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LTE physical layer (layer 1)

Service provided to higher layers
Generic Frame Structure

Downlink transmission scheme

Downlink Frame Structure (Type 1)

Slot Structure and Physical Resources

Downlink Physical Channels
Downlink Physical signals

Reference Signals

Downlink Transport Channels
Mapping Downlink Physical Channels to Transport Channels

Uplink transmission scheme

Uplink Frame Structure (Type 1)
Uplink Physical Channels
Uplink Physical Signals
Uplink Transport Channels
Mapping Downlink Physical Channels to Transport Channels

Symbol proccessing requirements
References

LTE physical layer (layer 1)

The radio interface described in 3GPP LTE specifications covers the interface between the User Equipment (UE) and the network 6. The radio interface is composed of the layer 1, 2 and 3. The TS 36.200 series (3GPP LTE Specifications) describes the layer 1 (physical layer) specifications. Layers 2 and 3 are described in the 36.300 series.

Figure 1 shows the 3GPP LTE radio interface protocol architecture around the physical layer (layer 1). The physical layer interfaces the Medium Access Control (MAC) sub-layer of layer 2 and the Radio Resource Control (LTE::RRC) layer of layer 3. The circles between different layer/sub-layers indicate Service Access Points (SAPs). The physical layer offers a transport channel to MAC. The transport channel is characterized by how the information is transferred over the radio interface. MAC offers different Logical channels to the Radio Link Control (RLC) sub-layer of layer 2. A logical channel is characterized by the type of information transferred.

Service provided to higher layers

The physical layer offers data transport services to higher layers. The access to these services is through the use of a transport channel via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service 6:

Error detection on the transport channel and indication to higher layers
FEC encoding/decoding of the transport channel
Hybrid ARQ soft-combining
Rate matching of the coded transport channel to physical channels
Mapping of the coded transport channel onto physical channels
Power weighting of physical channels
Modulation and demodulation of physical channels
Frequency and time synchronisation
Radio characteristics measurements and indication to higher layers
Multiple Input Multiple Output antenna processing
Transmit diversity
Beam forming
RF processing.

Generic Frame Structure

The generic frame structure is illustrated in Figure 2 7. Each 10 ms radio frame is divided into ten equally sized sub-frames. Each sub-frame consists of two equally sized slots. Each sub-frame can be assigned for either downlink or uplink transmission. Note that there are certain restrictions in the assignment as the first and sixth sub-frame of each frame include the downlink synchronization signals.

In addition, for coexistence with LCR-TDD, an alternative frame structure illustrated in Figure 3 is also supported when operating E-UTRA in TDD mode.

The size of various fields in the time domain is expressed as a number of time units Ts = 1/ (15000 x 2048) seconds. Downlink and uplink transmissions are organized into radio frames with Tf = 307200 x Ts = 10 ms duration. Two radio frame structures are supported:

Type 1, applicable to FDD
Type 2, applicable to TDD

Frame structure type 1 is optimized to co-exist with 3.84 Mcps UTRA systems. Frame structure type 2 is optimized to co-exist with 1.28 Mcps UTRA TDD systems, also known as TD-SCDMA. This document focuses on frame structure type 1.

Downlink transmission scheme

The downlink transmission scheme is based on conventional OFDM. In an OFDM system, the available spectrum is divided into multiple carriers, called subcarriers, which are orthogonal to each other. Each of these subcarriers is independently modulated by a low rate data stream. shows the generation of an OFDM symbols from a stream of serial QAM symbols. The N parallel streams are treated as samples in frequency domain, finally the N-point time domain blocks are obtained from the IFFT, which are subsequently serialsed to create a time domain signal.

Downlink Frame Structure (Type 1)

Downlink frame structure comprises of symbols mapped to different sub-carriers from multiple users. Each user is allocated a specific number of subcarriers for a specified time slot. These are referred to as physical resource blocks (PRBs) in the LTE specifications. PRBs thus are identified by two factors - time and frequency. Base station (eNodeB) is responsible for the resource scheduling to different users.

Frame structure type 1, as shown here is applicable to both full duplex and half duplex FDD. Each radio frame is long and consists of 20 slots of length, numbered from 0 to 19. A subframe is defined as two consecutive slots where subframe consists of slots and. For FDD, 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmissions in each 10 ms interval. Uplink and downlink transmissions are separated in the frequency domain. In half-duplex FDD operation, the UE cannot transmit and receive at the same time while there are no such restrictions in full-duplex FDD 7.

Slot Structure and Physical Resources

The smallest time-frequency unit for downlink transmission is called a resource element, which is one symbol on one sub-carrier. A group of 12 contiguous sub-carriers in frequency and one slot in time form a resource block (RB) as shown in (Ref 3GPP TS 36.211 V8.0.0 Figure 6.2.2-1). Data is allocated to each user equipment (UE) in units of RB.

For a frame structure type 1 using normal cyclic prefix (CP), a RB spans 12 consecutive sub-carriers at a sub-carrier spacing of 15 kHz, and 7 consecutive symbols over slot duration of 0.5 ms as shown in Table 2. A CP is appended to each symbol as a guard interval. Thus, a RB has 84 resource elements (12 sub-carriers x 7 symbols) corresponding to one slot in the time domain and 180 kHz (12 sub-carriers x 15 kHz spacing) in the frequency domain. The size of a RB is the same for all bandwidths; therefore, the number of available physical RBs depends on the transmission bandwidth. In the frequency domain, the number of available RBs can range from 6 (when transmission bandwidth is 1.4 MHz), to 100 (when transmission bandwidth is 20 MHz), as shown in table 2.

For a frame structure type 1 using normal cyclic prefix (CP), a RB spans 12 consecutive sub-carriers at a sub-carrier spacing of 15 kHz, and 7 consecutive symbols over a slot duration of 0.5 ms as shown in Table 2 (Ref 3GPP TS 36.211 V8.2.0 Figure 6.2.3-1). A CP is appended to each symbol as a guard interval. Thus, a RB has 84 resource elements (12 sub-carriers x 7 symbols) corresponding to one slot in the time domain and 180 kHz (12 sub-carriers x 15 kHz spacing) in the frequency domain. The size of a RB is the same for all bandwidths; therefore, the number of available physical RBs depends on the transmission bandwidth. In the frequency domain, the number of available RBs can range from 6 (when transmission bandwidth is 1.4 MHz), to 100 (when transmission bandwidth is 20 MHz), as shown in Table 3 5.

Configuration	N_sc^RB	N_symb^DL
Normal cyclic prefix, gap=15Khz	12	7
Extended cyclic prefix, gap=15Khz	12	6
Extended cyclic prefix, gap=7.5Khz	24	3

Table 2 Resource block parameters

Channel bandwidth BWChannel MHz	1.4	3	5	10	15	20
Transmission bandwidth configuration NRB	6	15	25	50	75	100

Table 3 Available Downlink Bandwidth is Divided into Physical Resource Blocks

In case of multi-antenna transmission, there is one resource grid defined per antenna port. An antenna port is defined by its associated reference signal. The set of antenna ports supported depends on the reference signal configuration in the cell 7, as explained in section 4.3.

Downlink Physical Channels

Three different types of physical channels are defined for the LTE downlink 11. Physical channels are used to convey information from higher layers in the LTE stack. This is in contrast to physical signals, which convey information that is used exclusively within the PHY layer.

LTE DL physical channels are:

Physical Broadcast Channel (PBCH)
The PBCH carries cell-wide broadcast message to all UEs. Robustness rather than maximum data rate is the chief consideration.

Physical multicast channel (PMCH)
Carries the Multicast Messages.
Physical Downlink Control Channel (PDCCH)
The PDCCH conveys UE-specific control information. Like PBCH, robustness rather than maximum data rate is therefore the chief consideration

Physical Downlink Shared Channel (PDSCH)
The PDSCH is utilized basically for data and multimedia transport. It therefore is designed for very high data rates.

Table 4 below shows the modulation scheme followed for different physical channels

Downlink Channels	Modulation Schemes
PBCH	QPSK
PMCH	QPSK
PDCCH	QPSK
PDSCH	QPSK, 16QAM, 64QAM

Table 4 Modulation Scheme for Downlink Physical Channels

Downlink Physical signals

Physical signals use assigned resource elements. However, unlike physical channels, physical signals do not convey information to/from higher layers.

The main DL physical signals are 7:

Reference signal (RS)
The downlink reference signal structure is important for cell search, channel estimation and neighbor cell monitoring. Reference signals are generated as the product of an orthogonal sequence and a pseudo-random numerical (PRN) sequence. Frequency hopping can be applied to the downlink reference signals. The frequency hopping pattern has a period of one frame (10 ms).

Primary and secondary synchronization signals (P-SCH, S-SCH)
During cell search, different types of information need to be identified by the handset: symbol and radio frame timing, frequency, cell identification, overall transmission bandwidth, antenna configuration, and cyclic prefix length.

Besides the reference signals, synchronization signals are therefore needed during cell search. The synchronization acquisition and the cell group identifier are obtained from different SYNC signals. Thus, a primary synchronization signal (P-SYNC) and a secondary synchronization signal (S-SYNC) are defined with a pre-defined structure.

Downlink Modulation schemes followed for different Physical Signals are shown in table below

Physical Signal	Modulation Schemes
RS	Orthogonal sequence of binary PN sequence
P-SCH	Cycle of 3 Zadoff-Chu sequence
S-SCH	Two 31 bit BPSK M sequences

Table 5 Modulation scheme for downlink reference signals

Reference Signals

Three types of downlink reference signals are defined:

Cell-specific reference signals, associated with non-MBSFN transmission, support a configuration of one, two, or four antenna ports and the antenna port number shall fulfil, , and , respectively.
MBSFN reference signals, associated with MBSFN transmission, are transmitted on antenna port.
UE-specific reference signals are transmitted on antenna port.

Mapping of Cell Specific Reference Signals

Reference signal is transmitted at OFDM symbol 0 and 4 of each slot. This depends on frame structure type and antenna port number 7.

Figure 5 Resource Element Mapping of Reference Signals

Downlink Transport Channels

The physical layer provides information transfer services to MAC and higher layers. The physical layer transport services are described by how and with what characteristics data are transferred over the radio interface. An adequate term for this is “Transport Channel”. The classification of what is transported, is related to the logical channels at MAC sub-layer and is clearly separated from Transport Channel

Downlink transport channel types are 11:

Broadcast Channel (BCH) characterised by:

fixed, pre-defined transport format;
requirement to be broadcast in the entire coverage area of the cell.

Downlink Shared Channel (DL-SCH) characterised by:

support for HARQ;
support for dynamic link adaptation by varying the modulation, coding and transmit power;
possibility to be broadcast in the entire cell;
possibility to use beam forming;
support for both dynamic and semi-static resource allocation;
support for UE discontinuous reception (DRX) to enable UE power saving;
support for MBMS transmission.

 NOTE: the possibility to use slow power control depends on the physical layer.

Paging Channel (PCH) characterised by:

support for UE discontinuous reception (DRX) to enable UE power saving (DRX cycle is indicated by the network to the UE);
requirement to be broadcast in the entire coverage area of the cell;
mapped to physical resources which can be used dynamically also for traffic/other control channels.

Multicast Channel (MCH) characterised by:

requirement to be broadcast in the entire coverage area of the cell;
support for MBSFN combining of MBMS transmission on multiple cells;
support for semi-static resource allocation e.g. with a time frame of a long cyclic prefix.

Mapping Downlink Physical Channels to Transport Channels

Figure 6 Mapping between downlink transport channels and downlink physical channels

Uplink transmission scheme

LTE has adopted SC-FDMA with cyclic prefix as uplink transmission scheme for both FDD and TDD mode of operation 11. SC-FDMA is a hybrid transmission scheme combining the low peak to average power ratio (PAPR) of single carrier schemes with the frequency allocation flexibility and multipath protection provided by OFDMA symbols. The PAPR characteristics are important for cost-effective design of UE power amplifiers. Still, SC-FDMA signal processing has some similarities with OFDMA signal processing, so parameterization of downlink and uplink can be harmonized.

Figure 9 Block Diagram of DFT-s-OFDM (Localized transmission)

Figure 9 shows the flow of SC-FDMA signal generation. The processing steps expains why SC-FDMA is sometimes described in specs as Discreet Fourier Transform spread OFDM (DFT-s-OFDM). The key processings steps shown in the figure 13 are formally defined in the physical layer specification 36.211 4:

Perform a DFT of length M/(symbol period) to create M point FFT bins spaced by 15 KHz
Map the M subcarriers to the desired locations (localized or distributed) within the system bandwidth
Perform an N point IFFT to create a time domain signal.
Insert the cyclic prefix between SC-FDMA symbols and transmit the IQ symbols after parallel to serial conversion.

Uplink Frame Structure (Type 1)

The Uplink Frame Structure Type 1 is same as Downlink in terms of frame, slot and sub-frame length as shown in Figure 14. An uplink radio frame consists of 20 slots of 0.5 ms each, and 1 subframe consists of 2 slots. The slot structure is shown in the figure below 7.

The transmitted signal in each slot is described by a resource grid of subcarriers and SC-FDMA symbols. The resource grid is illustrated in Figure 14.

The quantity N^UL_RB depends on the uplink transmission bandwidth configured in the cell and shall fulfill 6 < N^UL_RB < 110

Where 6 and 110 is the smallest and largest uplink bandwidth, respectively, supported by the current version of 3GPP LTE specification. The set of allowed values for N^UL_RB is given in table 6 12.

Channel bandwidth BWChannel MHz	1.4	3	5	10	15	20
Transmission bandwidth configuration NRB	6	15	25	50	75	100

Table 6 Transmission bandwidth configuration NRB in E-UTRA channel bandwidths

Figure 10 Uplink resource grid
The number of SC-FDMA symbols in a slot depends on the cyclic prefix length configured by higher layers and is given in Table 7.

Configuration	N^RB_sc	N^UL_symb
Normal cyclic prefix	12	7
Extended cyclic prefix	12	6

Table 7 Resource block parameters

Uplink Physical Channels

An uplink physical channel corresponds to a set of resource elements, which are used to transmit information originating from higher layers. The following uplink physical channels are defined:

Physical Uplink Shared Channel, PUSCH
User data is carried on the Physical Uplink Shared Channel (PUSCH) that is determined by the transmission bandwidth. The uplink transmission time interval is 1 ms (same as downlink). The PUSCH may employ QPSK, 16QAM or 64QAM modulation.
Physical Uplink Control Channel, PUCCH
The physical uplink control channel, PUCCH, carries uplink control information. The PUCCH is never transmitted simultaneously with the PUSCH from the same UE

Carries Hybrid ARQ ACK/NAKs in response to downlink transmission;
Carries Scheduling Request (SR);
Carries CQI (channel Quality Indicator) reports.

Physical Random Access Channel, PRACH
The PRACH carries the random access preamble. The physical layer random access preamble, illustrated in Figure 15 11, consists of a cyclic prefix of length and a sequence part of length. The parameter values are listed in Table 8 (where TS = period of a 30.72 MHz clock) and depend on the frame structure and the random access configuration. Higher layers control the preamble format.

Figure 11 Random access preamble format

Preamble format	T_CP	T_SEQ
0	3168.T_S	24576.T_S
1	21024.T_S	24576.T_S
2	6240.T_S	2 * 24576.T_S
3	21024.T_S	2 * 24576.T_S
4 (frame structure type 2 only)	448.T_S	4096.T_S

(frame structure type 2 only)
Table 8 Random access preamble parameters

The random access preambles are generated from Zadoff-Chu sequences with zero correlation zone, generated from one or several root Zadoff-Chu sequences. The network configures the set of preamble sequences the UE is allowed to use. There are 64 preambles available in each cell. The preamble sequence occupies TPRE = 0.8 ms and the cyclic prefix occupies TCP = 0.1 ms within one subframe of 1 ms. During the guard time TGT, nothing is transmitted. The preamble bandwidth is 1.08 MHz (72 subcarriers).

Uplink Physical Signals

An uplink physical signal is used by the physical layer but does not carry information originating from higher layers. The following uplink physical signals are defined:

Reference signal 7
Two types of uplink reference signals are supported:

Demodulation reference signal, associated with transmission of PUSCH or PUCCH
Sounding reference signal, not associated with transmission of PUSCH or PUCCH

The same set of base sequences is used for demodulation and sounding reference signals. Uplink reference signals for channel estimation for coherent demodulation are transmitted in the 4-th block of the slot assumed normal CP. The uplink reference 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 11.

Uplink Transport Channels

As in the DL, uplink transport channels act as service access points for higher layers. UL transport channel types are 11:

Uplink Shared Channel (UL-SCH) characterised by:

possibility to use beamforming; (likely no impact on specifications)
support for dynamic link adaptation by varying the transmit power and potentially modulation and coding;
support for HARQ;
support for both dynamic and semi-static resource allocation.

NOTE: the possibility to use uplink synchronisation and timing advance depend on the physical layer.

Random Access Channel(s) (RACH) characterised by:

limited control information;
collision risk;

NOTE: the possibility to use open loop power control depends on the physical layer solution.

Mapping Downlink Physical Channels to Transport Channels

Transport channels are mapped to physical channels as shown in Figure 1611.

Figure 10 Mapping between uplink transport channels and uplink physical channels

Symbol proccessing requirements

The ideal symbol length in OFDM systems is defined by the reciprocal of the subcarrier spacing and is chosen to be long compared to the expected delay spread. LTE has chosen 15 kHz subcarrier spacing, giving 66.7 µs for the symbol length. In a single-carrier system, the symbol length is closely related to the occupied bandwidth. For example, GSM has 200 kHz channel spacing and a 270.833 ksps symbol rate, giving a 3.69 µs symbol length that is 18 times shorter than that of LTE. In contrast, W-CDMA has 5 MHz channel spacing and a 3.84 Msps symbol rate, producing a 0.26 μs symbol length — 256 times shorter than LTE.

Each 15 kHz subcarrier in LTE is capable of transmitting 15 ksps,
Number of subcarriers = 1200 ( at 20 MHz bandwidth )
Symbol rate = 15 Ksps * 1200 subcarries = 18 Msps at 20 MHz bandwidth

Thus, LTE has a raw symbol rate of 18 Msps @ 20 MHz system bandwidth (1200 subcarriers, 18 MHz).
The maximum data rate is possible with the 64 QAM modulation scheme. The modulation scheme is selected on the basis of channel condition and channel types. Using 64QAM modulation format:
one symbol = 6 bits,
total capacity = 6 * 18 Msps = 108 Mbps.

Note that actual peak rates as described in the LTE specifications is arrived at after substracting the channel coding and control overheads and adding gains from features such as spatial multiplexing.