lte-planning sec04 100509 v01

© Informa Telecoms & Media

OFDM/OFDMA and LTE Concepts

OFDM/OFDMA AnD LTE COnCEpTs



ThE LTE RADiO inTERFACE

InTroducTIon To oFdM/oFdMA 4requirements of Modern communication Systems 4channel Bandwidth and Fading 6Flat Fading and Frequency Selective Fading 6defining narrowband and Wideband channels 8coherence Bandwidth 10Multi-carrier Solution 12oFdM Basic Principles 14Sub carrier orthogonality 16doppler Shift in radio channels 18coherence Time 18cyclic Prefix/Guard Time 20Peak-to-Average Power ratio (PAPr) 22Single carrier – Frequency division

Multiple Access (Sc-FdMA) 24LTE PHY Layer Parameters 28LTE Sub-carrier Spacing 28LTE Timing and Framing 30Frame Type 2, Tdd 32The resource Block 34comparison of resource Blocks, channel Size

and Sampling rate 36LTE channels and channel Mapping 38LTE Logical channels 40LTE Transport channels 42LTE Physical channels 44



channel Mapping 46Mapping channels to the resource Block 48channel Mapping on a 10MHz channel 50uplink Mapping of Physical channels 52uplink Mapping of the control channel 54overall Picture of uL Mapping 56Physical channels and Modulation Schemes 58Synchronisation and reference Signals 60Primary and Secondary Synch Sequences 62PSS and SS in the Frame Structure 64reference Signals 68LTE reference Signals 70dL cell Specific rS 72dL uE Specific rS 74uL uE Specific rS 76demodulation reference Signals (dM rS) 76Sounding reference Signals (SrS) 78Modulation, channel coding and Link Adaptation 80channel coding 84HArQ (Hybrid Automatic request) 86reporting of uE Feedback 88Power control in LTE 90The user Plane and control Plane Protocols 92

4



Requirements of Modern Communication systems

recent fixed and mobile broadband statistics suggest that the demand for data is increasing at an ever accelerating rate. Services such as Facebook, Youtube and other Web 2.0 type applications have traditionally been accessed from fixed broadband connections, however with the rising popularity of the smart phone, these applications are moving swiftly in to the mobile domain.

This puts pressure on the operators of mobile networks to ensure there is sufficient capacity for the existing voice traffic as well as all the new multimedia and social networking applications.

The demand for high capacity makes the radio engineer look to the radio channel to find additional capacity. In recent years the bandwidth of the channel has grown significantly from 200KHz GSM to 5MHz uMTS/HSPA and the modulation and coding schemes have grown steadily more complex and efficient. Given the current bandwidth and complexity of systems like HSPA it would be difficult to gain more capacity by simply increasing the channel bandwidth without making the technology prohibitively complex.

inTRODuCTiOn TO OFDM/OFDMA

5© Informa Telecoms & Media

Fig. 1 – Web 2.0 – A Driver for higher Communication speeds?

6



Channel Bandwidth and Fading

The figure opposite illustrates a typical urban environment through which radio signals propagate. The transmission of the signal from the source to the destination is carried over multiple paths. The main reason for this is the existence of the buildings, vehicles, and other obstacles which can reflect and scatter the transmitted signal. The received signal is a summation of all these signals from different paths. It is apparent that any receiver will be subject to multiple, time shifted copies of the same signal.

Flat Fading and Frequency selective Fading

Each of these paths experiences a different doppler shift and degree of attenuation. The frequency response is the representation in the frequency domain of the superposition of all these paths. With the multipath scenario, where the transmitted signals take place over different paths, the signals received from each path will add up at the receiver input

The power of the received signal will vary as it is dependent upon the relationship between the phases of each received component; whether the result is constructive or destructive addition of the phase values. This is generally known as fading

If the transmitted channel is sufficiently narrow then all the frequency components transmitted in the channel will be attenuated by the same amount, this is known as flat fading

The principle problem with increasing the bandwidth of the channel to accommodate higher capacity is that the channel becomes increasingly likely to suffer from frequency selective fading. This is where only a part of the overall transmitted spectrum suffers from the attenuation due to multipath fading.

Receiver

Transmitter

Pow

er

Frequency

Expected signalActual signal

Pow

er

Frequency

Expected signalActual signal


Fig. 2

a) Typical Multipath Environment

b) Flat Fading

c) Frequency selective Fading

8



Defining narrowband and Wideband Channels

Whether a channel is determined to be wide or narrow band depends on the relative magnitude of the symbol time and the delay spread characteristic of the channel environment. Temporal distortion of the signal is an effect of the multipath environment causing the same symbol to be received multiple times over a period of time. The time differences are due to the differing propagation delays experienced on different paths.

Typical delay spreads for indoor and outdoor environments are shown below.

Indoor – 40nS – 20nS; 12m – 60moutdoor – 1uS – 20uS; 300m – 6Km

A channel can be said to be narrow band when the symbol time (Ts) is significantly larger than any delay spread present (Td)

narrow band – Ts > Td

However if the delay spread is significantly larger than the symbol time then the channel may be considered wideband.

Wideband – Td > Ts

Following on from the discussion above, regarding flat and frequency selective fading, it can be said that a channel that is defined as wideband, it is more likely to suffer from frequency selective fading.

consider now, that the symbol time is a function of the channel bandwidth.

Ts = 1/Bw

Therefore as the channel bandwidth increases the symbol time will decrease.

e.g.

Bw = 1MHz; Ts = 1uSBw = 10MHz; Ts = 0.1uS

It is more probable therefore that high capacity, high bandwidth radio channels will experience frequency selective fading.

Transmitter

Receiver

Pow

er

TimeRMS delay spread

t5

t4t3

t2t1

t0

Narrow band ~ Ts > TdWideband ~ Td > Ts

Ts

Td


Fig. 3

indoor Delay spread

RMs Delay spread

narrowband or WideBand?

10



Coherence Bandwidth

coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered “flat” (i.e., a channel which passes all spectral components with approximately equal gain and linear phase). In other words, coherence bandwidth is the range of frequencies over which two frequency components have a strong potential for amplitude correlation.

coherence bandwidth is a function of the delay spread environment and can be calculated using the following expression;

1Bc = 2πτrms

Where; τrms is the rms delay spread of the channel.

The table below shows typical delay spreads for various environments and their coherence bandwidth. Knowing the coherence bandwidth for typical deployment environments allows an estimation of the probability that frequency selective fading will occur if the channel bandwidth of the system is know.

Environment Typical rms Delay Coherence BandwidthHilly area 3-10 μsec 53KHz-16KHz

urban 1-3 μsec 160KHz-53KHz

Suburban < 1 μsec > 160KHz

open area < 200 nsec > 795KHz

Indoors 10-50 nsec 16MHz-3.2MHz


Fig. 4 – Coherence Bandwidth

Coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered “flat” (i.e., a channel which passes all spectral components with approximately equal gain and linear phase)

1 Bc =

2πτrms

Environment Typical rms Delay Coherence BandwidthHilly area 3-10 μsec 53KHz-16KHz

urban 1-3 μsec 160KHz-53KHz

Suburban < 1 μsec > 160KHz

open area < 200 nsec > 795KHz

Indoors 10-50 nsec 16MHz-3.2MHz

12



Multi-Carrier solution

Given the problems outlined above the solution for today’s broadband wireless systems is to utilise multi-carrier systems known as oFdM (orthogonal Frequency division Multiplexing) or oFdMA (orthogonal Frequency division Multiple Access).

FDM (Frequency Division Multiple Access)Multi-carrier systems split the high speed stream of serial baseband data in to lower speed parallel streams. The lower bit rate on each sub-carrier results in a narrower radio channel that is resistant to the frequency selective fade.

OFDM (Orthogonal Frequency Division Multiplexing)However, these multi-carrier systems need to exhibit good spectral efficiency, each sub carrier must be placed close to its adjacent carrier with out causing interference. The channel spacing is 1/Ts where Ts is the symbol time of information modulated onto the carrier. Spacing the channels in this manner ensures that the centre of each carrier corresponds with a zero crossing point for each of the neighbouring sub-carriers. This means that the centre of the sub-carriers can be sampled, free from interference of the adjacent sub-carriers.

t1 t2 t3 t4 t5 t6

1/Ts


Fig. 5

Traditionally spaced FDM Channels

Orthogonally spaced FDM Channels (sub-carriers)

14



OFDM Basic principles

The block diagram opposite shows the basic principle of an oFdM transmitter/receiver. The incoming data stream is first converted from serial data to parallel data, the number of parallel data stream will depend on the bandwidth of the overall channel and the number of sub-carriers available to carry the data. Each of the parallel streams of data is then modulated on to each sub carrier which then undergoes an IFFT (Inverse Fast Fourier Transform) which transforms the frequency domain signal into a tome domain signal. The complex time domain signal is then added to produce a composite and complex waveform.

In receiver the signal must be sampled with sufficient frequency to ensure all the composite frequency components are captured. Where there are more sub-carriers the received signal must be sampled more frequently.

The term FFT (Fast Fourier Transform) points or samples, refers to the number of samples that must take place during a singe FFT symbol, hence the larger number of FFT points for higher bandwidth channels. The FFT symbol has a time equivalent to the baseband symbol time but is the composite of all the modulated sub-carriers. The captured and sampled signal is transformed to the frequency domain by applying an FFT. This effectively separates the sub-carriers so they may be demodulated independently.


Fig. 6 – simple OFDM Block Diagram

16



sub Carrier Orthogonality

Given the very tight spacing of the sub-carriers of the oFdM channel it is very important that the sub-carriers remain orthogonal from each other. disturbances in the time and frequency domain can reduced the orthogonality of the carriers resulting in an increase in BEr and generally poorer performance.

distortion in the frequency domain can come from doppler shift due to uE movement or from poor synchronisation of the uE sub systems to the system clock. The latter problem can be resolved by having the enB broadcast synchronisation signals on a regular basis, allowing the uE to adjust and maintain its synchronisation with the enB. This can also reduce the effect of doppler shift, however the coherence time of the channel will provide an indication of how likely the received signals will be affected by doppler shift.

Demodulatedsignal with

frequency offsetcausing ICI

Demodulatedsignal without

frequency offset(zero ICI)


Fig. 7 – sub Carrier Orthogonality

18



Doppler shift in Radio Channels

Frequency offset is an important consideration, particularly in oFdM systems. In mobile radio systems the velocity of the uE will cause an apparent offset from the centre carrier of the radio channel, yielding poorer performance and higher BEr. In oFdM systems it will also cause inter sub-carrier interference.

The following expression may be used to determine the frequency offset due to doppler shift.

fd = cosθ. f.υ

c

Where;

f is the frequency of operation

v is the velocity of the receiver

c is the speed of light

Coherence Time

An important performance attribute when considering the systems sensitivity to effects of frequency offset is the coherence time. The coherence time is a function of the amount of frequency offset present in the channel and is defined as;

The time over which a channel can be assumed to be constant.

Tc = 2√ 9

16π.fd2

Therefore a system that uses a symbol time which is less than the coherence time will not be distorted by the effects of doppler shift.

e.g.

Find the coherence time for a radio channel operating at 2.6GHz and a mobile travelling at 140kph. The angle of arrival is 0o

140kph = 38 m/s

fd = cosθ. 2.6 x 109 . 3.8

3 x 108

fd = 329.33 Hz

Tc = 2√ 9

16π.329.332

fc = 1.28 x 10-3 seconds


Fig. 8

Coherence TimeThe time over which a channel can be assumed to be constant

Doppler shift in Radio Channels

20



Cyclic prefix/Guard Time

The multi-path environment through which the radio signals are transmitted create temporal distortions in the data carried by the radio channel. The differing propagation duration for each of the multi-path components create inter-symbol interference (ISI). Inter-symbol interference in oFdM systems cannot be tolerated since it reduces the orthogonality between the sub-carriers and increases the BEr and reduces performance of the channel. All of the information important to the FFT (Fast Fourier Transform) is contained within the symbol time therefore it is critical that there is no distortion during this period.

Since the ISI cannot be eliminated from the channel, the information must be protected from its effect. The solution in oFdM systems is to extend the length of each symbol by a factor equivalent to the likely delay spread in the channel. This extension to the symbol is known as the cyclic prefix (cP) or guard time.

The cP, which appears at the beginning of each symbol and is actually a copy of the last part of that symbol. The inclusion of the guard period eliminates the effects of multi-path ISI at the expense of through put, since the cP carries no actual information and is discarded at the receiver once the rF signal has been successfully digitised.

Symbo l = 66.7µS

T cp= 4.7µ S

Total T ransmi tted Symbo l = 71.3µS

A B C CPA CPB CPC

A B C CPA CPB CPC

Td

Compete Symbol FFT Sampli ng T i me


Fig. 9

Creation of the Cyclic prefix

Cyclic prefix Operation

22



peak-to-Average power Ratio (pApR)

oFdM does present some technical challenges. The resulting composite waveform displays large variations in amplitude caused by the combination of a number of individual signals. This is illustrated in Figure 11. The effect is similar to that caused by the multipath environment – a resultant signal fluctuating in amplitude as a result of the combining of so many signals with discrete phase and amplitude differences.

This resultant composite signal has implications for A to d convertor and rF amplifier design. The dynamic range of the amplifier must be able to cope with the smallest and largest signal amplitudes – particularly the largest amplitude as it this that could cause over-driving of the amplifier. over driving an amplifier causes non-linear behaviour resulting in the generation of harmonics and Intermodulation Products (IPs) which will reside within the wanted spectrum, but will cause unwanted effects. The FFT process will be degraded as it attempts to deal with frequency components that should not be there, resulting in lost packets.

Symbol time 4Symbol time 3Symbol time 2

Carrier 1

Carrier 2

Carrier 3

Carrier 4

Compositesignal


Fig. 10 – peak to Average power (pApR)

24



single Carrier – Frequency Division Multiple Access (sC-FDMA)

3GPP has chosen Sc-FdMA for the uplink. not surprisingly, power consumption is a key consideration for uE terminals. The high PAPr and related loss of efficiency associated with oFdMA are major concerns. As a result, an alternative to oFdM was sought for use in the LTE uplink.

Sc-FdMA is well suited to the LTE uplink requirements. The basic transmitter and receiver architecture is very similar (nearly identical) to oFdMA, and it offers the same degree of multipath protection. Most important though is that the underlying waveform is essentially single-carrier, and therefore the PAPr is lower.

The figure opposite compares the oFdMA and Sc-FdMA structures. For clarity this example uses only four (M) subcarriers over two symbol periods with the payload data represented by quadrature phase shift keying (QPSK) modulation.

data symbols in the time domain are converted to the frequency domain using a discrete Fourier transform (dFT); then in the frequency domain they are mapped to the desired location in the overall channel bandwidth before being converted back to the time domain using an inverse FFT (IFFT). Finally, the cP is inserted. Because Sc-FdMA uses this technique, it is sometimes called discrete Fourier transform spread oFdM or (dFT-SoFdM).

The most obvious difference between the two schemes is that oFdMA transmits the four QPSK data symbols in parallel, one per subcarrier, while Sc-FdMA transmits the four QPSK data symbols in series at four times the rate, with each data symbol occupying M x 15 kHz bandwidth.

Q

I

QPSK modulatingdata symbols

Sequence of QPSK data symbols to be transmitted

1,1 1,1 1,1-1,1 -1,1 -1,1

1,-1

1,-1 1,-1

-1,-1

-1,-1 -1,-1

V V

Time

Time

OFDMA

symbo

l

OFDMA

symbo

l

SC-FDMA

symbo

l

SC-FDMA

symbo

l

Frequency Frequencyfc

CP CP

fc15kHz 60kHz

Constant subcarrierpower during

each SC-FDMAsymbol period

OFDMAData symbols occupy 15kHz for

one OFDMA symbol period

SC-FDMAData symbols occupy M*15kHz for

1/M SC-FDMA symbol periods


Fig. 11 – single Carrier – FDMA

26



Sc-FdMA signal generation begins with a special pre-coding process. The diagram opposite shows the first steps, which create a time-domain waveform of the QPSK data sub-symbols. using the four colour-coded QPSK data symbols from the previous diagram, the process creates one Sc-FdMA symbol in the time domain by computing the trajectory traced by moving from one QPSK data symbol to the next. This is done at M times the rate of the Sc-FdMA symbol such that one Sc-FdMA symbol contains M consecutive QPSK data symbols. once an IQ representation of one Sc-FdMA symbol has been created in the time domain, the next step is to represent that symbol in the frequency domain using a dFT.

To complete Sc-FdMA signal generation, the process follows the same steps as for oFdMA. Performing an IdFT converts the frequency-shifted signal to the time domain and inserting the cP provides the fundamental robustness of oFdMA against multipath. The diagram opposite shows the stages in common with oFdM.

Q

I

V(I)

+1

–1

One SC-FDMAsymbol period

One SC-FDMAsymbol period

V(I)

+1

–1

1,1-1,1

1,-1-1,-1

M d

ata

bits

in

Time domain

Mapdata to

constellation

Generatetime domainwaveform

Unique to SC-FDMA Common with OFDMA

PerformM-point DFT(time to freq)

Mapsymbols tosubcarriers

PerformN-point IFFT

N > M

Upconvertand

transmit

M d

ata

bits

out

De-mapconstellation

to data

Generateconstellation

PerformM-point IDFT(time to freq)

De-mapsubcarriersto symbols

PerformN-point DFT

N > M

Receiveand

downconvert

Frequency domain Time domain


Fig. 12

Generating the sC-FDMA signal

The sC-FDMA Block Diagram

28



LTE phY Layer parameters

LTE is designed to meet many differing requirements including urban, suburban, indoor and outdoor environments as well as coping with many different mobility conditions from stationary to high speed mobility up to 500Kph. cell sizes may also very from femto to large rural macro. The range of spectrum that LTE may be potentially deploy across is also very wide, 400MHz – 4GHz, the deployed system bandwidths that may be support also ranges from 1.4MHz to 20MHz.

Given the deployment flexibility of LTE the range of channel conditions that it is expected to perform under is extremely wide and varied. The critical parameters required to support this diversity are the sub-carrier spacing and the cyclic prefix.

LTE sub-Carrier spacing

The sub-carrier spacing is 15KHz. consider the previous discussions on coherence bandwidth and resilience to doppler effects, selection of sub carrier spacing of 15KHz for LTE radio interface is a compromise based on the expected operational environment and expected levels of performance.

Channel spacing = 1/Ts

Ts = 66.7µSFs = 1/66.7µS = 15KHz


Fig. 13 – LTE sub-Carrier spacing

30



LTE Timing and Framing

The basic unit of time in LTE is Ts, this is defined as 1/(15000*2408) = 32.56nS, where 15000 is the bandwidth of the sub-carrier and 2048 is the maximum number of FFTs supported. Every element of time is some multiple of this value.

The figure opposite shows the type 1 frame, or Frame Structure 1 (FS1), this is the timing structure used on the uplink and downlink of the Fdd (Frequency division duplex) channels. one slot is a 0.5mS period of time which contains 7 symbols of 66.67 µS. 2 slots make up one 1mS Sub-Frame, the sub-frame is sometimes referred to as the transmission time interval (TTI) particularly by the higher layers. There a 10 sub-frames or 20 slots in one 10mS frame. This structure is used in the time domain to map the physical channels. note that the physical channels also require a frequency domain component for complete mapping.

0 1 2 3 4 5 6

One subframe

One slot, Tslot = 15360, Ts = 0.5 ms

One radio frame, Tf = 307200, Ts = 10 ms

#0 #18 #19#1 #2 #3

66.67µS Symbols


Fig. 14 – Frame Type 1 FDD

32



Frame Type 2, TDD

The figure opposite shows the frame structure used on a Tdd (Time division duplex) channel. It has similar overall timing i.e. the overall frame length is 10mS and 10 sub-frames of 1mS each. However the structure of the sub-frames is different.

In the FS2 the sub-frame allows both an uplink and downlink transmission/reception opportunity. These are referred to as the dwPTS (downlink Pilot Time Slot) and upPTS (uplink Pilot Time Slot), these are separated in the sub-frame by a guard period (GP).

The frame has two different switch points i.e. the point at which a defined slot configuration begins to repeat, these are at 5mS and 10mS. In addition there are 7 different frame configurations. In any of these configurations sub-frame 0 and 6 carry downlink information only, and sub-frame carries uplink only. The table opposite shows the frame configurations.


Fig. 15 – Frame Type 2 TDD

Configuration switch-point periodicity

sub-frame number

0 1 2 3 4 5 6 7 8 9

0 5 ms d S u u u d S u u u

1 5 ms d S u u d d S u u d

2 5 ms d S u d d d S u d d

3 10 ms d S u u u d d d d d

4 10 ms d S u u d d d d d d

5 10 ms d S u d d d d d d d

6 10 ms d S u u u d S u u d

34



The Resource Block

Mapping of channels takes place in the time and frequency domains in LTE. The primary element that support the mapping process is the resource Block (rB). The rB has a fixed size and is common to all channel bandwidths/FFT sizes.

In the time domain the rB is one slot ( 7 x 66.67µS symbols). In the frequency domain there are 12 x 15KHz sub-carriers. 1 symbol and 1 sub-carrier is known as a resource element.

From the figure opposite it can bee seen that the rB occupies 12 x 15KHz = 180KHz of band width. In a 5MHz radio channel there will be 300 rB occupying 4.5MHz of spectrum. The number of FFTs required to process this is 512, assuming sub-carrier size of 15KHz, 512 x 15KHz = 7.68MHz. 7.68MHz if the space occupied by 512 FFT points and is not the transmitted bandwidth, 7.68MHz is also the sampling frequency required to recover information from the carrier to drive the FFT (time domain to frequency domain) in the receiver.

1 slot

Resourceblock

DL or ULsymbol

NscR

B

= 1

2 (1

80 k

Hz)

Time*5 MHz system with

frame structure type 1

Freq

uenc

y

NR

B x

NscR

B

= 3

00 (4

.5 M

Hz)

Zero

sZe

ros

M =

512

(7.6

8 M

Hz)


Fig. 16 – Defining a Resource Block

36



Comparison of Resource Blocks, Channel size and sampling Rate

The table opposite shows the number of rB required for channel bandwidths supported by LTE, it should be noted that the definition of channel bandwidth in this table refers to the nominal channel size defined by the spectrum regulating body, it is not necessarily the transmission bandwidth.

Since each rB contains 12 sub-carriers the number of occupied sub-carriers can be determined, multiplying the number of occupied sub-carriers by 15KHz will more accurately describe the transmission bandwidth of the various options.

The IdFT/dFT (Inverse discreet Fourier Transform) describes the number of FFT points required to successfully recover information from the carrier, it is always a value of 2n and determines the number of steps of processes required to construct/de-construct the composite oFdMA signal.

The sampling rate and samples per slot are determined from the FFT number and the sub-carrier bandwidth. E.g. in the 5MHz channel the sampling rate of 7.68MHz would result in 3840 samples every 1mS.


Fig. 17 – Table of Resource Block sizes and Channel Bandwidth

channel bandwidth (MHz) 1.4 3 5 10 15 20

number of resource blocks (nrB) 6 15 25 50 75 100

number of occupied subcarriers 72 180 300 600 900 1200

IdFT(Tx)/dFT(rx) size 128 256 512 1024 1536 2048

Sample rate (MHz) 1.92 3.84 7.68 15.36 23.04 30.72

Samples per shot 960 1920 3840 7680 11520 15360

38



LTE Channels and Channel Mapping

Information, both signalling and user, is transmitted through the protocol stack and over air using channels. There are 3 basic types of channel defined, Logical, Transport and Physical channels. Each channel is defined by a set of functions or attributes which determines the handling of the data over the radio interface.

Logical ChannelsLogical channels exist between the PdcP layer and MAc, they are principally defined by the type of information that they carry. There are logical channels that carry control data, and logical channels that carry user traffic.

Transport ChannelsTransport channels exist between the MAc layer and the Physical Layer and are define the manner in which the data will be transferred, i.e. the type of channel coding, whether the data is protected from errors, size of data packets, etc. The attributes of data transfer applied to the data in the transport channel is otherwise known as the transport format.

physical ChannelsPhysical channels are the actual implementation of the transport channels in the physical layer. The only exist in the physical layer and depend on the physical layer characteristics, i.e. channel bandwidth, FFT size, etc.

Trafficchannel

MAC

PHY

Controlchannel

Logical channelsDefined by Type of information i.e. traffic, control, e.g. BCCH, PCCH, CCCH, MCCH, DCCH

Transport channelsDefined by Transport attribute i.e. channel coding, CRC, interleaving, size of radio data packets, e.g. BCH, PCH, DL-SCH, MCH

Physical channelsDefined by actual physical layer characteristics, bandwidth, FFT size, e.g. PDSCH, PDCCH, PMCH, PBCH…


Fig. 18 – LTE Channels

40



LTE Logical Channels

There are two types of logical channel, control channels and traffic channels, they are described below.

Control Channelscontrol channels are used for transfer of control plane information only. The control channels offered by MAc are:

Broadcast Control Channel (BCCH)A downlink channel for broadcasting system control information. Information broadcast on this channel is shared by all the users in the cell, the information broadcast relates to the operator identity, cell configuration, access information etc

Paging Control Channel (PCCH)A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the uE.

Common Control Channel (CCCH)channel for transmitting control information between uEs and network. This channel is used for uEs having no rrc connection with the network. It would be used during the earliest phases of communication establishment.

Multicast Control Channel (MCCH)A point-to-multipoint downlink channel used for transmitting MBMS control information from the network to the uE, for one or several MTcHs. This channel is only used by uEs that receive MBMS.

Dedicated Control Channel (DCCH)A point-to-point bi-directional channel that transmits dedicated control information between a uE and the network. uEs having an rrc connection will exchange rrc and nAS signalling, it should be noted that application level signalling (SIP messages from the IMS) is not handled by the dccH.

Traffic ChannelsTraffic channels are used for the transfer of user plane information only. The traffic channels offered by MAc are:

Dedicated Traffic Channel (DTCH)A dedicated Traffic channel (dTcH) is a point-to-point channel, dedicated to one uE, for the transfer of user information. The dTcH will also carry signalling from the application layers, this may be SIP and rTSP signalling if the EPc supports IMS (IP Multimedia Subsystem)

Multicast Traffic Channel (MTCH)A point-to-multipoint downlink channel for transmitting traffic data from the network to the uE. This channel is only used by uEs that receive MBMS.

LTE Logical Channels

Logical Control Channels Logical Traf�c Channels

Broadcast Control Channel (BCCH)• System Information Messages

Paging Control Channel (PCCH)• Paging Messages, UE Location

not known

Common Control Channel (CCCH)• Early communication, no RRC

connection

Multicast Control Channel (MCCH)• Multicast control signalling

Dedicated Control Channel (DCCH)• Bi-Directional signalling, RRC

connection, RRC and NAS Signalling

Dedicated Traffic Channel (DTCH)• Point-Point bi-directional channel,

User data and application level signalling (SIP)

Multicast Traffic Channel (MTCH)• Point-Multi-point channel supporting

data transfer for the MMBS service


Fig. 19 – LTE Logical Channels

42



LTE Transport Channels

Transport channels are classified in to uplink and downlink channels and are described below.

Broadcast Channel (BCh)The BcH has a fixed and pre-defined transport format largely defined by the requirement to be broadcast in the entire coverage area of the cell since the information carried by this channel contains system information.

Downlink shared Channel (DL-sCh)This channel will carry downlink signalling and traffic and may have to be broadcast in the entire cell, given the nature of the data in this channel it will also support for both dynamic and semi-static resource allocation with the option to support for uE discontinuous reception (drX) to enable uE power saving, Error control is supported in this channel by means of HArQ and dynamic link adaptation by varying the modulation, coding and transmit power. Spectral efficiency can also be increased due to the possibility of using beamforming antenna techniques. The channel also supports MBMS transmissions.

paging Channel (pCh)This channel is associated with the PccH and will carry paging message to uEs not currently connected to the network. The PcH supports discontinuous reception (drX) to enable uE power saving where the sleep cycle is indicated by the network to the uE. The PcH may also have to be broadcast in the entire coverage area of the cell. The PcH is also mapped to physical resources which can be used dynamically also for traffic/other control channels.

Multicast Channel (MCh) The channel is associated with the multicast services from the upper layers and as such there is a requirement to broadcast both control and user data over the entire coverage area of the cell. It also support the Single Frequency network as semi-static resource allocation

uplink shared Channel (uL-sCh)The uL_ScH carries common and dedicated signalling as well as dedicated traffic information. It supports the same features as the dL-ScH.

Random Access Channel (RACh)The rAcH is a very specific transport channel, it carries limited control information during the very earliest stages of connection establishment. This a common uplink channel therefore there is the risk of collisions during uE transmission.

LTE Transport Channels

Downlink Transport Channels Uplink Transport Channels

Broadcast Channel (BCH) • �xed, pre-de�ned transport format;• broadcast in the entire coverage area

of the cell.

Downlink Shared Channel (DL-SCH) • HARQ;• dynamic link adaptation by varying

the modulation, coding and transmit power;

• broadcast in the entire cell;• beamforming;• dynamic and semi-static resource

allocation;• UE discontinuous reception (DRX) to

enable UE power saving;• MBMS transmission.

Paging Channel (PCH) • UE discontinuous reception (DRX) to

enable UE power saving • broadcast in the entire coverage area

of the cell;• mapped to physical resources which

can be used dynamically also for traf�c/other control channels.

Multicast Channel (MCH) • broadcast in the entire coverage area

of the cell;• MBSFN combining of MBMS

transmission on multiple cells;• support for semi-static resource

allocation e.g. with a time frame of a long cyclic

Uplink Shared Channel (UL-SCH) • beamforming• dynamic link adaptation by varying

the transmit power and potentially modulation and coding;

• HARQ;• dynamic and semi-static resource

allocation.

Random Access Channel (RACH)• limited control information;• collision risk;


Fig. 20 – LTE Transport Channels

44



LTE physical Channels

The physical channels are the actual implementations of the transport channels on the radio interface. They only exist within the physical layer and are highly dependant on the actual capabilities of the physical layer itself.

The physical channels are:

physical broadcast channel (pBCh)The system information is transmitted cyclically within BcH transport block and mapped to four subframes over a 40 ms interval, there is minimal synchronisation from the uE perspective since the 40 ms timing is blindly detected, i.e. there is no explicit signalling indicating 40 ms timing. Each subframe is assumed to be self-decodable, i.e. the BcH can be decoded from a single reception, assuming sufficiently good channel conditions.

physical control format indicator channel (pCFiCh)This channel informs the uE about the number of oFdM symbols used for the PdccHs and is transmitted in every subframe.

physical downlink control channel (pDCCh)This channel informs the uE about the resource allocation of PcH and dL-ScH, and Hybrid ArQ information related to dL-ScH and also carries the uplink scheduling grant.

physical hybrid ARQ indicator Channel (phiCh)carries Hybrid ArQ AcK/nAKs in response to uplink transmissions.

physical downlink shared channel (pDsCh)carries the dL-ScH and PcH.

physical multicast channel (pMCh)carries the McH, Mulitcast/Broadcast information

physical uplink control channel (puCCh)This channel carries uplink control information such as Hybrid ArQ AcK/nAKs in response to downlink transmission, carries Scheduling request (Sr) and, cQI reports.

physical uplink shared channel (pusCh)carries the uL-ScH, user data and application level signalling

physical random access channel (pRACh)carries the random access preamble sent by the uE to initiate and rrc connection.

There are also physical signals which are sent on the downlink but are not given any channel designation, they include;

reference signals – one signal transmitted per downlink antenna port•Synchronisation signals – primary and secondary synchronisation signals.•

LTE Physical Channels

Downlink Physical Channels Uplink Physical Channels

Physical broadcast channel (PBCH)• BCH transport block is mapped to

four subframes within a 40 ms • blindly detected, there is no explicit

signalling indicating 40 ms timing;• the BCH can be decoded from a

single reception.

Physical control format indicator channel (PCFICH)• Informs the UE about the number of

OFDM symbols used for the PDCCHs;

• Transmitted in every subframe.

Physical downlink control channel (PDCCH)• resource allocation of PCH and

DL-SCH, and Hybrid ARQ information related to DL-SCH;

• Carries the uplink scheduling grant.

Physical Hybrid ARQ Indicator Channel (PHICH)• Carries Hybrid ARQ ACK/NAKs

Physical downlink shared channel (PDSCH)• Carries the DL-SCH and PCH.

Physical multicast channel (PMCH)• Carries the MCH.• also for traf�c/other control channels.

Multicast Channel (MCH) - broadcast in the entire coverage area

of the cell;- MBSFN combining of MBMS

transmission on multiple cells;- support for semi-static resource

allocation e.g. with a time frame of a long cyclic

Physical uplink control channel (PUCCH)• Carries Hybrid ARQ ACK/NAKs ;• Carries Scheduling Request (SR);• Carries CQI reports.

Physical uplink shared channel (PUSCH)• Carries the UL-SCH.

Physical random access channel (PRACH)• Carries the random access

preamble.


Fig. 21 – LTE physical Channels

46



Channel Mapping

The diagram opposite shows the possible mapping of channels between logical, transport and physical channels.

It can be noted that, whilst the logical channels carry specific types of information, they can be mapped to common transport channels and in the case of the multicast control and traffic channels different transport channels can be used to carry the data.

In the case of the BccH logical channel, it will be noted that both the BcH and dL-ScH may be used to carry the system information. This depends on the type of system information being transmitted. critical system information messages such as those that carry scheduling information and need to be transmitted on a regular basis are transmitted as a fixed format message via the BcH and PBcH. Mapping system information to the dL_ScH allows some flexibility and additional capacity for less time dependant information.

The rAcH channel carries only the access preamble and has no instance above the MAc layer, therefore the channel is not mapped to a logical channel. once an rrc connection has been granted the rAcH is no longer used.

Some physical channels do not carry information above the physical layer therefore have no transport channel equivalents. Examples include PuccH, PdccH, PcFIcH, PHIcH, these carry information related to the coding of the physical blocks and HArQ mechanism.

Logical

BCCH CCCHPCCH DCCH DTCH MCCH MTCH

PCFICH Physical

Transport

PHICH

PUCCH

PDCCH PBCH PUSCH PDSCH PMCH PRACH

BCHPCH UL-SCH DL-SCH MCH RACH


Fig. 22 – Logical to Transport Channel Mapping

48



Mapping Channels to the Resource Block

The figure opposite shows the process of mapping the downlink control and shared channels to a resource block. The synchronisation and reference signals are also included.

note the PdccH occurs in the first few symbols of each sub-frame, the number of symbols is signalled by the PHFIcH. Also note the arrangement of the primary and secondary synchronisation signals and the PBcH. When this information is mapped to the 10mS frame it can be seen that the P-ScH, S-ScH and PBcH are transmitted in sub-frame 1 and the P-ScH, S-ScH is transmitted again in sub-frame 5. This means that primary and secondary synchronisation signals are retransmitted every 5mS. The PBcH is transmitted with 40mS periodicity.


Fig. 23 – Mapping of Downlink Control and sCh physical Channels to a Resource Block

50



Channel Mapping on a 10Mhz Channel

The figure opposite shows the downlink mapping on a 10MHz channel. The synch and broadcast data is located in the centre of the band to aid the uE cell search process.

600599598597596595

306305304303302301300299298297296295294293292291290289288287286285284283282281280279278277276275274273272271270269268267266265264263262261260259258

11

RB

109876543210

0 1 2

Slot 0

One subframe = 1 ms

One radio frame = 10 ms

Slot 1 Slot 2 Slot 10 Slot 19

3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6

Ant 0/Ant 1 reference• channel estimation• channel quality measurement

PDCCH• DL scheduling decision• UL scheduling grants• ACK/NACK information

P-/S-SCH• cell search • frequency and timing acquisition

PBCH• broadcasting channel• cell specific information


Fig. 24 – Detailed physical Channel Mapping for 5Mhz Channel

52



uplink Mapping of physical Channels

The uplink channels are mapped in a similar fashion to the downlink, the biggest difference here being the absence of sub-carriers since Sc-FdMA is used the resource block contains 7 time domain symbols (1 slot) and a single Sc-FdMA channel.

The mapping of the uplink shared channel is shown in the figure opposite. note the presence of the uplink reference signal in symbol 3 of every slot.


Fig. 25 – Mapping of uL shared Channel to Resource Block and Frame

54



uplink Mapping of the Control Channel

The figure opposite shows the mapping arrangement for the PuccH and its reference signals.

The PrAcH channel is also mapped into this sub-frame format although its presence and location must be signalled by the network.


Fig. 26 – Mapping of uL Control Channel to Resource Block

56



Overall picture of uL Mapping

The figure opposite shows the general arrangement for mapping uplink control and shared channels over time and frequency domains.

Frequency

Time

n PUSCH n Demodulation reference signal (for PUSCH)

n PUCCH n Demodulation reference signal for PUCCH format 0 & 1


Fig. 27 – Detailed Mapping of uL Data and Control Channels

58



physical Channels and Modulation schemes

There are generally 3 different types of information transmitted over the radio link, signalling, data and special reference signals.

Physical layer signalling has the primary requirement of reliability therefore the modulation schemes supported by the signalling channels are low level “robust” schemes. QPSK is the modulation scheme used in most cases although the PuccH has the option of using BPSK in circumstance where interference is very high.

data’s main requirement is one of speed and spectral efficiency. Most applications benefit from high data transfer rates and the network benefits from high spectral efficiency, therefore the highest order modulation scheme would generally be selected, 64QAM, however there are times when interference is high and the high order schemes cannot be maintained, therefore the shared channels also support 16QAM and QPSK.

The special signals don’t transmit explicit information, instead, complex signals which imply a channel condition or position in complex sequence generation are transmitted. The signals are used by the uE and the enB to determine channel conditions for MIMo processing and network synchronisation. The rS, P-ScH and S-ScH all transmit complex data sequences.


Fig. 28

The physical Layer Channels of LTEDL channels Full name purpose

PBcH Physical broadcast channel carries cell-specific information

PMcH Physical multicast channel carries the McH transport channel

PdccH Physical downlink control channel Scheduling, AcK/nAcK

PdScH Physical downlink shared channel Payload

PcFIcH Physical control format indicator channel defines number of PdccH oFdMA symbols per sub-frame (up to 4)

PHIcH Physical hybrid ArQ indicator channel carries HArQ AcK/nAcK

uL channels Full name purpose

PrAcH Physical random access channel call setup

PuccH Physical uplink control channel Scheduling, AcK/nAcK

PuScH Physical uplink shared channel Payload

The physical Layer signals of LTEDL signals Full name purpose

P-ScH* Primary synchronisation signal used for cell search and identification by the uE. carries part of the cell Id (one of three orthogonal sequences)

S-ScH* Secondary synchronisation signal used for cell search and identification by the uE. carries the remainder of the cell Id (one of 168 binary sequences)

rS reference signal (pilot) used for dL channel estimation. Exact sequence derived from cell Id (one of 3 x 168 = 504) pseudo random sequences)

uL signals Full name purpose

rS reference signal (demodulation and sounding)

used for synchronisation to the uE and uL channel estimation

60



synchronisation and Reference signals

synch sequence and Cell searchA uE entering a cell for the fist time must discover the time and frequency parameters that are required to successfully communicate with the enB. In other words the uE must synchronise with the enB. Synchronisation signals are broadcast from the enB on a frequent basis that enable the time domain and frequency domain parameters to be read by the uE, in addition this information can impart cell identification.

The requirements for synchronisation can be decomposed into three main functions.

1. Symbol timing acquisition, where the correct symbol start position is identified, to set the correct FFT window position.

2. carrier frequency synchronisation, which is needed to reduce or eliminate the effect of frequency errors arising from the mismatch of local oscillator to the transmitter and receiver, also other frequency distortions arising from temperature drift, ageing and doppler effects.

3. It is also necessary to have the sampling clock synchronised.

The uE is required to perform cell search either initially when entering the system after switch on and identifying a new cell (i.e. neighbour cell) once connected to the system.


Fig. 29 – synchronisation Requirements

Symbol timing acquisition 1.

Carrier frequency synchronisation 2.

Synchronised sampling clock 3.

62



primary and secondary synch sequences

There are 2 synch signals transmitted from the enB, the Primary Synch Signal (PSS) and the Secondary Synch Signal (SSS)

The PSS enables the uE to detect the slot timing and also provides a physical layer identity for the cell. The SSS provides the radio frame timing, the cell Id, cyclic Prefix (cP) detection and an ndication of Tdd or Fdd.

If the cell search is for initial entry in to the system the uE will detect PSS followed by SSS then go on to find and decode the Broadcast information in the cell, information broadcast will deliver other important cell parameters allowing the uE to modify its behaviour according to the selected cell.

If the uE has already entered the network the detection of adjacent cell PSS and SSS will be followed by the detection and measurement of the neighbour cell signal strength and quality.

New cellidentification

Initialsynchronisation

PSS DetectionSlot Timing

PHY Layer ID

PBCH DecodePBCH Timing Detection

System Information Access

RS DetectionMeasure and Report…

Signal QualitySignal Strength

RS DetectionMeasure and Report…

Signal QualitySignal Strength

SSS DetectionRadio Frame Timing

Cell IDCP Length DetectionTDD/FDD Detection


Fig. 30 – synch sequences and synch Activity

64



pss and ss in the Frame structure

The structure of the PSS and SSS is shown in the figure opposite. In both the Tdd and Fdd frame structure the PSS and SSS are transmitted periodically, twice in every 10mS frame. However the actual structure of the PSS and SSS as applied to the frame is slightly different depending on whether the frame is Tdd of Fdd and whether the long or short cP is used.

The Fdd frame locates the PSS and SSS in the last 2 symbols of the 1st and 11th slots of the radio frame. Allowing the uE to obtain slot boundary timing independently of cP length.

In the Tdd frame the PSS is located in the third symbol of the 3rd and 13th slots of the radio frame, the SSS is transmitted 3 symbols earlier.

10 ms radio frame

0.5 ms 1 slot

Normal CP

Extended CP

2

1

1 2 3 4 5 6

2 3 4 5 6

SSS PSS

7

3 4 5 7 8 9 10

1 ms subframe

10 ms radio frame

0.5 ms 1 slot

Normal CP

Extended CP

21

1

1 2 3 4 5 6

2 3 4 5 6

SSS PSS

7 1

1 2 3 4 5 6

2 3 4 5 6 7

3 4 5 6 7 8 9 10

1 ms subframe


Fig. 31

pss and sss Frame and slot structure in Time Domain in the FDD Case

pss and sss Frame and slot structure in Time Domain in the TDD Case

66



6 R

B

1 2 3 4 5 6 7 1 2 3 4 5 6 7 SSS

PSS

RS

Unused RE

1 ms subframe

10 ms radio frame


Fig. 31

pss and sss Frame structure in Frequency and Time Domain for an FDD Cell

68



Reference signals

Any information transmitted in to a radio channel will experience attenuation and distortion of the information as well as the additive accumulation of noise and ISI caused by the multipath radio environment. Therefore any information transmitted from A – B will require some decoding or equalisation to be applied to it. The detection processes can either be coherent or non-coherent.

coherent processes use explicit knowledge of the channel measured from known information passed through the channel. This advantage of this detection process is the simplicity of implementation at the expense of overhead data, which reduces the spectral efficiency of the channel.

non-coherent detection relies on some prior knowledge of a parametric model of the channel, exploiting the correlation properties of the channel or using blind estimation. Whilst these techniques may be more spectral efficient they are generally complex to implement.

LTE uses a coherent detection method by passing, so called, reference Signals (rS) through the channel at specific time and frequency intervals.

B

Noise

A Channel, H

Attenuation, distortion, ISI, fading


Fig. 32 – using Reference signals in the Channel

70



LTE Reference signals

There are a number of different reference signals used in LTE.

cell specific rS or common rS, these are available to al the uEs in a cell to perform basic •channel estimation functions.uE specific rS, embedded in the data structure for uL and dL for specific uE. The uL and •dL structures are different. In the uL there are 2 different types of uL rS, demodulation rS (dM rS) which are used to take channel estimates for coherent demodulation and Sounding rS (SrS) which are not directly associated with uL data or control. The SrS is used primarily for channel quality determination to enable frequency-selective scheduling on the uplink.There are also rS that are specific signals transmitted which are only used for the Multimedia •Broadcast Single Frequency network.

Time

Freq

uenc

y

R0

R0

R0

R0

R0

R0

R0

R0


Fig. 33 – General Arrangement of Rs in the LTE RB

72



DL Cell specific Rs

The figure opposite shows how the reference signals are arrange in the frequency and time domain.

The actual separation of the rS in the time domain is determined from the maximum doppler spread expected in the channel. LTE is designed to work up to 500Kph, assuming 2GHz spectrum the maximum doppler shift would be ~950Hz, according to the nyquist sampling theorem the signal should be sampled with an interval no less than twice the inverse of the frequency shift. Therefore there should be at least 2 rS per slot (where a slot in 0.5mS) in the time domain.

The separation of the rS in the frequency domain is related to the amount of delay spread present in the channel. The rMS delay spread is assumed to be no worse that 991nS therefore the coherence bandwidth for 90% and 50% of the rMS spread expected is somewhere between 20KHz and 200KHz. The rS are distributed every 3rd sub-carrier (over 2 symbols), therefore the expected frequency variations may be resolved.

The LTE dL has been designed to work with multiple antennas, therefore there are different rS patterns for each antenna ports that may be in use. The position of the rS in the time and frequency domains is carefully chosen to ensure there is no overlap between the antenna ports. This allows the receiver to take up to 4 separate dL channel estimates.

The rS its self is a pseudo random sequence from a length – 31 Gold sequence with different initialisation values depending on the type of rS. The rS can also carry one of 501 different cell identities and each rS has a cell specific frequency shift applied to it, to reduce the time-frequency collisions that may occur in a frequency re-used system.

Time

(a)Antenna port (0) Antenna port (1)

Freq

uenc

y

R0 R1R0 R1

R0 R1R0 R1

R0 R1R0

R0 R1R0

R1

R1


Fig. 34 – DL Rs Freq-Time Locations for 2 port Tx Antenna

Pattern of RS Depends on the Antenna Port used•

Time and Freq separation determined from Doppler •and Delay Spread

RS is formed from a length-31 Gold sequence•

74



DL uE specific Rs

rS which are specific to uE may also be used, they are embedded in the resource Blocks (rB) which are transmitted to a specific uE. More accurately they occur in the rB to which the PdScH is mapped for uE which are configured to operate in this mode. The mode is configured by higher layer rrc signalling.

uE specific rS may be used to enable the application of beam-forming antennas, where a single beam is formed to transmit data to the uE. Where beam-forming antennas are used the channel response for different uEs will be different there for the use of Eu specific rS is very useful.

The position of the uE specific rS in the rB is shown in the diagram opposite, the location of the rS in the frequency and time domain is chosen so as not to collide with the cell specific rS.

Time

Freq

uenc

y

R5

R5

R5

R5

R5

R5

R5

R5

R5

R5

R5

R5


Fig. 35 – uE specific DL Rs positions

Specific to a UE•

Used to assist DL Beam-forming•

UE RS position orthogonal to cell specific RS•

76



uL uE specific Rs

As with the downlink (dL), the uplink (uL) specifies the use of reference signals to enable the coherent detection of the channel. The rS can be used to support channel estimation, channel quality estimation for uL scheduling, power control, timing estimation and direction-of-arrival estimation for downlink beam-forming.

There are two types of rS:

demodulation rS (dM rS) associated with transmission of data on the PuScH and control •data on the PuccH. Primarily used to derive the channel estimate for coherent demodulationSounding rS (SrS) used to determine the uL channel quality and derive the frequency •selective scheduling on the uL

Demodulation Reference signals (DM Rs)

The uL rS are once again based on the Zadoff-chu sequences, similar to those used in the PSS and SSS. There are 30 base-sequences available whose length is determined by the number of rBs allocated to a uE. Within a base-sequence there are 12 possible orthogonal (good cross correlation) time shifted versions of the sequence. A cell will be allocated on of the 30 base sequences and the BS will allocate one of the 12 possible time shifts to the uEs. In a non-MIMo case the same time-shifted sequence could be used for all uEs since there transmissions are separated in the frequency and time domains.

Where MIMo is used in the channel then further means are required to separate the uE transmissions. In this case the uEs sharing the MIMo channel will be allocated different time shifted sequences from the same base-sequence in the cell.

The dM rS appear on the uL channel in the 4th symbol of each allocated slot and span the entire allocated bandwidth. This is true for allocations of PuScH and PuccH.

Group U29

ZC Seq ZC Seq U0

ZC Seq U1

ZC Seq U2

ZC Seq U12

Group U29

ZC Seq

Group U29

ZC Seq

Group U29

ZC Seq

Cell allocation UE allocation

PUSCH

DMRS

PUSCH

PUSCH

DMRS

PUSCH

Time

10 2 3Symbol

4 5 6

Freq

uenc

y

Res

ourc

e bl

ock


Fig. 36

DM Rs sequence Generation and Allocation

Mapping DM Rs to the physical Channels

78



sounding Reference signals (sRs)

The SrS have nothing to do with the specific transmission of data they allow the channel quality to be estimated and enables frequency selective scheduling. In addition the measured information may be used to enhance power control, or to support various start up functions for uE with new uL allocations. E.g. initial McS, initial power control, timing advance and frequency selective allocations for the first sub-frame slot.

The SrS occupies the last symbol of the sub-frame, and may occupy a bandwidth greater than that used by the data transmission, depending on specific control data sent to the uE. The transmission of the SrS may be aperiodic where a specific request is made for SrS or periodic, where the period may be any value 2,5,10,20,40,80,160 or 320mS.

The structure of the SrS signal is such that it can allow allocations of SrS sounding that overlap in the frequency domain. This is necessary to allow frequency selective scheduling between uEs.

PUSCH

DMRS

SRS

PUSCH

Time

2nd slot of a sub-frame

Symbol3 4 5 6210

Freq

uenc

y

Res

ourc

e bl

ock


Fig. 37 – Allocation of pusCh showing sRs Location

80



Modulation, Channel Coding and Link Adaptation

The LTE radio interface supports several modulation and coding schemes and allows the schemes to be adapted according to the quality of the radio link. The LTE radio link is developed primarily for the transmission of packet data therefore the link rate is allowed to rise and fall as the quality of the link rises and falls. For constant bit rate services such as voice, methods such as power control can be used to adapt the power output to keep link quality and therefore the link rate constant.

LTE supports QPSK, 16QAM and 64QAM with various code rates depending on the quality of the channel.

16QAM4 bits/Baud

64QAM6 bits/Baud

QPSK2 bits/Baud

10-2

10-1

Typical SNR Performance of LTE Modulation and Coding

0 5 10 15 20 25

QPSK, r = �∕� QPSK, r = ½ QPSK, r = �∕� QPSK, r = 4∕5 16QAM, r = �∕� 16QAM, r = ½

16QAM, r = �∕� 16QAM, r = 4∕5 64QAM, r = �∕� 64QAM, r = ½ 64QAM, r = �∕� 64QAM, r = 4∕5

BLER

SNR


Fig. 38

Modulation schemes supported by LTE

Typical snR performance of LTE Modulation and Coding

82




Fig. 38

Modulation and Coding Rate with spectral Efficiency CQi index Modulation Approximate

code rateEfficiency (information bits per symbol)

0 no transmission – –

1 QPSK 0.076 0.1523

2 QPSK 0.12 0.2344

3 QPSK 0.19 0.3770

4 QPSK 0.3 0.6016

5 QPSK 0.44 0.8770

6 QPSK 0.59 1.1758

7 16QAM 0.37 1.4766

8 16QAM 0.48 1.9141

9 16QAM 0.6 2.4063

10 64QAM 0.45 2.7305

11 64QAM 0.55 3.3223

12 64QAM 0.65 3.9023

13 64QAM 0.75 4.5234

14 64QAM 0.85 5.1152

15 64QAM 0.93 5.5547

84



Channel Coding

Cyclic Redundancy Check (CRC)A crc coding process is applied to each Transport Block (TB) – 24-bit crc applied to dL-ScH, PcH, and McH transport blocks and 16-bit crc applied to BcH and dcI code blocks.

segmentation code block segmentation is applied to dL-ScH, PcH, and McH transport blocks (i.e., data that are turbo encoded), with an additional 24-bit crc computed on each code-block (in cases where segmentation produces more than one code-block).

Encoding A Turbo code is applied to dL-ScH, PcH, and McH data to be carried over a downlink physical channel is scrambled prior to modulation. convolutional code is applied to BcH and dcI data (single code block).

channel coding used over the LTE air interface is based on the uTrAn release 6 turbo-coding schemes. other schemes are under consideration with the main drivers being

Improvement in power efficiency (low Eb/no)•Lower complexity decoder in the uE•code rates lower than 1/3.•Extension of maximum code block size•removal of tail•

All the above objectives are in pursuit of a reduction in overhead, an improvement in rF performance, and reduction in equipment costs.

coding schemes being studied by 3GPP include:

duo-binary turbo codes•Inter-block permutation turbo code (IBPTc)•rate-compatible/quasi cyclic LdPc code (rc/QcLdPc)•concatenated zigzag LdPc code•Turbo single parity check (SPc) low-density parity check (LdPc) code•Shortened turbo code by insertion of temporary bits•

Rate Matching rate matching is applied on a code-block basis to dL-ScH, PcH, McH, BcH, and dcI data. This function performs appropriate puncturing according to the AMc parameters.

Figure 39 is a schematic diagram of the above processes.

PDSCH Transport block (TB) processing

PBCH, Ref. Signals,P-SCH, S-SCH,

PCFICH, PDCCH,PHICH, PMCH

No. of antennas

#CB#CB

#CB#CB

# layers

TB2

TB1

UE…

UE1

ResourceElement Mapper(Subframe builder)

RF Front-End CP Insertion IFFT

LayerMapping

Precoding

Codeblock(CB)

Segmen-tation

TBCRC

CBCRC

Turboencoder(internal

interleaver)

Subblockinterleaver

Subblockinterleaver

Subblockinterleaver

• Rate matching• HARQ

functionality

Scrambling Modulation


Fig. 39 – Channel Coding process in LTE

86



hARQ (hybrid Automatic Request)

HArQ is commonly used in emerging communication systems to provide a high reliability over wireless channels. HArQ is essentially a combination of Automatic request (Arc) and Forward Error correction (FEc) techniques. Among two different types of HArQ are chase combining and incremental redundancy (Ir), which are also known as HArQ Type-I and HArQ Type-II (or Type-III), respectively. In the chase combining scheme the receiver sends a retransmission to the transmitter if the initial packet fails to be successfully decoded. Then the transmitter resends the same packet again so that the receiver combines the previously received packet with the new packet. In the Ir scheme instead of resending the same packet, the transmitters in general add more redundancy than the previous packet and recreate a different packet delivering the same information. The receiver needs to keep the previous erroneous packet (packet with bad crc) in the memory and combine it with the newly received packet for achieving a higher coding gain.

Discarddatax

ARQ data #1

CRC Data #2 CRC Data #1 CRC Data #1

CRC Data #1

Bufferdatax

ARQ data #1

CRC Data #2 CRC Data #1 CRC Data #1

CRC Data #1 CRC Data #1buffered

CRC Data #1combined


Fig. 40 – LTE hARQ

normal ARQ Operation

hybrid-ARQ Operation

UL-SCH, DL-SCH support HARQ•

1 Bit HARQ Field•

DownlinkAsynchronous •

ACK/NACK on PUCCH and PUSCH•

uplinkSynchronous •

ACK/NACK on PHICH•

88



Reporting of uE Feedback

The uE can be configured to report the quality of the channel to assist the enB with selecting the most appropriate modulation and coding scheme. The reports are derived from the downlink signal quality based o the downlink reference signals. The report signal quality is not a direct indication of the SInr in the channel, instead the chanel Quality Indicator (cQI) refers to the highest level of modulation and coding it can decode with an error rate not exceeding 10%. This method of reporting allows any advanced signal processing and channel decoding techniques to be employed.

The reporting may consist of the following elements:

cQI(channel quality indicator) is an indication of the downlink mobile radio channel quality as experienced by this uE. Essentially, the uE is proposing to the enB an optimum modulation scheme and coding rate to use for a given radio link quality, so that the resulting transport block error rate would not exceed 10%. 16 combinations of modulation scheme and coding rate are specified as possible cQI values. The uE may report different types of cQI.

A so-called “wideband cQI” refers to the complete system bandwidth. Alternatively, the uE may evaluate a “sub-band cQI” value per sub-band of a certain number of resource blocks which is configured by higher layers. The full set of sub-bands would cover the entire system bandwidth. In case of spatial multiplexing, a cQI per code word needs to be reported.

PMI(precoding matrix indicator) is an indication of the optimum precoding matrix to be used in the base station for a given radio condition. The PMI value refers to the codebook table. The network configures the number of resource blocks that are represented by a PMI report. Thus to cover the full bandwidth, multiple PMI reports may be needed. PMI reports are needed for closed loop spatial multiplexing, multi-user MIMo and closed-loop rank 1 precoding MIMo modes.

rI(rank indication) is the number of useful transmission layers when spatial multiplexing is used. For transmit diversity the rank is equal to 1.

The reporting may be periodic or aperiodic and is configured by the radio network. Aperiodic reporting is triggered by a cQI request contained in the uplink scheduling grant. The uE would send the report on PuScH. In the case of periodic reporting, PuccH is used if no PuScH is available.


Fig. 41 – Channel Reporting

CQi – Channel Quality indicatorDL channel quality as experienced by UE•

UE proposes optimum modulation and coding scheme•

Wideband CQI – complete system bandwidth•

Sub-band CQI – number or resource blocks•

pMi – precoding Matrix indicatorIndicates optimum precoding matrix•

Refers to codebook table•

Closed loop, MU-MIMO, Closed loop rank 1•

Ri – Rank indicationNumber of useful transmission layers for spatial •multiplexing

TX diversity Rank is 1•

Periodic or aperiodic•

CQI request on DL – UE reports on PUSCH•

UE reports on PUCCH if no PUSCH available•

90



power Control in LTE

Like many mobile radio systems LTE supports dynamic or adaptive power control. The reason for power control systems is to reduce the power emissions from devices and therefore reduce the overall interference across the network.

The system for LTE power control is shown on the opposite page. The scheme basically involves parameters that are determined by the current occupied bandwidth, network determined components for the cell and uE, the radio link pathloss and a power control command from the network.

The uE will read this information from the system information blocks or in dedicated messages during connection setup. Many of the parameters are determined by the upper layers and signalled during resource allocation. Some parameters such as the power control command are dynamic and can by modified on a regular basis.

PPUSCH(i) = min {PMAX, 10log10 (MPUSCH(i)) + PO_PUSCH(j) + α(j).PL + ∆TF(i) +ƒ(i)}

PMAX is the maximum allowed power that depends on the UE power class

α(j) = 0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 depending on certain configurations

PL is the downlink pathloss estimate calculated in the UE

∆TF is related to the Transport Block Size (TBS) and the number of resource elements

ƒ(i) = ƒ(i–1) + δPUSCH(i–KPUSCH)where δPUSCH is a UE specific correction value, also referred to as a TPC command

MPUSCH(i) is the bandwidth of the PUSCH resource assignment expressed in number of resource blocks valid for subframe i

PO_PUSCH(j) = PO_NOMINAL_PUSCH(j) + PO_UE_PUSCH(j) where

PO_NOMINAL_PUSCH(j) is a 8-bit cell specific signalled from higher layers

PO_UE_PUSCH(j) is a 4-bit UE specific component


Fig. 42 – power Control in LTE

92



The user plane and Control plane protocols

The user planeFigure 43 show the user Plane protocols, Packet data convergence Protocol (PdcP), radio Link control (rLc) and Medium Access control (MAc). These protocols will originate and terminate in the enB and uE

PDCP LayerThe PdcP will receive user data from the nAS and forward it to the rLc layer, and vice versa. It also provides retransmission, sequencing, and duplicate packet detection for handover when rLc operates in acknowledged mode.

ciphering, header de/compression and timer based packet discard are some of the other functions that this layer provides.

RLC LayerThe principal function of rLc is to provide a layer 2 datalink-like function. The rLc layer will receive data user data from the PdcP and forward it for scheduled transmission to the MAc layer and vice versa.

This layer can provide ArQ based error detection/correction, segmentation and reassembly of packets, sequenced delivery of upper layer information (not during handover) and duplicate detection.

rLc supports 3 modes of data transfer acknowledged mode, un-acknowledged mode, and transparent mode (AM, uM, TM). Each transfer mode will be selected depending on the required QoS of the upper later services.

MAC LayerThe MAc layer is primarily responsible for ensuring user data is mapped to the correct channels for transmission on the physical layer, this process is known as logical to physical channel mapping.

other functions include multiplexing/de-multiplexing of information from multiple radio bearers, HArQ error correction, priority handling and scheduling, transport format selection and padding. The MAc layer can also report traffic volume measurements to upper layers.

Transport channels

Physical channels

User plane Control plane

PHY36.201 PHY General36.211 PHY Channel and Modulation36.212 Multiplexing and Channel Coding36.213 PHY Procedures36.214 Measurements

MAC

36.321 MAC Protocol Speci�cation

Logical channels

RLC

36.322 RLC Protocol Speci�cation

Radio bearers

PDCP-controlPDCP-user

36.323 PDCP Protocol Speci�cation

RRC

36.331 RRC Protocol Speci�cation

NAS (ESM, EMM)

TCP/UDP

IP

APPs


Fig. 43 – LTE protocol stack

lte-planning sec04 100509 v01

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