frequency fc. At the reception side, the signal is first converted into baseband signal with the RF demodulator, and then multiplied by the same spreading code used at the transmitter side. Since the spreading code is composed of 1 and -1, the result of multiplying the received signal with the same sequence has the effect of removing the code contribution in the received signal and keeps only the information message. Because the information message has the symbol rate, and not the chip rate, the signal is “despread”. This procedure of "despreading" is only possible if the receiver is perfectly synchronized in time with the transmitter with an accuracy less than a chip (see Figure 4).

Figure 4: Spectrum despreading / Narrowband jammer effect The "spread" signal resists well to interferences, which have a smaller spectral width. This robustness comes simply because the information is "spread" on a frequency band rather important: only a small part of the spread signal spectrum is affected. After dispreading the useful signal, the jammer is spread, making the reception more efficient.

Code with length L

Received signal

Jammer

Spreading signal

RF frequency Baseband

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1.4 Spreading codes used in CDMA The choice of the spreading codes is directly influenced by their correlation properties, and specifically by their autocorrelation and crosscorrelation properties. From a statistical point of view, the autocorrelation is a measure of the correspondence between a code and a time shift version of it. Let’s have a code ci with a length M and ci = [ci(0), ci(1),..., ci(M-1)]. We define its autocorrelation function Rci (t) as follows:

Rci(t) = ci(0)*ci(t)+ci(1)*ci(1+t)+...+ci(M-1-t)*ci(M-1)

Furthermore, the crosscorrelation represents the degree of correspondence between two different codes. The mathematical expressions of the crosscorrelation function Rcicj(t) of two codes ci and cj with a length M such that ci = [ci(0), ci(1),..., ci(M-1)] and cj = [cj(0), cj(1),..., cj(M-1)], is written:

Rcici(t)= ci(0)*cj(t)+ci(1)*cj(1+t)+…+ci(M-1-t)*cj(M-1) If the crosscorrelation function between two different codes ci and cj (i≠j) satisfies Rcicj (0) = 0, we say that the two codes are orthogonal. When the codes are not completely orthogonal, a crosscorrelation residue appears between the useful user code and other users codes. This "residue" implies a kind of interferences which is called “multiple access interference”. In most radio systems based on CDMA, the codes used to perform the spreading process are two kinds of codes: the Walsh-Hadamard or “orthogonal” codes and the "pseudo-random" codes. 1.4.1 Walsh-Hadamard codes Orthogonal codes used in CDMA are chosen among those proposed by Walsh in 1923. There are different ways of generating Waslsh sequences. The generation of Hadamard matrix is one of the most famous. Hadamard matrixes are always square and can be recursively generated: H2M = HM HM

HM -HM

where M is of power of 2. For example, if we want to have 4 orthogonal codes c1 , c2 , c3 et c4, we have to generate an order 4 Hadamard matrix. So we get: H1 = +1 H2 = +1 +1 +1 -1 H4 = +1 +1 +1 +1 +1 -1 +1 -1 +1 +1 -1 -1 +1 -1 -1 +1 where c1 = [+1 +1 +1 +1], c2 =[+1 -1 +1 -1], c3 =[+1 +1 -1 -1], c4 =[+1 -1 -1 +1]. Because of this method of generation, the Walsh codes are sometimes called the Walsh-Hadamard codes.

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A Walsh-Hadamard code ci has the following characteristics: 1) the first element ci is always +1, for all i ; 2) ci has the same number of + 1 and -1 (excepted c1) ; 3) The intercorrelation function of two codes satisfies:

Rcicj (0) =0, for all i≠j ; 4) The crosscorrelation function Rci(0) of the code ci satisfies:

Rci (0) = M

� Drawback of orthogonal codes : We assume that the receiver knows the beginning of the code to perform the corresponding crosscorrelation function Rcicj(0). But in practice, the receiver must perform a synchronization phase before proceeding the extraction of the signal. Furthermore, in a multipath channel, even if the receiver is perfectly synchronized in time, for example on the most powerful path, the other paths are not received at the same time and the orthogonality between different copies of the signal is lost. The result is a kind of interference which may be more or less inconvenient and is called “inter-path interference”. In these conditions, a major drawback of Walsh-Hadamard codes is that they have poor autocorrelation properties. Furthermore, we can see that the same problem occurs between two distinct Walsh-Hadamard codes, which means that these codes do not have good crosscorrelation properties. For example, if we consider the code c3 = [+1 -1 -1 +1] one chip shifted to the right, c'3 = [+1 +1 -1 -1], the crosscorrelation function between this code and c2 = [1 + 1 -1 -1] gives

Rc2c’3(0) = (+1 +1)+(+1 +1)+(-1 -1)+(-1 -1) = 4 In other words, c2 and c'3 codes are not orthogonal. 1.4.2 Pseudo-random codes A pseudo-random code is a binary code whose statistical properties are similar to those of a random binary sequence, but a pseudo-random code is periodic and the larger is its period, the more similar to a true random binary signal it is. A binary shift register is one of the most common ways to generate pseudo-random codes (see Figure 6). Its operation is the following: once initialized the different states of the register, the bit output is calculated at each clock time by adding modulo 2 all bits present in each state. The bits are then circular shifted to restore the states and calculate the next output bit. The generated sequence depends on the initial state of the register, the number of registers n and the values (αi): either 1 or 0.

Figure 6: Shift register scheme with 4 states feedback (1+X2+X4)

Output

Input

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This register whose full name is “linear feedback shift register (LFSR)” is periodic because, whatever the initial values, ie the values (αi), we find the same values after a finite number of clock time. So the sequence period is not greater than 2n. A binary sequence with a period P = 2n -1, generated with an LFSR register is called a “m-sequence” or maximum length sequence. The generator of m-sequences can also be represented using a polynomial notation:

f(x) = Xn+αn-1Xn-1+ αn-2X

n-2+...+α1X+1 The interest of m-sequences in their use CDMA derives the following properties:

• the m-sequences are balanced. The number of 1 is equal to the number of 0 plus one. In the frequency domain, this property provides a symmetric spectrum with a DC value close to zero. • the autocorrelation function of an m-sequence (binary) ui can have two values over a period P = 2n – 1: Rvi(t) = P if t = 0 et -1 if t ≠ 0, where vi = 1 - 2ui

Autocorrelation peak becomes more pronounced when the period P is increasing and therefore the autocorrelation function becomes closer to that of the autocorrelation function of a white noise (this function is zero for t ≠ 0). The crosscorrelation properties between two m-sequences are not good because some quite pronounced peaks may appear. So, two m-sequences generated or not by the same primitive polynomial are not orthogonal. To improve the crosscorrelation properties of m-sequences, another kind of codes was proposed: the Gold codes. The main quality of Gold codes is related to the fact that the crosscorrelation function between two codes is uniformed and bounded. Gold proposes to add modulo 2 two m-sequences with the same length, generated from two different primitive polynomials. If the period of the two sequences is 2n -1, the resulting sequence will also have a period 2n -l. Considering all the shifts between the two m-sequences, we can generate 2n -1 different codes, and therefore, with two shift registers, we can generate a total of 2n +1 codes (the 2n -1 previous codes plus the two generating m-sequences). Gold shows that some pairs of m-sequences have crosscorrelation peaks whose amplitude is equal to or less than the minimum crosscorrelation value between all pairs of m-sequences with the same length. This pair of m-sequences is called “preferred pair”. Gold also demonstrates that the crosscorrelation function of a “preferred pair” is predictable and can take three values: -f (n), -1 and f(n)-2, where f(n) = 1+2(n+1)/2 for n even and 1+2(n+2)/2 for n odd. 1.5 Spreading codes used in UMTS In UTRA / FDD and UTRA / TDD, the spread spectrum is performed using two codes: one of Walsh-Hadamard and the other of Gold. The function of each code depends on the link (up or down) (see Figure 7).

Figure 7: Channelization code et scrambling code

Data [bps]

Channelization code (Walsh-Hadamard)

Chips [cps]

Scrambling codes (Gold)

Chips [cps]

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1.5.1 Channelization codes Channelization codes, also called orthogonal codes or spreading codes, allow the transition from symbols to chips, ie from a narrowband signal to a wideband signal. The number of chips per information symbol is called the Spreading Factor SF. The spread function is performed by multiplying, at the chip rate, the real (I component) and imaginary (Q component) of each symbol with the allocated channelization code (length SF). In the downlink, channelization codes identify the information symbols of each user in a cell. In the uplink, they help to differentiate the information symbols from the channel or dedicated physical data channels (DPDCH) and dedicated physical control channels (DPCCH) belonging to the same user. In the downlink, it is possible to use orthogonal channelization codes to identify users, because the network decides when the information is transmitted to the mobile (the spread signals are synchronous). In the uplink, the transmission from the mobile to the base station is not synchronized and signals that have been spread with the channelization codes are no longer orthogonal from one user to another. Also, we prefer to use scrambling codes to distinguish users due to their crosscorrelation properties. Moreover, in the uplink, different users in the same cell can use the same channelization code to transmit: it is the scrambling code that identifies them. The symbol rate Rs is:

Rs = Rc/SF where Rc is the chip rate which is constant and equal to 3.84 MHz. Because of the wide variety of services and therefore, dataflow, it is necessary to generate channelization codes with a variable SF while preserving their orthogonality. To do this, we use the OVSF method (Orthogonal Variable Spreading Factor). The codes generated are called OVSF codes. OVSF codes have the particularity of being an orthogonal codes family in the sense of correlation. They are defined by a generator tree. The principle of this tree is similar to the Walsh-Hadamard matrices of (Figure 8):

Figure 8: OVSF codes generation One root voice generates two branches. The code of a branch is composed of the code of the root and its complementary. This principle is used to generate the OVSF code tree used for the UTRA (see Figure 9).

Root 2 branches

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Figure 9: OVSF tree

In direct sequence spreading (DS-CDMA), the code length is equal to the spreading factor SF. Furthermore, the factor k, which determines the number of bits in the DPCH frame, verifies the following equation:

SF = 256/ 2k with k = 0 .. 6. This means that SF can take the values: 4, 8, 16, 32, 64, 128, 256 in FDD. This tree (see Figure 9) shows the direct link between the number of codes available for a given spreading and the spreading factor. Indeed, the factor SF simultaneously determines the code length but also the number of codes available for a spreading SF. To use codes, it is necessary to respect certain rules. In a single cell, the OVSF codes can be used all at the same time because they are not all orthogonal to each other. The code of one branch is strongly linked to its root. So, a rule for the use of CDMA codes is defined:

when a code is allocated, all the codes from its branches can not be used (see Figure 10).

Figure 10: OVSF code allocation

In figure 10, the code C16,1, 1 is allocated, thus all the codes from it may not be used: C32,1, C32,2, etc.. But root codes of C16,1 1 in the tree can not also be used. Indeed, if we used C4,1, for example, the codes from these branches would no longer be usable, so C16,1 could not be used. This rule for the use of CDMA codes provides a strong constraint on the availability of high rate channels. This rule indicates that the number of simultaneous users in downlink for a service of 2 Mbps (SF = 4) can not exceed 4 users.

This code can not be used in the cell

Used spreading factor

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1.5.2 Scrambling codes The signal spread by the channelization codes is multiplied chip by chip by another scrambling code. This code is a pseudo-random codes whose crosscorrelation properties are higher than those obtained by the channelization codes. Scrambling codes, which are not orthogonal, are derived from Gold sequences (see Figure 11). For both uplink and downlink, each scrambling code chip is represented by a complex value.

Figure 11 : Scrambling code generator In the downlink, the scrambling codes are used to identify a cell. There is therefore a unique scrambling code per cell. With the scrambling code, it is possible to reuse the OVSF tree in the same geographical area. Furthermore, the use of the scrambling codes makes the statistical properties of interference close to those of a Gaussian noise, (this interference is caused by neighbouring cells that emit on the same carrier frequency). The scrambling code can therefore help the receiver to detect the transmitted symbols. In the uplink, the scrambling codes are used not only to improve the statistical properties of the channelization codes but also to differentiate each user in the same cell. A summary of the use of codes is given in Figure 12.

Figure 12: Use of channelization codes (OVSF) et scrambling codes (SC) in

UMTS cells

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2. UMTS radio transmission chain 2.1 Radio characteristics The main radio characteristics of the UMTS/FDD system are given in the following tab.

Multiple access technique FDMA/CDMA

Duplexing mode FDD

Frequency hopping Not neccessary

Chip rate 3.84 Mcps

Spreading Factor From 512 to 4 in DL and from 256 to 4 in UL

Carrier frequency spacing 5000 kHz

Frequency spectrum in Europe (MHz) 1920-1980 in UL and 2110-2170 in DL

Carrier duplex space between UL and DL 190 MHz

Data modulation BPSK in UL and QPSK in DL

Radio modulation QPSK

Power control frequency 1500 Hz

Frame duration 10 ms

Slot duration 10/15 = 0.667 ms The time unit in a UMTS network is given in number of chip time which is constant and equal to 1/3.84 MHz = 0.26 µs. The following metrics are also referenced to determine the physical channels duration (see Figure 13):

• A radio frame is a period of 15 slots (or 38,400 chips or 10 ms). This temporal metric is fundamental for the coding and the channel interleaving.

• A slot is a fixed length time interval equal to 2560 chips. The number of bits per slot is variable.

Figure 13: Frame and slot

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2.2 Modulation of Dedicated physical channels 2.2.1 Dedicated physical channels in the uplink There are two kinds of dedicated physical channels in the uplink: the Dedicated Physical Data Channel DPDCH and the Dedicated Physical Control Channel DPCCH. In a single radio link, there may be zero, one or more DPDCH and always a single DPCCH. Several DPDCH is used by the mobile in case of multicode transmission. The DPDCH transmits the dedicated transport channels information, while the DPCCH carries the signal or control information generated by the physical layer and the TFCI bits provided by the MAC layer. The control information contained in the bit fields of the DPCCH included (see Figure 14):

• The pilot bits known to the receiver of the base station. The BS uses these bits to estimate the impulse response of the propagation channel. The pilot bits are also used to estimate the signal to interference ratio of in the radio link. • An identifier of the combination of transport formats represented by the bits TFCI (Transport Format Combination Indicator). By using these bits, the base station determines the attributes that the data bits conveyed by the DPDCH were processed by the transport channels (multiplexing and channel coding operations). • The feedback bits FBI (FeedBack Information). • Power control commands represented by TPC (Transmit Power Control) bits. The mobile uses these bits to request the node B to increase or decrease the transmission power of DPCH in order to guarantee a certain level of quality in the received signal. There may be 1 or 2 TPC bits in a slot, bits that are transmitted and estimated slot by slot, so base station power control is performed with a frequency of 1500 Hz

Figure 14: Dedicated physical channels in the uplink The DPDCH and DPCCH are transmitted in parallel, the first on the I component, the second on the Q component. The information on each component is spread with a different channelization code allowing the UTRAN to distinguish the information transmitted by the data channel and the information carried by the control channel. The upper layers of the mobile choose the SFvalue and choose the channelization code in the OVSF code tree. It is then used to spread each symbol. The selection of the OVSF code of DPDCH is restricted to CSF,SF/4 while the DPCCH channel is always spread with code C256,0. The complex scrambling operation is applied with the sequence Cn (i), i = 0 ... 38399 (see Figure 15).

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Figure 15: Example of an uplink processing chain

The modulation that consists of transmitting data and control information in the independent I and Q components is sometimes called dual-channel QPSK modulation. It's equivalent in practice to the combination of two BPSK modulations - the one being applied to the binary information of the DPDCH and the other binary information of the DPCCH. In most of cases, the number of bits per slot of the DPDCH or DPCCH depends on the binary datarate that each of these two channels transmits, and it depends on the spreading factor SF following the relationship:

N = 2560/SF 2.2.2 Dedicated physical channels in the downlink In contrast to the uplink, there is only one type of dedicated physical channel in the downlink called DPCH (Dedicated Physical Channel). This channel transmit the transport channel DCH information (see Figure 16) - information which may be data traffic or signaling traffic generated by the upper layers. It also transmits control information generated by the physical layer itself, and it can be considered as a time division multiplexing of a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH). The data bits are separated over two fields of bits inside the DPDCH slot: Data 1 and Data 2. In the control (DPCCH) part, we find the same bits of information as in the uplink with the exception of FBI bits.

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Figure 16: Dedicated physical channels in the downlink The number of bits in the DPCH slot is computed by the expression:

N = 5120/SF

Figure 17: Example of a processing chain in the downlink The even bits of the DPCH are distributed along the I component, while the odd bits are transmitted on the Q component (see Figure 17). The symbols are then created whose rate is twice less than the bit rate. The same channelization code is used to spread the symbols on the two components

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2.3 Signal pulse shaping After spreading and scrambling, the resulting complex signal, composed of chips, is distributed on a real and an imaginary branch known as I and Q components. Then it is filtered with a digital filter and converted into an analog signal with a digital-to-analog converter. The signal is filtered again before being modulated in quadrature and transposed on a carrier frequency f0. The digital filter is a raised cosine (RRC for Root Raised Cosine) filter with a factor β = 0.22 called roll-off factor. This factor determines the shape of the output spectrum, because the energy of the filtered signal is concentrated in a band BW=Rc(1+β), where Rc is the chip rate. The impulse response of the RRC filter is an approximation of the Nyquist filter whose aim is to reduce the problems of inter-symbol interference duringthe signal sampling.

RRC filter impulse response as a function of β

RRC filter frequency response as a function of β

Once filtered, the analog signal is then modulated and transposed on a carrier using a QPSK modulation. This kind of modulation allows the transmission of two bits per symbol, or two chips per symbol (the binary signal has already been spread). The phase of the modulated signal takes different values that represent a single pair of chips.

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Glossary

3GPP 3rd Generation Partnership Project

ADS Advanced Design System

ADSL Asymmetric Digital Subscriber Line

BCH Broadcast Channel

BER Bit Error Ratio

BLER Block Error Ratio

BPSK Binary Phase-Shift Keying

BW Bandwidth

CCPCH Common Control Physical Channel

CDMA Code Division Multiple Access

CPICH Common Pilot Channel

DL Down Link

DPCCH Dedicated Physical Control Channel

DPCH Dedicated Physical Channel

DPDCH Dedicated Physical Data Channel

DS Direct Sequence

FDD Frequency Division Duplex

GSM Global System for Mobile communications

IMT International Mobile Telecommunications

MMS Multimedia Messaging Service

MS/ME Mobile Station / Mobile Equipment

NODE B equivalent to the Base Station in GSM

OVSF Orthogonal Variable Spreading Factor

QPSK Quadrature Phase - Shift Keying

RF Radio Frequency

RRC Root-Raised-Cosine

SNR Signal to Noise Ratio

SC Scramble Code

SCH Synchronization Channel

SF Spreading Factor

TDD Time Division Duplex

TDMA Time Division Multiple Access

UL Up Link

UMTS Universal Mobile Telecommunications System

UTRAN UMTS Terrestrial Radio Access Network

VSA Vector Signal Analyzer

WCDMA Wide band Code Division Multiple Access

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Simulation 1 – 1: UMTS Spreading code Q1 Open the simulation scenario “Spreading_Code” (File - Open Project – “UMTS_prj” - networks – “Spreading_Code.dsn”), pay attention first to the different variables of the system and justify the choice of these parameters:

TimeChip = 1/3.84e6, SF = 4, TimeSymbol = TimeChip * SF Detail now the functionality of the two blocs “Bits” and '”GPPFDD_OVSF”, and explain the scenario. Q2 Run simulation ( “Simulate” or F7), the results can be viewed by opening a window “New Data Display Window”. It is possible to recharge a predefined data representation (File - Open – “Spreading_Code.dds”). Compare the shape of the digital signal before and after spreading, which spreading codes are used? Simulation 1 – 2: UMTS orthogonal spreading codes Q1 Open the simulation scenario “Spreading_Code_Ortho”. What are the characteristics of OVSF codes? Q2 What is the rule for allocating OVSF codes in a single cell? Q3 Run simulation and observe the results. What is the chip rate used in the transmission? What is symbol rate? Q4 Watch now in the receiving part, explain the use of blocks “Correlator”, “Downsample” and “Quant”. Q5 Try now to add a fourth user in the scenario. Give at least two possible OVSF codes for this new user. Run simulation and check the good implementation of the system. Simulation 1 – 3: UMTS orthogonal spreading codes Q1 Open the simulation scenario “Spreading_Code_Autocorr”. At the reception side, we see that a signal is mixed with several time shifted versions. What is the name of this phenomenon in wireless communications? Q2 Launch simulation and compare the results: why can’t we recover the right information in the presence of multipaths? Which “bad property” of OVSF codes is linked with this problem?

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Q3 What codes are used in UMTS to improve the statistical properties of OVSF codes? Simulation 1 – 4: UMTS scrambling code Q4 Open the simulation scenario “Spreading_Code_Scrambler_LFSR”. What is the new element introduced in the chain of transmission? Q5 The length of used register is equal to 18 while the initial state is equal to 1185. What is the primitive polynomial of the register? What is the period of the generated sequence? Q6 Run simulation and compare results with those of 1 - 3, comment.

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21

Simulation 1 – 5: Codes and Multi-user in UMTS Q1 Open the simulation scenario “Multi_Access_DL”, what key parameter is used to multiplex several users in UMTS downlink? Justify your answer. Q2 Open the simulation scenario “Multi_Access_UL”, what key parameter is used to multiplex several users in UMTS uplink? Justify your answer. Q3 Run the two simulations and compare the error rate in the presence of noise. Explain this difference. Simulation 2 – 1: UMTS Up Link Constellation Q1 Open the simulation scenario “Constellation_UL” and start the simulation. Watch the constellation of a single DPDCH or DPCCH channel. What are the possible states (a gain of 0.5 is applied to both channels to maintain the 0dB signal)? Is it a BPSK or QPSK modulation in respect of each channel? (the scrambling code used in a UMTS system is in the complex form “± 1 ± j”) Q2 Watch now a constellation of a two channels DPDCH + DPCCH multiplexing. By making a small calculation, find all the possible states in the case of a combination of two signals with an equivalent power gain. Q3 In the case of a combination of the two signals with different power gain, a 6dB attenuation is applied to the DPCCH channel (Q component). What is the effect of a complex scrambling on the modulated signal? Simulation 2 – 2: UMTS Down Link Constellation Q1 Open the simulation scenario “Constellation_DL”. In order to better visualize the states distribution on the constellation, a background noise is added to the signal. Compare the three signal sources and explain their differences. Q2 Launch simulation, what is the modulation mode used in DL? Q3 Compare the constellations achieved with the combination of the correlated and non-correlated sources, pay attention to the probability of occurrence of different states, what do you see?

Q4 Explain this difference.

22

Simulation 2 – 3: UMTS pulse shaping filter Q1 Open the simulation scenario “Filter” and locate the two blocks of QPSK modulation, what is their difference? Q2 Launch simulation and watch the impulse response of this filter, which advantage presents a RRC filter compared to classical low-pass filter? Q3 Watch now the modulated signal in the VSA. Compare the temporal signal with and without filtering. Then measure the length of a chip Tc with a marker. (What signal do you choose to make such a measure?)

Signal without filter Signal with filter

Q4 Now use a digital demodulation to study the signal. Click on “MeasSetup” –“Demodulator” –“Digital demod” to activate this element. Then go in “MeasSetup” – “Demod Properties” – “Format” – “Preset to Standard” and choose “Cellular” – “W-CDMA”. Place in a first window the signal constellation (“IQ Meas Time”). Compare the result with the two signals, what do you see? (No background noise in this scenario.) Q5 Display in a second window the signal spectrum (Spectrum). What is the factor β used by the RRC filter? Compute the theoretical frequency band of the filtered signal, and compare with the value measured by the VSA (Occupied Band Width). Why do we need to filter the signal noting that the bandwidth of a UMTS channel is 5 MHz?

Q6 Change now the factor β from 0.22 to 1 and restart the simulation. Do not forget to change the configuration of digital demodulation in VSA (“MeasSetup” – “Demod Properties” – “Filter” – “Alpha / BT”). Measure the new frequency band occupied by the signal and compare with the constellation of the previous questions. What is the influence of this parameter on the modulated signal?