04 - ltend - physical layer

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K Labs S.r.l. all right reserved

LTE Physical layer

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LTE Physical layer

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ROHC is Robust Header Compression, a standardized header-compression

algorithm used in WCDMA as well as several other mobile-communicationstandards.


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Layer 1 provides data transport services to the higher layers. These services

are accessed through transport channels via the MAC sub-layer. Thephysical layer provides transport channels to the Layer 2 MAC sub-layer,

and the MAC provides logical channels to the Layer 2 RLC sub-layer.

Transport channels are characterized by how the information is transferred

over the radio interface, whereas logical information is characterized by

the information type. The circles in the diagram between different layers

or sub-layers indicate service access points (SAPs). The physical layer also

interfaces to the Layer 3 RRC layer.


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LTE Physical layer

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The physical layer specifications are split into four main sections.TS36.211 physical channels and modulation

This specification describes the uplink and downlink physical signals andphysical channels, how they are modulated, and how they are mapped intothe frame structure. Included is the processing for the support of multipleantenna techniques.TS 36.212 multiplexing and channel coding

This specification describes the transport channel and control channel dataprocessing, including multiplexing, channel coding schemes, coding of L1and L2 control information, interleaving, and rate matching.TS 36.213 physical layer procedures

This specification describes the characteristics of the physical layerprocedures including synchronization procedures, cell search and timingsynchronization, power control, random access procedure, CQI reportingand MIMO feedback, UEsounding, HARQ, and ACK/NACK detection.TS 36.214 physical layer measurements

This specification describes the characteristics of the physical layermeasurements to be performed in Layer 1 by the UE and eNB, and howthese measurement results are reported to higher layers and the network.This specification includes measurements for handover support.

TS 36.133 radio resource managementAlthough not strictly a part of the physical layer, the requirements for radioresource management (RRM) are summarized here since they are closelylinked to the physical layer measurements.


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LTE Physical layer

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The LTE air interface consists of physical signals and physical channels,

which are defined in TS 36.211 Physical signals are generated in Layer 1and used for system synchronization, cell identification, and radio channel

estimation. Physical channels carry data from higher layers including

control, scheduling, and user payload.

In the downlink, primary and secondary synchronization signals encode the

cell identification, allowing the UE to identify and synchronize with the

network.

In both the downlink and the uplink there are RSs, known as pilot signals in

other standards, which are used by the receiver to estimate the amplitude

and phase flatness of the received signal. The flatness is a combination oferrors in the transmitted signal and additional imperfections that are due

to the radio channel. Without the use of the RS, phase and amplitude shifts

in the received signal would make demodulation unreliable, particularly at

high modulation depths such as 16QAM or 64QAM. In these high

modulation cases, even a small error in the received signal amplitude or

phase can cause demodulation errors.

Note: There are no formal acronyms to describe the primary and

secondary synchronization signals; the terms PSCH and S-SCH come from

earlier technical reports but are still used informally despite suggesting

channel rather than signal.


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LTE Physical layer

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Alongside the physical signals are physical channels, which carry the user andsystem information. Notice the absence of dedicated channels, which is a

characteristic of packet-only systems. The channel structure of LTE is closer to HSPAthan it is to the original W-CDMA, which is based on channels dedicated to singleusers.

LTE physical channelsDL channelsPBCH Physical broadcast channel- Carries cell-specific informationPMCH Physical multicast channel- Carries the MCH transport channelPDCCH Physical downlink control channel- Scheduling, H-ARQ Infos.PDSCH Physical downlink shared channel- PayloadPCFICH Physical control format indicator - Number of OFDM symbols used fortransmission of PDCCHs in a subframe (1,2,3, or 4).

PHICH Physical hybrid ARQ indicator channel- Carries HARQ ACK/NACK

UL channelsPRACH Physical random access channel- Call setupPUCCH Physical uplink control channel- Scheduling, ACK/NACKPUSCH Physical uplink shared channel Payload

The physical downlink/uplink control channel carries scheduling assignments andother control information. A physical control channel is transmitted on anaggregation of one or several consecutive control channel elements (CCEs), wherea control channel element corresponds to 9 resource element groups.

The PHICH carries the hybrid-ARQ ACK/NAK. Multiple PHICHs mapped to the sameset of resource elements constitute a PHICH group, where PHICHs within the samePHICH group are separated through different orthogonal sequences


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LTE Physical layer

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Throughout this specification, unless otherwise noted, the size of various

fields in the time domain is expressed as a number of time unitsTs=1/(15000 * 2048) seconds


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FS1 is optimized to co-exist with 3.84 Mbps UMTS systems. This structure

consists of ten 1 ms sub-frames, each composed of two 0.5 ms slots, for atotal duration of 10 ms. The FS1 is the same in the uplink and downlink in

terms of frame, sub-frame, and slot duration although the allocation of the

physical signals and channels is quite different. Uplink and downlink

transmissions are separated in the frequency domain.


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LTE Physical layer

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The structure of FS2 is much more flexible than the structure of FS1. An

example of an FS2 structure is shown in Figure. This example is for 5 msswitch-point periodicity and consists of two 5 ms half-frames for a total

duration of 10 ms.

Sub-frames consist of either an uplink or downlink transmission or a special

sub-frame containing the downlink and uplink pilot timeslots (DwPTS and

UpPTS) separated by a transmission gap guard period (GP). The allocation

of the sub-frames for the uplink, downlink, and special sub-frames is

determined by one of seven different configurations. Sub-frames 0 and 5

are always downlink transmissions and sub-frame 1 is always a special sub-

frame, but the composition of the other sub-frames varies depending onthe frame configuration. For a 5 ms switch-point configuration, sub-frame

6 is always a special sub-frame. With 10 ms switch-point periodicity, there

is only one special sub-frame per 10 ms frame.


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duration of 10 ms.











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duration of 10 ms.











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LTE Air Interface

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Time: time adjusted by the common SFN initialisation time, in units of

10ms to match the length of radio frame and accuracy accordingly;period(SFN): SFN period.

NOTE: When eNB is connected via TDM interfaces, these could be used to

synchronize frequency the eNB. The characteristics of these interfaces are

described in 25.411.

In case eNB is connected via TDM interface, it may be used to synchronize

frequency the eNB. The characteristics of the clock in the eNB shall be

designed taking into account that the jitter and wander performance

requirements on the interface are in accordance with network limits foroutput wander at traffic interfaces of either Reference [7], [8] or network

limits for the maximum output jitter and wander at any hierarchical

interface of Reference [9], whichever is applicable.

In case eNB is connected via Ethernet interface and the network supports

Synchronous Ethernet, the eNB may use this interface to get frequency

synchronization. In this case the design of the eNB clock should be done

considering the jitter and wander performance requirements on the

interface are as specified for output jitter and wander at EEC interfaces of

Reference [10], defined in section 9.2.1/G.8261. Further considerations on

Synchronous Ethernet recommendations and architectural aspects are

defined in clause 12.2.1 and Annex A of G.8261.


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LTE Physical layer

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The smallest time-frequency unit used for downlink/uplink transmission is

called a resource element, defined as one symbol on one subcarrier. Agroup of 12 subcarriers contiguous in frequency and one slot in time form a

RB. Transmissions are allocated in units of RB.


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One downlink slot using the normal CP length contains seven symbols.

Variations on this configuration for FS1 are summarized in Table. The CP ischosen to be slightly longer than the longest expected delay spread in the

radio channel.

For LTE, the normal CP length has been set at 4.69 s, enabling the system

to cope with path delay variations up to about 1.4 km. Note that this figure

represents the difference in path length due to reflections, not the size of

the cell.

Longer CP lengths are available for use in larger cells and for specialist

multi-cell broadcast applications. This provides protection for up to 10 km

delay spread but with a proportional reduction in the achievable datarates. Inserting a CP between every symbol reduces the data handling

capacity of the system by the ratio of the CP to the symbol length. For LTE,

the symbol length is 66.7 s, which gives a small but not insignificant seven

percent loss of capacity when using the normal CP.


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LTE Physical layer

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An entire 10 ms frame is required for the control channels to repeat. The

frame structure is referenced to Ts which is the shortest time interval ofthe system defi ned as 1/(15000x2048) seconds or 32.552 ns.

For this example, the physical mapping of the DL physical signals is as

follows:

RS are transmitted at OFDMA symbol 0 of the first subcarrier and symbol

4 of the fourth subcarrier of each slot. This is the simplest case for single

antenna use. The position of the RS varies with the antenna port number

and the CP length.

P-SCH is transmitted on symbol 6 of slots 0 and 10 of each radio frame; itoccupies 62 subcarriers, centered around the DC subcarrier.

S-SCH is transmitted on symbol 5 of slots 0 and 10 of each radio frame; it

occupies 62 subcarriers centered around the DC subcarrier.

PBCH is transmitted on symbols 0 to 3 of slot 1; it occupies 72 subcarriers

centered around the DC subcarrier.

Note that the PMCH, PCFICH, and PHICH are not shown in this example.


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LTE Physical layer

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Tables show the normal and extended CP lengths by symbol number. For

the normal CP configuration, the subcarrier spacing is 15 kHz and the CPlength is 160 x Tx (for OFDMA symbol number 0) and 144 (for OFDMA

symbols numbered 1 to 6 ). The extended CP lengths also are used to cope

with the longer path delays in large cells or for eMBMS in which multiple

cells are combined.


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LTE physical channelsDL channelsPBCH Physical broadcast channel- Carries cell-specific informationPMCH Physical multicast channel- Carries the MCH transport channelPDCCH Physical downlink control channel- Scheduling, H-ARQ infosPDSCH Physical downlink shared channel- PayloadPCFICH Physical control format indicator - Number of OFDM symbols used fortransmission of PDCCHs in a subframe (1,2,3, or 4).






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LTE Physical layer

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The downlink reference signals consist of known reference symbols inserted in

the first and third last OFDM symbol of each slot. There is one reference signaltransmitted per downlink antenna port. The number of downlink antenna

ports equals 1, 2, or 4. The two-dimensional reference signal sequence is

generated as the symbol-by-symbol product of a two-dimensional orthogonal

sequence and a two-dimensional pseudo-random sequence. There are 3

different two-dimensional orthogonal sequences and 170 different two-

dimensional pseudo-random sequences. Each cell identity corresponds to a

unique combination of one orthogonal sequence and one pseudo-random

sequence, thus allowing for 504 unique cell identities 168 cell identity groups

with 3 cell identities in each group).Frequency hopping can be applied to the downlink reference signals. The

frequency hopping pattern has a period of one frame (10 ms). Each frequency

hopping pattern corresponds to one cell identity group.

The downlink MBSFN reference signals consist of known reference symbols

inserted every other sub-carrier in the 3rd, 7th and 11th OFDM symbol of sub-

frame in case of 15kHz sub-carrier spacing and extended cyclic prefix

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LTE Physical layer

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Properties of Zadoff-Chu sequences:

1. They are periodic with period NZC if NZC is prime. xu(n + NZC) = xu(n)

2. Given NZC is prime, Discrete Fourier Transform of ZadoffChu sequence is

another ZadoffChu sequence conjugated, scaled and time scaled.

3. The autocorrelation of a prime length ZadoffChu sequence with a cyclically

shifted version of itself also has zero auto-correlation. i.e. it is non-zero only at

one instant which corresponds to the cyclic shift.

4. The cross correlation between two prime length ZadoffChu sequences, i.e.

different u, is constant 1/sqrt(NZC)

5. The constant amplitude propriety limits the PAPR and generates boundedtime-flat interference to other users.

5. ZC of any lenght have ideal cyclic autocorrelation (i.e. a Delta Dirac

autocorrelation function)

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Figure shows the difference between the periodic autocorrelation of a

truncated Pseudo Noise sequence (CDMA) and a ZC sequence. Both are839 symbiols long in the example. The periodic autocorrelation of ZC

sequence is exactly 0 for every shift whereas PN periodic autocorrelation

shows significant peaks, some above 0.1.

LTE Physical layer

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The three PSS sequences have low-frequency offset sensitivity, defined as

the ratio of the maximum undesired autocorrelation peak in the timedomain to the desired correlation peak computed at a certain frequency

offset. This allow a certain robustness of the PSS detectionduring the initial

synchronization procedure.

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SSC1 and SSC2 are two different cyclic shifts of a single lenght-31 M-

sequence. The cyclic shift indices of the M-seq are derived from a functionof the phy layer cell identity group. The two codes are alternated between

the first and the second SSS transmission in each radio frame. This enable

the UE to determine the 10 ms radio frame timing from a single

observation of a SSS. For each transmission, SSC2 is scrambled by a

sequence that depends on the index of SCC1. The sequence is then

scrambled by a code that depends on the PSS. The scrambling code is one-

to-one mapped to the phy layer identity whithin the group corresponding

to the target eNodeB.

The SSS id secoded after PSS and so the channel response is known at theUE. So the SSS deconding can be done with coherent decoding techniques.

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The physical cell identity, or L1 identity (Phy_ID in this document), is an

essential configuration parameter of a radio cell, it corresponds to a uniquecombination of one orthogonal sequence and one pseudo-random

sequence, and 504 unique Phy_IDs are supported leading to unavoidable

reuse of the Phy_ID in different cells [3].

When a new eNodeB is brought into the field, a Phy_ID needs to be

selected for each of its supported cells, avoiding collision with respective

neighbouring cells (the use of identical Phy_ID by two cells results in

interference conditions hindering the identification and use of any of them

where otherwise both would have coverage). Traditionally, the properPhy_ID is derived from radio network planning and is part of the initial

configuration of the node. The Phy_ID assignment shall fulfil following

conditions,

collision-free: the Phy_ID is unique in the area that the cell covers

confusion-free: a cell shall not have neighbouring cells with identical

Phy_ID


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LTE Physical layer

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An entire 10 ms frame is required for the control channels to repeat. The

frame structure is referenced to Ts which is the shortest time interval ofthe system defi ned as 1/(15000x2048) seconds or 32.552 ns.

For this example, the physical mapping of the DL physical signals is as

follows:

RS are transmitted at OFDMA symbol 0 of the first subcarrier and symbol

4 of the fourth subcarrier of each slot. This is the simplest case for single

antenna use. The position of the RS varies with the antenna port number

and the CP length.

P-SCH is transmitted on symbol 6 of slots 0 and 10 of each radio frame; itoccupies 62 subcarriers, centered around the DC subcarrier.

S-SCH is transmitted on symbol 5 of slots 0 and 10 of each radio frame; it

occupies 62 subcarriers centered around the DC subcarrier.

PBCH is transmitted on symbols 0 to 3 of slot 1; it occupies 72 subcarriers

centered around the DC subcarrier.

Note that the PMCH, PCFICH, and PHICH are not shown in this example.


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LTE Architecture and Air Interface

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Power Boosting

Power of the reference signal can be increased (up to 6 dB) Increase of coverage, channel estimation and reception quality

Signaling of RS Tx Power in P-BCH with 1 4 bits (for further study)

For the cell-specific RSs, a cell-specific frequency shift is also applied, given by

NCellID mod6.

This shift can avoid time-frequency collision between common RS from up to

six adjacent cells. Avoidance of collisions is particulary important in cases when

the transmission power of the RS is boosted, as is possible in LTE up to a

maximum of 6 dB relative to the surrounding data symbols.

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Power Boosting

Power of the reference signal can be increased (up to 6 dB) Increase of coverage, channel estimation and reception quality

Signaling of RS Tx Power in P-BCH with 1 4 bits (for further study)

For the cell-specific RSs, a cell-specific frequency shift is also applied, given by

NCellID mod6.

This shift can avoid time-frequency collision between common RS from up to

six adjacent cells. Avoidance of collisions is particulary important in cases when

the transmission power of the RS is boosted, as is possible in LTE up to a

maximum of 6 dB relative to the surrounding data symbols.

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Cap 2 - pag. 31

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UE-Specific RS may be transmitted in addition to the cell-specific RSs. They

are embedded only in the RBs to which the PDSCH is mapped for Ueswhich are specifically configured (by higher layer RRC signalling) ti receive

their downlink data transmissions in this mode. If UE-specific RSs are used,

the UE is expected to use them to derive the channel estimate for

demodulating the data in the corresponding PDSCH RBs. Thus the UE-

specific RS are treated as being transmitted using a distinct antenna port,

with its own channel response from the eNodeB to the UE.

A typica usage of UE-Specific RSs is to enable beamforming of the data

transmissions to specific UEs. In this case the UE experience a different

channel response due to the combining of multiple beams at the receiver.

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Cap 2 - pag. 33

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4 symbols for PDCCH are available only in case of narrow system

bandwidth (


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Cap 2 - pag. 35

Figure shows the downlink mapping across frequency and time. The central

sub-carrier of the downlink channel is not used for transmission but isreserved for energy generated due to LO feed through in the signal

generation process.


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PCFICH: Physical Control Format Indicator Channel

CFI=4 is reserved for future use.The 32-bit CFI values are modulated QPSK and mapped in 16 pre-defined

Resource Elements. CFI values are transmitted in Frequency Diversity on

different REG distributed on 6 RB.

The RE mapping configuration of CFICH is, as PBCH, independent of the

antenna scheme used.

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NOTE: the concept or REG (4 RE) is used also for PCFICH and PHICH

In order to provide robustness against transmissio errors, the PDCCH

information bits are coded and then scrambled with a cell-specific

scrambling sequence. This reduce the possibility of confusion with PDCCH

transmission from neighbour cells.

The scrambled bits are mapped to blocks of four QPSK symbols (REGs).

Interleaving is used to these symbol blocks, to provide frequency diversity,

followed by mapping to the available physical resource elements on the set

of OFDM symbols indicated by the PCFICH.

This mapping process exclude the resource elements reserved for RS andthe other control channels (PCFICH and PHICH).

The PDCCHs are transmitted on the same set of antenna ports as the PBCH,

and transmit diversity is applied if more than one antenna port is used.

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The factor-3 repetition coding is applied for robustness, resulting in 3

instances of the orthogonal Walsh code being transmitted for everysingle ACK/NACK message.

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In order to obviate the need of additional signalling to indicate which

PHICH carries the ACK/NACK for a specific PUSCH transmission, the PHICHindex is implicitly associated with the index of the lowest uplink RB used for

the corresponding PUSCH transmission.

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LTE physical channelsDL channelsPBCH Physical broadcast channel- Carries cell-specific informationPMCH Physical multicast channel- Carries the MCH transport channelPDCCH Physical downlink control channel- Scheduling, ACK/NACKPDSCH Physical downlink shared channel- PayloadPCFICH Physical control format indicator - Number of OFDM symbols used fortransmission of PDCCHs in a subframe (1,2,3, or 4).






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Cap 2 - pag. 42

Uplink reference signals [for channel estimation for coherent demodulation]

are transmitted in the 4-th block of the slot [assumed normal CP]. The uplinkreference signals sequence length equals the size (number of sub-carriers) of

the assigned resource.

The uplink reference signals are based on [prime-length] Zadoff-chu sequences

that are either truncated or cyclically extended to the desired length.

Multiple reference signals can be created:

- Based on different Zadoff-Chu sequence from the same set of Zadoff-Chu

sequences;

- Different shifts of the same sequence.

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Uplink reference signals [for channel estimation for coherent demodulation]

are transmitted in the 4-th block of the slot [assumed normal CP]. The uplinkreference signals sequence length equals the size (number of sub-carriers) of

the assigned resource.

The uplink reference signals are based on [prime-length] Zadoff-chu sequences

that are either truncated or cyclically extended to the desired length

Multiple reference signals can be created:

- Based on different Zadoff-Chu sequence from the same set of Zadoff-Chu

sequences;

- Different shifts of the same sequence.

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LTE Physical layer

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Example mappings for the PUSCH. The number of symbols in a slot

depends on the CP length. For a normal CP, there are seven SC-FDMAsymbols per slot. For an extended CP there are six SC-FDMA symbols per

slot.

Demodulation reference signals are transmitted in the fourth symbol of the

slot on all subcarriers of allocated PUSCH resource blocks. These are used

for uplink channel estimation to enable the eNB to demodulate the signal.


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Cap 2 - pag. 45

Uplink control information mainly consists of uplink channel quality

reporting and ACK/NACK indication for the related downlink transmission.The uplink transport format does not need to be signalled since everything

has already been decided by the base station.

No related HARQ information needs to be signalled in the uplink since

synchronous HARQ is being used.

Control Channel Information multiplexed onto PUSCH

Due to the single carrier constraint the user and control data must be

transmitted jointly.

So if there is uplink data transmission, the control information ismultiplexed to the PUSCH, whereas the PUCCH does not exist.

Control data will benefit from adaptive coding and modulation and there

will be no resource fragmentation.

Separate decoding should allow for independent processing to simplify

processing.

Same power must be used for control and data, but different coding

might be applied.


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LTE Physical layer

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Example mappings for the PUCCH. The number of symbols in a slot

depends on the CP length. For a normal CP, there are seven SC-FDMAsymbols per slot. For an extended CP there are six SC-FDMA symbols per

slot.

Demodulation reference signals are transmitted in the fourth symbol of the

slot on all subcarriers of allocated PUSCH resource blocks. These are used

for uplink channel estimation to enable the eNB to demodulate the signal.


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Cap 2 - pag. 47

Advantages of Border Positioning:

-Maximum frequency diversity by F. Hopping-Out-Of-Band emission are smaller if the UE is only transmitting a single RB

per slot compared to multiple Bs.

-Using control regions on the band edges maximizes the achievable PUSCH

data rate in the central position of the spectrum.

-Control regions on the band edges impose fewer constraints on the UL

data scheduling.

Two RBs are referred to as a PUCCH Region.


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Figure shows the FS1 physical uplink mapping for one UE assuming a

constant allocation. Because the uplink is shared by multiple users and thedata rate is directly linked to the bandwidth, the allocation for one UE will

almost always be much less than the channel bandwidth. The

demodulation reference signal in the uplink is not transmitted beyond the

allocation of each UE, unlike the reference signal in the downlink, which is

always transmitted across the entire operating bandwidth, even if the

downlink channel is not fully allocated.

This allows any UE to make channel measurements in the downlink to

optimize scheduling opportunities; however, for the uplink transmitting RS

at the maximum system bandwidth would be impractical for reasons ofbattery consumption and coordination with other UE. When no PUCCH or

PUSCH is scheduled in the uplink, the eNB can request transmission of the

sounding reference signal (SRS), which allows the eNB to estimate the

uplink channel characteristics.


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Inter-slot frequency hopping implies that the physical resources used for

uplink transmission in the two slots of a subframe do not occupy the sameset of subcarriers.


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Detailed specifications for the physical signals and channels, along with

their modulation and mapping, are documented throughout TS 36.211


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Block Procedures:

CRC calculation

24 bit CRC

Code block segmentation and code block CRC attachment

Channel coding

tail biting convolutional coding;

turbo coding.

Rate matching

Code Block Concatenation

The code block concatenation consists of sequentiallyconcatenating the rate matching outputs for the different code

blocks.

Data and Control Multiplexing

Channel Interleaving


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Gold Codes can be generated with very low implementation complexity, as

they can be derived from the mid-2 addition of two maximun-lenghtsequences (otherwise known as M-sequences), which can be generated

from a simple shift-register.

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Antenna Mapping jointly processes the modulation symbols corresponding

to transport blocks, and maps the result to the different antennas.LTE supports up to four transmit antennas.

Antenna mapping can be configured in different ways to provide different

multi-antenna schemes including transmit diversity, and spatial

multiplexing.

The layer mapping provides de-multiplexing of the modulation symbols of

each codeword (coded and modulated transport block) into one or

multiple layers.

The pre-coding extracts exactly one modulation symbol from each layer,

jointly processes these symbols, and maps the result in the frequency andantenna domain.


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