UMTS Radio Theory

Content:

n The Basic Principles of Wireless Communication

l3G services

lMultiple Access Technologies

lSpectrum Planning

lSpreading Technology

lCoding And Interleave Technology

lModulation

n UMTS Radio mechanism

Radio Transmission Technology Requirements

nData：

l144 kbps  High speed and driving

l384 kbps  Modest speed and walking

l2 Mbps     Low speed and indoor

nVoice

l4.75Kb/s -- 12.2Kb/s

l64kb/s (Video Phone)

nInformation transmission at variable rate according to bandwidth requirements；

nDelay requirements of different service

3G services

Duplex Mode

nTDD mode－uplink and downlink has the same frequency

lAdaptable to any frequency band

lSuitable for both asymmetric and symmetric services

nFDD mode－uplink and downlink has the different frequency

lPaired frequency bands are needed

lSuitable for symmetric services

Multiple Access Technologies

Multiple access technologies enable various users access public communication line but without interference.

nWhy Multiple Access?

lIncreased capacity: serve more users

lReduced capital requirements since fewer media can carry the traffic

lDecreased per-user expense

nTypes of Transmission Medium:

lTwisted pair

lCoaxial cable

lFiber optic cable

lAir interface (radio signals)

nThree methods are frequently used:

lFDMA

lTDMA

lCDMA

Each pair of users enjoys a dedicated, private circuit through the transmission medium, unaware that the

other users exist.

FDMA (Frequency Division Multiple Access)

nFDMA

lTraffic channels are assigned to different users at different frequency band, such as TACS, AMPS.

nTDMA

• Traffic channels are assigned to different users at different time, such as  GSM, DAMPS.

CDMA (Code Division Multiple Access)

nCDMA

lTraffic channels are assigned to users at same time, same frequency band, but with different code.

CDMA Application

nUsers are distinguished by scrambling codes and OVSF codes

nSelf-interference system

nCDMA system is restricted to interference (GSM system is restricted to frequency resources) 

Spectrum Planning

Spreading Technology

n Where,

       C is capacity of channel, b/s

       B is signal bandwidth, Hz

       S is average power for signal, W

       N is average power for noise, W

nIt is the basic principle and theory for spread spectrum communications.

nUser information bits are spread over a wide bandwidth by multiplying high speed spread code(chip)

nSpread signal bandwidth W wider than original signal bandwidth Rb

Spreading Mode:

nDirect sequence spread spectrum（DS-SS）

lBase band data is spread by multiplication of pseudo-noise sequence and base-band pulse,  the pseudo-noise sequence generated by the pseudo-noise generator

lBER subject to Multiple Access Interference and near-far effect

lPower control can overcome the near-far effect, but it  is limited by power detection accuracy

lWCDMA uses DS-SS

nFrequency hopping spread spectrum（FH-SS）

lData is transmitted in the random channel  by the carrier frequency hopping

lBefore FH again, data is transmitted using traditional narrowband modulation

lNo near-far effect

Characteristics of Spreading Communication

nHigh anti-multi-path- interference capability

nAnti-sudden-pulse

nHigh security

nLower transmitting power

nEasy to implement large-capacity Multiple Access Communication

nOccupy band wide

nComplex realization

Coding And Interleave Technology

Purpose of Channel Coding:
By adding redundant information in the original data stream, receivers can detect and correct the error signal, and improve data transmission rates.

nChannel coding

lError-correcting ability obtains by adding redundancy in the original data

lConvolutional coding and Turbo coding （1/2，1/3） are widely applied.

lIncrease noneffective load and transmission time

lSuitable to correct few non-continuous errors

Principle of Interleave Technology

nadvantage

lInterleave is to change the sequence of data to random the unexpected  errors

lAdvance the correcting validity

ndisadvantage

lIncrease the processing delay

lEspecially, Several independent random errors may intertwined for the unexpected error.

Encoding and Interleaving

Modulation

nDefinition

lModulation is the process where the amplitude, frequency, or phase of an electronic or optical signal carrier is changed in order to transmit information.

lUsing symbol stand for one or more bits to improve communication effectiveness

nClassification

lAnalog Modulation

lDigital Modulation

Analog Modulation

nThe purpose of analog modulation is to impress an information-bearing analog waveform onto a carrier for transmission.

lCommon analog modulation methods include:

nAmplitude modulation (AM)

nFrequency modulation (FM)

nPhase modulation (PM)

Digital Modulation

nThe purpose of digital modulation is to convert an information-bearing discrete-time symbol sequence into a continuous-time waveform (perhaps impressed on a carrier).

lBasic digital modulation methods include

nAmplitude shift Keying (ASK)

nFrequency shift Keying (FSK)

nPhase shift Keying (PSK)

UMTS Data transmission Procedure

Channel Coding of UMTS

Convolutional Code

nMainly used in the voice channel and control signal channel

nCoding rate is ½ and 1/3.

Characteristics of Convolutional code

nEasy decode

nShort delay

nGenerally use the Viterbi Algorithm

nChannel bit error rate is 10－3 magnitude

nSuitable to realtime service

le.g. speech and video service.

Turbo Code

nUsed in Data service channel

nCode Rate is 1/3

nCan be implemented in the transmission for large block and long delay services

nTurbo coding structure is based on two or more weak error control code combinations. The information bits are interleaved in the two Encoder, and generate two information flow. At last, this information can be multiplexed and punctured

nDecoding needs cycle iterative calculation

Characteristic of Turbo Codes:

nComplex decoding

nUse the LOG-MAP arithmetic

nChannel bit error rate is 10－6 magnitude

nVery suitable to non-realtime package service which is BER sensitive & delay insensitive, e.g.  WWW, FTP, E_mail, multimedia transmission.

Spreading Technology of UMTS

Channelization Code

nAdopt OVSF code

lDefinition: Cch,SF,k, describe channelization code, where

  SF : spread factor， k : code number, 0 < k

Scrambling Code

nUMTS Scrambling code is  pseudo random binary sequence

lIt has similar noise array character, seemingly random but with regularity.

lCan make the user data further random , strengthened by scrambling a code to keep secret the user data,  at the same time easy to carry out multiple access communication.

nUMTS scrambling code is generated from Gold sequence

lGold sequence has excellent self-correlation.

lCross-correlation is very week between two codes.

lIt is used to identify cell and user for multiple access.

Characteristic of Scrambling Code

nThere are  224 Uplink Scrambling Codes, they are used to distinguish different users in one cell.

nThere are  218-1  Downlink Scrambling Codes, used to distinguish different cells

lScrambling codes usually used are the first 8192 codes, which are code 0，1，……，8191. They are divided into 512 aggregations，each aggregation has 1 primary scrambling code (PSC) and 15 secondary scrambling codes (SSC).

lThe 512 primary scrambling codes are divided further into 64 primary scrambling code groups , with 8 primary scrambling codes in each group.

Code Function

nChannelization code ---- for separation of physical channels in the uplink and separation of users in the downlink

nScrambling code ---- for separation of users/terminals in the uplink and cells/sectors in the downlink. 

Spreading code & scrambling code 

nCch：spread code

lRelative to service rate，extended to 3.84Mchips/s

lA kind of orthogonal code

nCscrambling：scrambling code

lHave no effect on signal bandwidth

lDownlink for identifier cell，uplink identifier terminal

lA pseudo-random sequence

nPG=Wc/Rb  (Wc : Chip rate , Rb : Service bit rate)

lTransmitter/receiver can obtain gain after spread/despread

lThe narrower original signal bandwidth, the larger Pg , the better

The higher PG, the more anti-interference capability system has.

Relation between Eb/N0  and PG

Modulation of UMTS

nBPSK (Binary Phase Shift Keying) in Uplink channles

nQPSK (Quadrature Phase Shift Keying) in  Downlink channels

n16QAM (16-state Quadrature Amplitude Modulation) in HSDPA

End of UMTS Radio Theory Course