Radio Interface - Técnico Lisboa - Autenticação · PDF file•...

Mobile Comms. Systems

Radio Interface


• The transmission of bidirectional information in duplex systems (uplink - UL - and downlink - DL -channels) can be done by dividing in:

• frequency: UL and DL channels occupy different frequency bands – FDD (Frequency Division Duplex);

• time: UL and DL channels occupy different time windows – TDD (Time Division Duplex).

Multiple Access TechniquesMuAT (1/23)


• FDD is characterised by:• allowing simultaneous transmission in both

ways;• requiring filters with a good rejection in the

adjacent band;• requiring, in general, the use of duplex filters.


[Source: Rappaport, 1996]

UL DL


• TDD is characterised by:• doing a sequential transmission in both ways;• requiring synchronism;• not requiring duplex filters.



UL DL


• The use of a duplex division technique may depend on the multiple access technique used for the system.

• In order to have efficient systems, each user should have a channel allocated to him/her only when it is requested, and when it is needed.



• There are 3 basic multiple access techniques:• frequency: allocation of a carrier – FDMA

(Frequency Division Multiple Access)• time: allocation of a time-slot – TDMA (Time

Division Multiple Access)• code: allocation of a code – CDMA (Code

Division Multiple Access)



• FDMA is the classic scheme, used in analogue systems, in which each user occupies a carrier during the connection.




• The main characteristics of FDMA are:• single channel per carrier;• continuous transmission;• low inter-symbol interference;• low information overhead;• simple user equipment;• high BS cost;• use of duplex filters;• perceptible handover.



• In TDMA, each user has access to the whole band, a time-slot being periodically allocated to him/her, in a frame structure.




• TDMA requires the use of a hierarchy of frames and time-slots.




• The main characteristics of TDMA are:• multiple channels per carrier;• burst transmission;• synchronism required;• narrow- or wideband systems;• high inter-symbol interference;• high information overhead;• complex user equipment;• low BS cost;• duplex filters not required;• efficient handover.



• Time-slots synchronism is essential.


[Source: Shankar, 2002]


• CDMA is based on a spread spectrum technique, in which each user has access to a code.




• User’s information is associated to a code:• the transmission scheme is


[Source: Yacoub, 1992]


• in transmission, one has




• the reception scheme is




• in reception, one has




• CDMA leads to a high increase in bandwidth.




• The main characteristics of CDMA are:• “unlimited” channels per carrier;• (very) wide bandwidth;• very high inter-symbol interference;• interference and capacity are user dependent;• low sensitivity to fast fading;• power control is essential;• low (control) information overhead;• still a new technology.



• GSM uses TDMA




• GSM’s time-slot structure is:




• UMTS uses CDMA, with 2 types of codes:• channelisation;• scrambling.


Data

Channelisation code

Scrambling code

Bit rate Chip rate Chip rate


• The two codes have different functionalities.

1 chip = 240.4 ns


NoYesSpreadingGold or S(2)OVSFFamily

DL: 512UL: > 1 000 000

Spreading factorNumber

38 400 chipDL: 4 – 512 chipUL: 4 – 256 chip

Duration

DL: sectorseparation

UL: MTseparation

DL: MTseparation

UL: channelseparation

UseScramblingChannelisation


• The OVSF (Orthogonal Variable Spreading Factor) code tree defines the spreading.


[Source: Holma and Toskala, 2004]

SF = 1

SF = 2 SF = 4

SF = 2k, k = 2,3, ..., 16


• There are 3 basic types of channels:• radio channel: channel associated to a carrier

frequency;• physical channel: channel transporting any kind

of system information, associated to a carrier, time-slot, or code;

• logical channel: channel transporting a specific kind of system information.

Channel DescriptionChDe (1/23)


• Concerning content, one distinguishes:• traffic channels: contain users’ information, e.g.,

voice, data, and video;• control channels: contain system’s information,

e.g., signalling, synchronism, control, and identity.

• Concerning addressing, one distinguishes:• common channels: exchange of information

between the BS and TMs in general;• dedicated channels: exchange of information

between the BS and one or several (specific) TMs.



• Radio channels:• come from the division of the spectrum band

into Nrc intervals;• are separated by Δfcs;• have a bandwidth of Δfrc.


f

fc -1 fc fc +1

Δf lb

Δf sc Δf sc

crΔfrc

Δfcs Δfcs

fc-1 fc fc+1


• GSM radio channels have:Δfcs = 200 kHzΔfrc = 200 kHzwith

• 900 MHz bandUL: [890, 915] MHzDL: [935, 960] MHz

• 1800 MHz bandUL: [1 710, 1 785] MHzDL: [1 805, 1 880] MHz



• GSM radio channels mask:


[Source: Steele, 1992]

MT:2WMT:5W

BS:20WBS:2W


• GSM radio channels spectrum:


[Source: Mouly and Paulet, 1992]


• UMTS radio channels have:Δfcs = 5 MHzΔfrc = 4.4 MHzwith

• FDD modeUL: [1 920, 1 980] MHzDL: [2 110, 2 170] MHz

• TDD modeUL/DL: [1 900, 1 920] ∪ [2 010, 2 025] MHz



• UMTS radio channels spectrum:


[Source: Lempiäinen and Manninen, 2001]


• Physical channels transport information in the radio interface, depending on the type of multiple access:

• in analogue systems, they correspond to radio channels;

• in GSM, they correspond to time-slots in a carrier;

• in UMTS, they correspond to codes with different characteristics, depending on the link way.



• UMTS physical channels are:• DL:

• P-CCPCH – Primary Common Control Physical Channel

• S-CCPCH – Secondary Common Control Physical Channel

• DPDCH – Dedicated Physical Data Channel

• DPCCH – Dedicated Physical Control Channel

• PDSCH – Physical Downlink Shared Channel



• UL:• PRACH – Physical Random Access Channel• DPDCH – Dedicated Physical Data

Channel• DPCCH – Dedicated Physical Control

Channel• PCPCH – Physical Common Packet Channel



• In UMTS, the scheme of physical channels is:


PRACHPCPCHPDSCH

BSDPCCHMTDPDCH

S-CCPCHP-CCPCH


• Logical channels are associated to traffic and control contents, defining the transfer of a specific type of information between the MT and the network.

• The complexity of logical channels has been increasing along the several generations.



• GSM logical channels are:• TCH – Traffic Channels (UL/DL):

• CS• TCH-S – Speech (11.4, 22.8 kb/s)• TCH-D – Data (9.6, 4.8, 2.4 kb/s)

• PS (GPRS)• PDTCH – Packet Data Traffic Channel



• CCH – Control Channels:• BCCHs – Broadcast Control Channels

• BCCH – Broadcast Control Channel (DL)• FCCH – Frequency Correction Channel

(DL)• SCH – Synchronisation Channel (DL)

• CCCH – Common Control Channels• PCH – Paging Channel (DL)• AGCH – Access Grant Channel (DL)• RACH – Random Access Channel (UL)



• DCCH – Dedicated Control Channels (UL/DL)

• SDCCH – Stand-alone Dedicated Control Channel

• FACCH – Fast Associated Control Channel

• SACCH – Slow Associated Control Channel



• In GSM, the scheme of logical channels is:


DCCHRACHAGCH

BSPCHMTCCCHBCCHs

TCH


• UMTS logical channels are:• BCCH – Broadcast Control Channel (DL)• PCCH – Paging Control Channel (DL)• CCCH – Common Control Channel (UL/DL)• CTCH – Common Traffic Channel (DL)• DCCH – Dedicated Control Channel (UL/DL)• DTCH – Dedicated Traffic Channel (UL/DL)



• In UMTS, the scheme of logical channels is:


DCCHDTCH

BSCTCHMTCCCHPCCHBCCH


• In UMTS, transport channels establish the interface between the UE and the RNC.


RNCNode BUE

Logical Channels

Transport Channels

Physical Channels


• UMTS transport channels are:• DL

• BCH – Broadcast Channel• PCH – Paging Channel• FACH – Forward Access Channel • DCH – Dedicated Channel (UL/DL)• DSCH – Dedicated Shared Channel

• UL• RACH – Random Access Channel • DCH – Dedicated Channel (UL/DL)• CPCH – Common Packet Channel



• UMTS DL channel mapping is:


Physical channels

PDSCHDPCCHDPDCHS-CCPCH

P-CCPCH

Transport channels

DSCHDCHFACHPCHBCH

Logical channels

DTCHDCCHCCCHCTCHPCCHBCCH


while in UL:


Physical channels

PCPCHDPCCHDPDCHPRACH

Transport channels

CPCHDCHRACH

Logical channels

DCCHDTCHCCCH


• One can consider that the transition between GSM (2nd generation system) and UMTS (3rd generation system) was done through cdmaOne(2nd generation system) in the USA, by introducing CDMA.

Systems ComparisonSyCo (1/5)



1 5008002Power control [Hz]

MultiMonoMonoChannels per service

CDMACDMATDMAAccess

PowerPowerFrequencyShared resource

Based on codes

Based on CSGPRSPacket

transmission

DataVoiceVoiceReference service

51.250.2Chan. [MHz]

UMTScdmaOneGSM



1020Frame [ms]

High, associated to

QoS

No (voice)

Sensitivity to Radio Resource

Management

3.841.2288Chip rate [Mc/s]

YesPossibleInter-frequency handover

NoYes (via GPS)BSs synchronism

UMTScdmaOne



WidebandSlow freq. hopping

Frequency diversity

Radio Resource Management

Frequency planningQuality measure

LowHighSensitivity to planning

13 - 12Frequency reuse

CodesFrequenciesPlanning

yesNoCoverage/capacity trade-off.

SharedDedicatedPower per channelCodesTime-slotsPhysical channels

UMTSGSM



3.84, 1.283.84Chip rate [Mc/s]35 MHz2 × 60 MHzTotal bandwidth

TSUM

UnpairedPairedBand

1, ..., 164, ..., 512Spreading FactorMulti-code,

multi-slot, OVSFMulti-code,

OVSFMulti bit rate

HardSoft/HardHandover

TDDFDDDuplex

TDMA/CDMA (FDMA)

WCDMA (FDMA)Access

Mode TDDMode FDD


• MuAT - Multiple Access Techniques• ChDe - Channel Description• SyCo - Systems Comparison

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