Umts fundamentals

1 Beneficiario COLFUTURO 2003
1. Appendix: UMTS
fundamental concepts
1.1 What is UMTS?
UMTS (Universal Mobile Telecommunications System) represents the
choice for the 3rd
Generation Global Mobile Communications System in
several countries/regions including almost all European countries, Japan
and Australia. 3rd
Generation mobile communication systems are intended
to provide advanced global services to the customer, either circuit-
switched (e.g. speech and new services like video calls) or packet-
switched, new mobile multimedia services (e.g. streaming/mobile TV,
location based services, Downloads, multi-user games and many more)
giving more flexibility for the operator to introduce these new services to
its portfolio and from the user point of view, more service choices and a
variety of higher, on-demand data rates compared with current 2-2.5G
mobile systems. The “global” feature means that the system is designed
to reach global coverage (if required) through the use of Satellite Links,
Macro-cells, Micro-Cells and Pico-Cells.
From the Standards point of view, UMTS is a mobile communications
system standardized by the 3GPP and the specifications are at the present
time in their Release Number 6. The mobile operators that bought 3G
licenses in Europe have already deployed their UMTS W-CDMA (Wideband
CDMA: the chosen multiple access technology for UMTS) based networks,
although the coverage is still not comparable with the currently huge,
transnational coverage of GSM-GPRS networks. In fact, at least in the first
years of deployment, UMTS networks are going to rely on GSM networks
to reach zones where there is still no UMTS coverage, using a technique
called Inter-RAT (RAT: Radio Access Technology) handovers.
About the UMTS services, some commercial services have been available
for the general customers, in the concrete case of The Netherlands, since
the last year.
1.2 Technical characteristics
Technically speaking, the radio-access part (also called “the air interface”)
is the most important difference regarding to the so-called 2-2.5G
systems (e.g. GSM, GSM+GPRS). Instead of using the FDMA-TDMA
combination (i.e. carriers and timeslots per carrier) as the access
technology like in GSM, UMTS uses Wideband-CDMA, a technology based
on the Direct Sequence (DS) Spread Spectrum principle. Direct Sequence
makes reference to the usage of a special code to separate the signals
(opposite to “frequency hopping” which is the other Spread Spectrum

A.F. COSME. UMTS CAPACITY SIMULATION STUDY
2
method and which is used for instance in Bluetooth technology). Spread
Spectrum means that because of the special signal processing method of
CDMA, the original information signal is spread in the frequency domain
within the wider frequency range of the W-CDMA channel.
In W-CDMA Systems, users belonging to a cell are separated by codes
(i.e. special sequences of bits) and not by timeslots as in TDMA (timeslots
in W-CDMA systems timeslots are not used for user separation, but to
support periodic functions, e.g. UE reception of power control commands
each timeslot).
Another characteristic of W-CDMA is that the users share the complete
frequency spectrum of near 5 MHz per UMTS channel all the time during
their communication.
In general, in CDMA systems, the way the uplink (i.e. mobile station to
base station transmission) and downlink (i.e. base station to mobile
station transmission) connections are separated is referred either by FDD
(Frequency Division Duplex) or TDD (Time Division Duplex) modes
respectively.
For the UMTS public mode (W-CDMA), the choice has been the FDD mode,
which uses different frequencies for both uplink and downlink (i.e. the
mobile transmits in one frequency and receives in another). FDD is used
for large outdoor cells because it can support more users than TDD mode.
TDD uses the same frequency but different timeslots for each type of
connection (UL-DL) and W-CDMA in TDD mode is intended to provide
private indoor low-range communications.
In practice, an operator needs 2 to 3 channels (2x5x2 or 2x5x3 MHz) to
be able to build a high-speed, high-capacity network, probably using a
layering approach, such as the so-called Hierarchical Cell Structure (HCS)
scenarios, using different carriers for micro-cells and macro-cells. In the
next graph, we can see the allocated spectrum in the concrete case of The
Netherlands [Umtsworld].
Figure 1-1: Allocated UMTS spectrum in The Netherlands
About the code usage, it is important to mention that CDMA requires two
kinds of codes for its operation: channelization (spreading) code and

scrambling code. The usage of these codes depends on the direction of
the communication (in Uplink, the transmitter is the mobile whereas in
Downlink the transmitter is the Base station). The purpose of the
channelization (spreading) codes in both UL and DL directions is to
separate channels from a single transmitter, whereas the purpose of the
scrambling codes is to separate transmitters (also applies to both UL and
DL directions).
The main difference in the frequency domain between both kinds of codes
is that the Scrambling codes don’t modify the bandwidth of the
Information Signal, whereas the channelization codes do. As this is
something very important in order to understand how UMTS works, in the
next section the code usage is explained both in UL and DL.
1.3 Channelization (Spreading) codes
In UL direction (UE transmits and Node B receives), channelization codes
are used to separate physical data and control (i.e. signaling) channels
from the same terminal. In DL (Node B transmits, UE Receives),
channelization codes are used to separate connections to different users
within one cell (users of the cell are sharing the “code tree” of that cell,
that is, the pool of DL code resources of the tree). Once a channelization
code is applied to the information signal, the Bandwidth of the information
signal changes (in frequency domain) to a higher bandwidth, in other
words is “spread” over the UMTS bandwidth channel (hereby the name of
“spread spectrum”).
In time domain, the effect is the change of rate of the information signal.
To distinguish from the Information Rate, 3GPP calls the Rate of the
channelization code as the Chip Rate, although physically the chips are
also bits (although of higher frequency (smaller period) than the data or
information bits). Therefore, after the channelization code is applied to the
information signal, the result is a signal with a bit rate equal to the chip
rate (the reference chip rate in UMTS is fixed to 3.84 Megachips / sec, and
varying the number of chips per information bit we obtain different user
speeds). The next figure helps to clarify the effects of the channelization
code in both frequency and time domain.

4
Figure 1-2: Effects of the channelization code in Time and Frequency domain
The codes used for the channelization operations must have a special
property called orthogonality. Orthogonal code means that the inner
product of the code with the codes from the other users (called cross-
correlation property) or the product of the code with a shifted version of
the code itself (called auto-correlation) has to be as small as possible.
These codes are also known as OVSF (Orthogonal Variable Spreading
Factor) codes.
For orthogonality to work, the signals must be properly synchronized in
time. That’s why in DL for instance, due to multi-path propagation, some
of the orthogonality property is lost. This is had into account in CDMA
capacity equations with the so-called orthogonality factor, which is a
factor that varies between 0 (full orthogonality, no interference) and 1 (no
orthogonality, full interference)*
.
The number of chips used for each data bit is known as the spreading
factor (SF). Also, in the frequency domain, SF = W / R, where W =
Bandwidth of the spread signal [Hz] and R = Bandwidth of baseband data
[Hz].
Summarizing, in Time Domain:
SF = Chip Rate / Data Rate coded channel (A1.1)
And also, in Frequency Domain:
SF = W / R (A1.2)
*: This definition is in line with the definition in Wines simulator documentation. However in some
other references, for instance [Holma], 0 means no orthogonality and 1 means full orthogonality.
Where Data Rate coded channel means that this data rate has into account
the overhead introduced by coding techniques and it doesn’t corresponds
Bandwidth
after spreading
f
t
1
-1
Voltage
Power
Bandwidth of baseband

directly to the information rate (unless the coding factor is 1 of course).
This is important to know because it is a common source of mistakes in
calculations.
If we have a low spreading factor it means that it is consuming more code
resources from the code tree and the bit rate is higher, for instance with
SF = 8, the data rate of the spread signal would be 480 Kbps, whereas
with SF=256, the data rate of the spread signal would be 15 Kbps.
Therefore, in Downlink, the number of codes (given by the maximum SF)
is a scarce resource that can be in shortage and therefore must be
carefully considered in any capacity analysis.
1.4 Scrambling codes
Scrambling codes separate different mobiles (in uplink) and different
Node-B cells/sectors (in downlink). This is a code that does not affect the
transmission bandwidth which was already transformed by the usage of
the channelization code. The codes used for scrambling codes are known
as Gold codes and there are two versions (long and short) depending on
the features of the terminal/Node B either one or another version is used.
In Uplink, the number of codes available is in the order of millions of
codes (that guarantees no code shortage when trying to separate the
transmitting users), but in Downlink this number is limited to 512;
otherwise the cell-search procedure shouldn’t be possible to solve in a
reasonable time.
Finally, in the reception side the same transmitter’s channelization code is
applied and that allows the receiver to reconstruct the original transmitted
signal. W-CDMA also involves a certain degree of security, in the sense
that without the transmitter’s channelization code available, it is almost
impossible to reconstruct the original signal, thus preventing tampering
attacks in the air interface.
Summarizing, the following schematic illustrates in a simple way the
process of transmission and reception in UMTS involving all the elements
mentioned.
Spreading
code
Data bits
Scrambling
code
Transmission medium Σ
Recovered
bits
RX
SIDE
TX
SIDE
Scrambling
code
Spreading
code
Figure 1-3: Simplified Transmission and Reception process in UMTS

6
As the radio access part has changed with respect to previous 2-2.5G
systems as GSM-GPRS, new methods have to be developed to estimate
capacity and coverage of the W-CDMA system.
1.5 The processing Gain, SIR (Signal to Interference
Ratio) and Eb/No concepts in UMTS [Vourekas]
Consider a single-cell CDMA system with N users where ideal power
control is applied and consequently the signal from all the users reaches
the node B demodulator with the same intensity S (figure 1-4).
The demodulator of the Node B processes one desired signal S, and N-1
interfering signals with total power equal to S*(N-1). The desired signal is
shown in the graph as a continuous line and the rest in dotted lines. The
interference sums up to (N-1)* S.
The signal-to-interference power ratio, denoted SIR, is then:
( ) ( )1
1
1 −
=
−
=
NSN
S
SIR (A1.3)
The bit energy to noise ratio, denoted as Eb/No, is obtained by dividing the
signal power by the information (baseband) bit rate, and the interference
power by the total RF frequency.
Figure 1-4: Derivation of the SIR and Eb/No relationship
N users

( ) ( ) R
W
N
W
SN
R
S
N
E rf
rf
o
B
⋅
−
=
−
=
1
1
1
(A1.4)
In the last part of equation (A1.4), the first term is equal to the signal to
interference ratio (as defined in equation A1.3) and the second term is
defined as the processing gain:
R
W
nBitRateInformatio
dBandwidthTotalSprea
G
rf
p == (A1.5)
Comparing (A1.5) with (A1.2), we see that the definition of Gp is
equivalent to the definition of SF. The processing gain is a Gain achieved
at the receiver during the de-spreading process and it is due to the fact
that the W-CDMA receiver can sum-up coherently the multiple copies of
the original data generated by the multi-path propagation, by means of a
special receiver technique known as Rake Receiver. Therefore, making the
equivalence between SF and Gp, we can say that the high data rate
transmissions have low processing gain (low spreading factor).
From the equations (A1.3), (A1.4) and (A1.5), we derive a relationship
between the SIR and Eb/No that also involves the processing gain. So after
de-spreading process:
P
o
b
GSIR
N
E
⋅= (A1.6)
In another form:
o
b
P N
E
G
SIR ⋅=
1
(A1.6.1)
Equation (1.6.1), when the quantities are expressed in dBs, becomes:
p
o
b
G
N
E
SIR −= (A1.7)
The following figure summarizes graphically this physical process.

8
RF input
Rc
W= 5MHz
Spreading
Code
W
f0
Data, Rb
f
0
RF output
W= 5MHz
W
f0
Data, Rb
Rc
Spreading
Code
f
0
EEcc//IIoo ++ GGpp == EEbb//NNoo,, iinn ddBB
Rx
antenna
Tx
antenna
Noise &
interfering
signals
W
f0
EEbb//NNoo
EEcc//IIoo
This is
negative!
oorr SSIIRRttaarrggeett ++ GGpp == EEbb//NNoo iinn ddBB
Figure 1-5: Physical meaning of SIR and Eb/No [Vourekas]
To put a practical example: consider a speech signal with a bit-rate of
12.2kbps. So Rb=12.2 kbps and Rc= 3.84 Mchips/sec. Then the processing
gain of the signal is:
( ) dB
R
R
G
b
c
p 251015.3log10
102.12
1084.3
log10log10 2
3
6
=⋅⋅=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⋅
⋅
⋅=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⋅=
After despreading, the baseband (own) signal needs to be typically a few
dB above the interference and noise power. This required signal power
density above the noise power density after despreading is designated as
Eb/No. This quantity is of capital importance because the quality targets
are always expressed as a function of Eb/No as can be seen in the
analysis presented in [Castro] where the Bit Error Rate probability is
derived in terms of this figure. As the quality targets are expressed as
function of Eb/No , the CDMA equations regarding capacity also use this
important figure as it is going to be shown in a later section.
The required signal power density below the interference power density
before despreading is designated as SIR (Signal to Interference Ratio),
and it is also known as Ec/Io (In fact, Ec/Io and Ec/No are the same thing.
3GPP just had to use different nomenclature than the IS-95 community).

For speech service Eb/No is typically in the order of 5dB. That means that
after despreading the resulting baseband signal must be 5dB above noise
in order to be successfully reconstructed at the decoder. Therefore, the
required wideband SIR must 5 dB minus the processing gain. This follows
also from equation A1.7.
SIR“target” = 5dB-25dB = -20dB.
In other words:
dBdBdB
NEGSIR obp
52520
/
=+−⇒
⇒=+
(Gains or ratios that are expressed in dBs can be added and subtracted.
In the dB scale multiplication is translated into addition).
But, what exactly does SIR of –20dB mean? It means that the signal can
be buried far below the interference. In fact for our example the chip
power density signal is 100 times smaller than the noise +interference
level.
co
o
c
o
c
o
c
EN
N
E
N
E
dB
N
E
⋅=⇒=⇒−=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⇒−=⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⋅ 100
10
1
2log20log10 2
Thus the required wideband SIR is so tolerant that the signal can be
buried in interference of a power density that is 100 times larger! Still is
that SIR good enough for the signal to be recovered. Compare this with
the 9 to 18 dBs of SIR required for good voice quality in GSM systems
[Holma].
As we have seen so far, within any given channel bandwidth (chip rate)
we will have a higher processing gain for lower user data bit rates
than for high. With high data rates some robustness of the WCDMA
against interference is clearly compromised.
Summarizing, we have to remember the equation (all quantities in dBs):
SIRtarget + Gp = Eb/No (A1.8)
Expressed in dB the received SIR is negative. It is then multiplied with the
processing gain, an addition in dB scale. If now the processing gain is not
large enough the resulting Eb/No will be too small and will not “rise” above
the interference. In case the Eb/No < 0 there is no detection at all.

10
This fact gives us the first impression of why the Interference levels in the
network are so important in the radio planning process of UMTS systems,
because if the interference level is high in some cells (because the
interference contribution of many users sharing the air interface and
probably using different data rates), then the Eb/No level of some links is
not going to be enough to make their signal to “rise” above the
interference level and therefore the call would be dropped (i.e. the
capacity in terms of number of supported users per cell is modified) and
the cell-size (coverage) would be reduced (phenomena known as “cell
breathing” effect). This is the main reason why in UMTS capacity and
coverage planning cannot be separated processes, as it can be done
for instance in other mobile systems such as GSM where first predictions
of the path loss are evaluated in order to ensure the coverage of the
desired area, and then capacity is dimensioned as a second step (capacity
in a GSM cell it is given by the number of available channels, which is a
function of the reuse factor and the number of carriers per cell [Umts-
forum6] and therefore the sensitivity level at the base stations (i.e. the
minimum power level of the incoming signal at the receiver in order to be
detected) can be assumed as a constant.
On the contrary, in UMTS the sensitivity of the base stations is a random
variable that depends on the number of users and the bit rates / services
being used at any given time, then it is clear that capacity influences
coverage and a separate planning of capacity and coverage cannot be
performed, as the interference should be taken into account already in the
coverage prediction.
1.6 UMTS Architecture (Rel99)
The following section aims to introduce shortly the network elements and
interfaces of the UMTS architecture (Release 99), including UTRAN and
Core Network. The Core Network however, it is presented here just for the
sake of the architecture completeness but its analysis is out of the scope
of this study.

Figure 1-6: UMTS Architecture (Rel-99)
UE (User Equipment)
The UE, as defined in [21.905] is the mobile equipment with one or
several UMTS Subscriber Identity Modules (USIMs). Therefore, the UE
consists of two parts, the ME which is the radio terminal itself, and the
USIM which is the “smartcard”, analog to the SIM cards of the GSM
phones but with some advanced extra-features (secure downloading of
applications, possible inclusion of payment methods, etc).
UTRAN (UMTS Radio Access Network)
UTRAN is a logical grouping that includes one or more Radio Network
Subsystem (RNS). Two of them (RNS1, RNS2) are depicted in the figure
4. A RNS is a sub-network within UTRAN and consists of one Radio
Network Controller (RNC) and one or more Node Bs. For simulation
purposes, only one RNS is simulated. In the following section, the main
components of the RNS are explained.
Node B
The Node-B is analog in functionality to the BTS in GSM networks. Its
main function is to provide the radio link between the UE and the UMTS
network. It performs radio functions related to the “air interface”, which is
the logical interface (known as Uu interface in 3GPP specifications)
between the UE and the Node B. Higher layer functions (e.g. Medium
Access Control) and control of the Node Bs is performed by the RNC.
Some of its main tasks are the implementation of Radio Resource
Node B
Node B
....
Uu
(air interface)
Node B
....
RNC
RNC
Iur
Iub
Iub
Iub
Iu-CS
Iu-PS
MSC/V GMSC
CS
Networks
(PSTN,
SGSN GGSN
PS
Networks
(Internet..)
PS-Domain
CS-Domain
HLR
UTRAN CN
...
UE
Simulation
Scope
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