UTRA TDD Overview

Telecommunications and Multimedia Unit

UTRA TDD Overview

Agostinho [email protected]

Rui Sarmento [email protected]


Outline• Introduction

– Definition, Principles and Characteristics• UTRA TDD Physical Channels Structure

– Frame, Slot and Burst• Mapping of Transport Channels to Physical Channels• Power Control• Resource Allocation

– Code Allocation Strategies (FDD)– Dynamic Channel Allocation (TDD)

• UTRA TDD Interference Scenario


Introduction• Two implementations are proposed for the UTRA (UMTS

Terrestrial Radio Access) physical layer:– Frequency Division Duplex (FDD)

– Time Division Duplex (TDD)

DownlinkUplink5MHz 5MHz

190MHz

Frequency

Time

5MHz

Downlink

Uplink

Time

Frequency

Guard Period


UTRA FDD

– Requires the allocation of two frequency bands: one for the uplink and another for the downlink.

– FDD radio units need duplexers in order to separate the incoming and outgoing signals at the antenna.

– FDD does not allocate efficiently the available bandwidth for all types of services.


UTRA TDD

– TDD mode can use the same frequency band for both the uplink and the downlink by allocating distinct time slots to the two links .

– Each time slot can be allocated either to the uplink or to the downlink.

– TDD terminals do not need duplexer hence have less hardware complexity than FDD terminals.

– TDD requires better time synchronization between the users than FDD

• the base station cannot be allowed to transmit at the same time as the mobiles stations.

• a guard period must be included in the protocol to make sure only one link is active at the same time.


UTRA TDD Characterics• Utilization of unpair band;

• Asymmetric uplink/downlink capacity allocation;• Discontinuos transmission;

• Interference between uplink and downlink;• Reciprocal channel.

Downlink

Uplink

Time

Frequency

Guard Period


UTRA TDD• Combination of TDMA and CDMA

– TD/CDMA• Direct Sequence CDMA (SS Technique)

Frequency

Time

10 ms

5 MHz

Uplink (MS -> BS)

Downlink (BS -> MS)


UTRA TDD

Frequency

Time

10 ms

5 MHz

Time

Code


UTRA TDD Physical Channel StructureTDD Frame 10ms (15 timeslots)

MS TX PartBS TX Part

Sp

read

ing

Cod

es

0,666 ms Slot

0,666*15=10ms

Burst

DATAMIDAMBLEDATA

GUARD PERIOD


UTRA TDD Frame Structure• Switching point configurations

– Multiple-switching-point (symetric DL/UL allocation)

– Multiple-switching-point (asymetric DL/UL allocation)

– Single-switching-point (symetric DL/UL allocation)

– Single-switching-point (asymetric DL/UL allocation)

10ms

10ms

10ms

10ms


Asymmetrical Capacity Allocation• Part of the slots must be fixed for

– Downlink: BCH and SCH– Uplink: RACH

• Other slots could be allocated according to need (DCA)


Physical Channels• A Physical Channel in TDD is a BURST• A BURST is a combination of DATA part, a

MIDAMBLE and a GUARD PERIOD• Several BURST can be transmitted at the same time

from one transmitter. The DATA part must use different OVSF channelisation codes (the same scrambling code)

• The duration of a BURST is one time slot


Bursts Types• Burst Type 1

– Burst Type 1 can be used for up- and downlink;• Burst Type 2

– Burst Type 2 can be used for up- and downlink;• Burst Type 3

– Burst Type 3 is used for uplink only;Training Sequence

Traffic Burst, PRACH Burst, SCH Burst


Mapping of the Transport Channels to Physical Channels


Power Control • Power Control

– Low SF – low processing gain;– Uplink – Open Loop;– Downlink – SIR based closed inner loop.


Dynamic Resource Allocation• FDD

– Code Allocation Strategies• TDD

– Channel Allocation– Resource Unit (RU) Allocation - timeslot and code

(frequency)• Resource Allocation to cells (Slow DCA)• Resource Allocation to bearer Service (Fast

DCA)


Dynamic Channel Allocation • Resource Allocation to cells (Slow DCA)

– RNC• Resource Allocation to bearer Service (Fast DCA)

– Base Station


Dynamic Channel Allocation (DCA)

Fast DCA Nk

Fast DCA N(k+1)

Fast DCA A1

Fast DCA A2

Slow DCA A Slow DCA N

RNC

BS A1

BS A2

BS Nk

BS N(k+1)


TDD Interference Scenario

BS 1 BS 2

MS 1 MS 2

T T T T R R R R R R R R R R

R T T T T T T T T TR R TRBS 1

MS 1

R R R R R R R T T T T T T T

R RT T T T T T T T R R RRMS 2

BS 2

High power

BS1 blocks MS2 in BS2


Packet Access• A packet service session contains one or several

PACKET CALLS depending on application;• PACKET CALLS constitues a BURSTY SEQUENCE

OF PACKETS;


Packet Data Traffic• Characteristics

– Session arrival process;– Number of packet calls per session;– Reading time between packets calls;

• Reading time starts when the last packet of the packet call is completely received by the user and ends when user makes a request for the next packet call.

– Time interval between two packets inside a packet call;

– Packet size.


Packet Data Traffic


Non-real Time Packet Service• Characteristics from air interface point of view

– Packet data is BURSTY;– Packet data tolerates longer delay then real-time

services;– Packets can be retransmited by Radio Link

Control (RLC).


WCDMA Packet Access• Packet allocations in WCDMA are controlled by the

PACKET SCHEDULER (PS)• Packet Scheduler Functions

– Divide the avaiable air interface


Packet Data Service• Model of Operation in WCDMA (uplink)

– Packet data can be transmited in three ways• Packet transmission on the RACH;• Packet transmission on a dedicated channel;• Packet transmission on a dedicated channel

(when there is already a dedicated channel available).


Packet Transmission on the RACH• Packet is included in the message part of the access

burst;• If there is a small amount of data to transmit;

– Short message service;– Short text-only e-mails.

• No explicit reservation is carried out;• No explicit channel assignment is needed;• Risk of collisions on the common RACH;• Not power-controlled.


Packet Transmission on Dedicated Channels• MS first sends a resource request message,

indicating what type of traffic is to be transmitted;• Network evaluates whether the MS can be assigned

the necessary resource;– YES

• A resource allocation message (RAM) is transmitted on the FACH.

• RAM consists of a set of transport formats and the specification of a dedicated channel to use for the packet transmission,

• (out of this set) MS will use one transport format to transmit the data on a DCH;


Packet Transmission on Dedicated Channels (cont.)

• EXACTLY which transport format the MS may use and at what time the MS may initiate its transmission is EITHER transmitted TOGETHER with RAM (traffic load is low) OR is indicated in a separate capacity-allocation message AT LATER TIME;

• The second alternative is used in cases where the load is high AND the MS is not allowed to immediately transmit the the packet;


Packet transmission on a dedicated channel (when there is already a dedicated channel available).

• It is used when there is already a dedicated channel available;

• MS can then either issue a capacity request on the DCH, when the MS has a large amount of data to transmit OR simply start

Documents

UTRA TDD Overview