UMTS Technology OverviewUMTS Technology Overview

22nd June 2000Andrew Coyte

AIRCOM 3g Software Project ManagerAndrew.Coyte@aircom.co.uk

Presentation structurePresentation structure• IMT-2000

• Multiple Access using Spread Spectrum

• Soft Blocking

• Services

• Soft Handover

• Conclusions

The IMT-2000 conceptThe IMT-2000 concept

The ITU World Radio Conference in 1992 identified 230 MHz, in the 2GHz band, on a world-wide basis for IMT-2000, including both satellite and terrestrial components.

Users in the 21st century should be able to seamlessly roam amongst the various networks and radio environments because of intelligent multiple mode handsets that are able to automatically determine the network and the radio environment they are processing and transparently select the appropriate mode.

The ITU-R (ITU Radiocommunication Sector) has set out the performance, service, technological and inter-working requirements of such a system. Proprietary solutions, or solutions developed by international standards bodies were then put forward for consideration by The ITU-R in June 1998.

The IMT-2000 Concept (2)The IMT-2000 Concept (2)The aim was that one such proposal would be accepted as a truly global

standard. In fact, different technologies and services are desirable in different parts of The World - largely for commercial reasons.

On 10th December 1999, the ITU adopted six sets of specifications for the Terrestrial component of the IMT-2000 solution.

UMTS is the European Telecommunication Standards Institute's (ETSI) IMT-2000 technology proposal. The UMTS proposal itself includes two technologies, both of which are included in the six ITU-adopted solutions. The rest of this presentation focuses primarily on 'IMT-DS'.

IMT -DSW CDMA

(UTRA FDD)D irect Spread

IMT -MCCDM A2000M ulti Carrier

IM T -T CUTRA TDDTimeCode

IMT -SCUW C-136

Sing le C arrier

IMT -F T DECT

Frequencyand

T ime

EvolvedG SM Evolved ANSI-41 IP Based

netw orks

IMT -2000 Harm onisatio n

Core Netw orks

Netw ork-to -Netw o rkInterfacesInter-Netw ork roam ing

IMT-2000 Terrestrial Standards (Dec 1999)IMT-2000 Terrestrial Standards (Dec 1999)

Multiple Access using Spread Spectrum (1)Multiple Access using Spread Spectrum (1)

In first and second-generation mobile communications systems, multiple simultaneous users are separated by timeslots (TDMA) or by carrier frequency (FDMA).

In CDMA, different users occupy the same bandwidth at the same time, but are separated from each other by a set of orthogonal waveforms or codes.

We can define the properties required for these orthogonal codes mathematically [1] :

• The cross-correlation should be zero or very small

• Each sequence in the set has an equal number of 1s and –1s, or the number of 1s differs by the number of –1s by at most 1.

• The scaled dot product of each code should be equal to 1.

Multiple Access using Spread Spectrum (2)Multiple Access using Spread Spectrum (2)

c1(t)

m 1(t)

c2(t)

m 2(t)

m 1(t) c1(t)

m 2(t) c2(t)

c2(t)

c1(t)

Transmitters Channel Receiver

M edium

Multiple Access using Spread Spectrum (3)Multiple Access using Spread Spectrum (3)

Each users message [m1(t), m2(t)] is transmitted simultaneously but spread with an orthogonal code [c1(t), c2(t)]. By re-applying the codes to the combined signal, the receiver can completely recover the two separate messages.

Note that the spreading code runs at a much higher rate than the user’s message (baseband data). The clock rate of the spreading code is called the CHIP RATE. The ratio of the chip rate to the baseband data rate is called processing gain (W / R).

Soft Blocking (1)Soft Blocking (1)

Consider the situation in the frequency domain.

The user’s baseband data signal spectrum is narrowband.

The spreading code spectrum is wideband.

The baseband signal is spread out across the bandwidth of the spreading code signal, when it is multiplied by the spreading code in the time domain.

Baseband signal

Frequency

Power

Baseband signalbandwidth = 8 kHz(assume simplevoice service)

Soft Blocking (2)Soft Blocking (2)The frequency spectrum of the baseband signal after spreading is shown below. Because all users share the same spectrum in CDMA systems, every user

appears as noise to every other user.

Power

Frequency

Spread signal

Carrier bandwidth = 4.096 M Hz

Background no ise

Signal after spreading

Soft Blocking (3)Soft Blocking (3)If we have N users all transmitting in the band together, the radio spectrum as seen

by the receiver will look like this:

The diagram shows that CDMA communication channels operate at a negative signal-to-noise ratio.

Spread signal - N usersFrequency

Power

User N

User 4User 3User 2User 1

..

Soft Blocking (4)Soft Blocking (4)When a particular orthogonal code is applied to this ‘crowded’

spectrum, the signal is recovered and once again has a positive signal-to-noise ratio.

Frequency

Power

De-spread signal

Positive SNR

Soft Blocking (5)Soft Blocking (5)It can be seen that as more and more user’s mobiles transmit in the

band, a point will be reached where there is too much noise for any additional mobiles to be supported – they will not be able to transmit at a high enough power to overcome the noise. At this point, new users are blocked from initiating new connections.

When this occurs depends on the location of each active user and how much power they are transmitting (which in turn depends upon what service they are using and also the environment they are in – pedestrian, moving train, city street, etc.).

It is important to remember that noise experienced at the base station is caused by mobiles in the home cell and mobiles in neighbouring cells. This effect is called cell loading.

Services (1)Services (1)In CDMA Systems, we use the communications link metric Eb/No or

“Energy per bit per noise power density” to express a the radio link requirement

Achieved Eb/No relates to the conventional signal-to-noise ratio by the following equation [1]:

Eb Energy per bit

No Noise power density

S Average signal modulating power

N Total noise power

W Transmitted bandwidthRW

NS

NEb

0

R Bit rate (baseband)

Services (2)Services (2)In UMTS, services will be available which operate at different data rates

- e.g. 384 kbit/s mobile video, 8 kbit/s voice etc. The system will also support services that can change data rate dynamically during a session.

As the bit rate R increases for a particular connection, so the transmitter signal power S must increase in order to maintain Eb/No.

In the uplink, noise N will comprise of thermal noise, noise from other communication systems and all other ‘local’ users of the same system (i.e. home and neighbouring cells).

Services (3)Services (3)The following service parameters will be specified (separately for uplink

and downlink) by the network operators:

• Required Eb/No for coherent detection in the receiver

• Acceptable Bit Error Rate (BER)

• Acceptable delay

• Range of supported data types

• Data Transfer mode • Circuit switched (voice and data)

• Packet switched (for unconstrained delay packet data transfers)

Soft Handover (1)Soft Handover (1)CDMA-type mobiles continuously search for neighbouring cells and

report their received levels back to the current cell. When a neighbouring cell’s signal is strong enough, the network will instruct the mobile to add the new cell to it’s active set.

The subscriber’s connection is maintained between the mobile terminal and all cells in it’s active set. When the communications link is maintained between the mobile and more than one cell, the user is in Soft Handover.

Soft Handover (2) - Set MaintenanceSoft Handover (2) - Set MaintenanceSource cell A Target cell B

Pow er (dB)

User Equipm ent

distance (km )

TA D DTD R O P

Cell A in active setCells A & B in

active set(Soft Handover)

Cell B in active set

Soft Handover (3)Soft Handover (3)

Soft handover has the following advantages over GSM style ‘hard’ handovers:

• A much smoother transition is experienced as the user moves between cells – calls are less likely to be dropped (but not impossible)

• Both the mobile and the base station equipment can combine the different data streams to reduce bit error rate (BER)

• The User Equipment can transmit at lower power on cell boundaries than otherwise

required, because it is received by multiple base stations.

The disadvantages of Soft Handovers are:• Additional radio resources (channels) are required in the network

• The planning and design of networks is more complex

ConclusionsConclusionsBenefits of CDMA systems:• Simple frequency planning - only one frequency!

• Imperceptible Soft Handovers

• More efficient use of radio spectrum (channels are defined by codes, not by frequency channels with guard bands)

• Good resistance to frequency selective fading

Disadvantages• More difficult to plan - cell breathing, soft handover, etc.

• Near far problem - users close to BS can ‘block’ remote users

• Partial correlation - channelisation codes need to be synchronised to maintain orthogonality

• Traffic modelling for packet switched services is very difficult

• Difficult ‘branches’ of CDMA - cdmaOne, cdma2000 and IS-95