5.4 Medium and high power regime: towards the Mott transition
5.4.2 Spe tral hara teristi s
= 1
15,000 x 2048
= 32.5 nsec
Timing Units
Almost all numbers, durations and calculations related to E-UTRA are derived from a fundamental parameter known as Ts – or the Basic Time Unit.
Ts represents the ‘sampling time’ of the overall channel and is itself derived from basic channel parameters.
The underlying calculations for E-UTRA are based on a service that operates in a 20 MHz channel, with 2048 subcarriers set at 15 kHz spacing. E-UTRA deployments at all other bandwidths are based on these parameters.
Ts is calculated to be the reciprocal of the subcarrier spacing multiplied by the total number of subcarriers in the FFT, or:
Ts = 1/15,000 x 2048 seconds = 0.0325 μsec
Frame, subframe and slot lengths, cyclic prefix durations and many other key parameters are calculated as multiples of Ts.
Crucially, the value of Ts does not vary even when E-UTRA operates in channel bandwidths that are smaller then 20 MHz. As Ts stays constant, all of the key parameters used to define E-UTRA services also stay constant.
The consistency reduces the overall complexity of E-UTRA and enables system manufacturers to scale their devices more easily to a variety of channel bandwidths and frequency bands.
Further Reading: 3GPP TS 36.211
The LTE Radio Interface
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Band Popular name Frequencies (MHz)
1 IMT Core 1920–1980/2110–2170
2 PCS 1900 1850–1910/1930–1990
3 GSM 1800 1710–1785/1805–1880
4 AWS (US) 1710–1755/2110–2155
5 850 (US) 824–849/869–894
6 850 (Japan) 830–840/875–885 7 IMT Extension 2500–2570/2620–2690
8 GSM 900 880–915/925–960
9 1700 (Japan) 1750–1785/1845–1880 10 3G Americas 1710–1770/2110–2170
FDD
TDD Band Popular name Frequencies (MHz)
33 TDD 1900 1900–1920
34 TDD 2.0 2010–2025
37 (1915)1910–1930
38 2570–2620
PCS Centre Gap IMT Extension Centre Gap
LTE Frequency Bands
Preparation for the deployment of EPS systems has begun in many countries, with operators, vendors and regulators starting to discuss the spectrum requirements for these systems.
The standards currently identify 13 bands for FDD operation, ranging from frequencies in the range 800 MHz through to frequencies in the range 2.5 GHz. There also eight bands identified for TDD operation ranging from 1900 MHz to 2.5 GHz. Considerable scope has been left in the standards to add more frequency bands as global requirements evolve.
Further Reading: 3GPP TS 36.211, 36.104
LTE/SAE Engineering Overview
1 ms Subframe (2 slots)
Normal CP
1 ms Subframe (2 slots)
The Physical Layer Frame Structure
There are two basic frame types employed in E-UTRA, which are common to both uplink and downlink.
Type 1 frames are employed for FDD full- and half-duplex systems, while Type 2 frames are reserved for TDD operation only.
Type 1 frames last for 10 ms and are composed of 20 slots of 0.5 ms each. Two slots are combined to form a subframe, which lasts for 1 ms. For FDD, 10 subframes are available for downlink transmissions and 10 for uplink transmissions in each 10 ms interval. Uplink and downlink transmissions are
separated in the frequency domain.
Type 1 slots contain either 7 or 6 symbols, depending upon which CP type is used. The length of the CP prefixed to each symbol may vary depending upon where that symbol sits within the slot. With the normal CP, symbol 0 in each slot has a CP equal to 160 x Ts or 5.21 μsec, while the remaining symbols in the slot have slightly shorter CPs of just 144 x Ts or 4.7 μsec. When using the extended CP, all symbols are prefixed with a CP of 512 x Ts or 16.67 μsec.
Scheduling occurs across a subframe period. Up to the first three symbols in the first slot of each subframe can be defined as a ‘control region’ carrying control and scheduling messages. The remaining symbols of the first and all symbols in the second slot within the subframe are then available for user traffic.
Type 2 frames are used by TDD systems, which are required to be backwards-compatible with the Chinese TD-SCDMA IMT-2000 standard; they last for 10 ms and consist of two 5 ms half-frames.
Each half-frame carries six subframes and three specialized fields. Subframe 0 and the DwPTS (Downlink Pilot Time Slot) field are reserved for downlink services and subframe 1 and the UpPTS (Uplink PTS) field are reserved for uplink – the other fields can be assigned dynamically between uplink and downlink.
Further Reading: 3GPP TS 36.211
The LTE Radio Interface
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1 2 3 4 5 6 0 1 2 3 4 5 6
1 ms Subframe (2 slots)
Subcarrier 1
Subcarrier 12 180 kHz
Resource Descriptions
Capacity allocation in E-UTRA is based on a concept known as the RB (Resource Block). A PRB (Physical Resource Block) consists of 12 subcarriers (in the frequency domain) for one slot period (in the time domain).
On both uplink and downlink channels, 12 subcarriers correspond to 180 kHz of bandwidth. The minimum possible capacity allocation period is the TTI (Transmission Time Interval) of 1 ms. This equates to the allocation of two consecutive resource blocks.
For capacity allocation purposes, the resources available during a frame period are organized into resource grids, which consist of a number of consecutive resource blocks.
The theoretical minimum definable capacity allocation unit is the resource element, which is defined as one subcarrier during one symbol period. Within each resource grid the resource elements that will be carrying reference signals are assigned first; the remaining elements are then available to have user data or control messages mapped to them.
In data transfer terms, one resource element is the equivalent of one modulation symbol on a
subcarrier, so if QPSK modulation was being employed, one resource element would be equal to 2 bits, with 16QAM 4 bits and with 64QAM 6 bits of transferred data.
If MIMO is employed on the downlink then separate resource grids are created for each antenna port – each port maps to a different MIMO stream.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
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Modulation Schemes
Error Coding Schemes
CRC
BPSK QPSK
16QAM
64QAM
Signalling functions only
Optional on uplink
1/3 Turbo Coding Traffic and most control channels
1/3 CC BCH only
Transport Block 24 bit CRC
Error Protection and Modulation Schemes
The range of modulation schemes used in E-UTRA comprises BPSK, QPSK, 16QAM and 64QAM.
BPSK is only employed for a limited set of signalling and reference functions, while 64QAM is optional on the uplink.
The range of error coding options used in E-UTRA devices is far more limited than those available to, for example, an HSPA device. For shared and multiplex traffic channels, the only error coding option mentioned in the current version of the applicable 3GPP specification (TR 36.212) is 1/3 Turbo coding.
Broadcast traffic channels are only permitted to use 1/3 Tail Biting convolutional coding. Various control channels have been assigned either convolutional coding, block coding or simple repetition as their error coding options.
In addition to error coding, transport blocks containing user and control traffic may also optionally have a CRC (Cyclic Redundancy Check) block attached. Transport blocks on connections that have CRC selected have a 24-bit CRC block appended to the end of the data container.
The familiar UMTS error monitoring levels of BER (Bit Error Rate), derived from the error coding service, and BLER (Block Error Rate), derived from CRC, continue to be available in E-UTRA.
Further Reading: 3GPP TS 36.211, 36.300
The LTE Radio Interface
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1 2 3 4 5 6 0 1 2 3 4 5 6 0
1 ms Subframe (2 slots)
Subcarrier 1
Downlink Reference Signals
In any mobile radio system it is necessary to provide mobile devices with a means of measuring and monitoring the strength and quality of the signal they receive and of calibrating their own output to ensure that the correct frequencies are being employed.
In an OFDM-based system, where different UEs are assigned the use of different sets of closely packed subcarriers, the need for reliable frequency calibration is critical.
E-UTRA’s version of OFDMA/SC-FDMA employs a physical reference signal, embedded in the main body of the transmitted signal to provide an opportunity for channel estimation and frequency calibration on the downlink. A process of UE sounding performs a similar function on the uplink.
Reference signal symbols are placed in predetermined locations within each resource grid. An example of this for both uplink and downlink is shown in the diagram.
On the downlink, three types of downlink reference signals are currently defined: cell-specific reference signals, MBSFN (Multicast/Broadcast Single Frequency Network) reference signals, associated with MBSFN transmission, and UE-specific reference signals.
In most circumstances only the first of these reference signal types will be used. The reference signal takes the form of a modulated symbol chosen from a pseudo-random sequence of symbols, which is inserted into the transmitted signal following a predetermined sequence.
Cell-specific reference signals, as well as providing a ‘known signal’ upon which to base channel estimations, are modulated to identify the cell to which they belong. When MIMO is employed, separate reference signals are transmitted by each air interface port. In Type 1 frames the downlink reference signal is transmitted during symbol periods 0 and 4, although the subcarrier on which it is transmitted varies following a predetermined pattern.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
1 ms Subframe (2 slots)
Uplink Reference Signals
A ‘demodulation’ reference signal is embedded in uplink user traffic and control transmissions. The demodulation signal provides the receiving eNB with a ‘known signal’ element upon which to perform channel estimations and from which it can calculate timing adjustments.
In Type 1 frames, the uplink demodulation reference signal is always sent in symbol 3 of each slot.
Channel estimations for received uplink signals are made by the eNB based on measurements taken of the reference signal symbols embedded in uplink transmissions.
If there is no uplink transmission taking place, however, the eNB cannot take measurements. In these circumstances a UE may be instructed to perform uplink sounding, which consists of the UE
transmitting a reference signal within an uplink resource allocation specifically set aside for the purpose.
UEs may also be instructed to undertake sounding to enable the eNB to perform ‘frequency-specific scheduling’. This term describes a procedure whereby the eNB measures the sounding signal
transmitted by a UE across some or all subcarriers and then chooses the resource block that consists of the best performing set of frequencies.
Further Reading: 3GPP TS 36.211, 36.300
The LTE Radio Interface
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Cell-specific Reference Signal – Physical Layer Group (1 of 168)
PSS – Cell ID within Physical Layer Group (1 of 3)
SSS – other synchronization parameters
Air Interface Synchronization
Air Interface Synchronization
E-UTRA employs two synchronization channels, Primary and Secondary, which, although based on different technologies, perform much the same functions as the PSS (Primary Synchronization Signal) and SSS (Secondary Synchronization Signal) in R99 UMTS.
To facilitate fast cell search, E-UTRA cells are assigned one of 504 physical layer cell identities. The 504 available identities are grouped into 168 groups, each of which contains three cell identities.
The physical layer cell identity group to which a cell belongs is explicitly flagged by the modulation of the cell-specific reference signal it transmits.
The PSS provides coarse frequency synchronization, on a whole channel basis, and also allows a UE to discover symbol, slot and subframe synchronization. The physical cell ID index of the ID assigned to the transmitting cell is carried by the P-SCH. There are three cell IDs per physical cell ID group, index numbers 0,1 and 2.
The SSS is modulated to allow a UE to discover the boundary of each 10 ms frame, and provides details of the physical cell ID group to which the cell belongs. This information can also be gained from the cell-specific reference signal transmitted in the cell.
On a wider scale, the synchronization of the eNB within the E-UTRAN is an area of future study in the current versions of the specifications.
The only directive given so far is that eNBs should be permitted to obtain synchronization via a wide range of sources. This means that an eNB will be free to obtain a synchronization signal from either wired (via cable or fibre link) or wireless via GPS (Global Positioning System) or some other radio link.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
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PRACH, uplink traffic or UE sounding
Timing Advance – steps of 0.52 μsec (16 x Ts)
Power Adjustment – EPRE in steps of 1 dB
Timing and Power Control
The E-UTRA uplink employs a slotted frame structure which is shared by multiple simultaneous users.
To ensure that transmission from each active UE arrives within its allotted slot period, however near to or far from the cell site each UE is, the eNB is required to measure and adjust each UE’s uplink timing offset.
A timing advance system, similar in principle to that employed by GSM, is used to manage this aspect of UE transmission. Received transmissions from each UE are measured by the eNB and any deviation from the required arrival time is noted. A TA (Timing Advance) message is returned to each UE
specifying the amount of change, positive or negative, that is required. Timing advance adjustments are made in multiples of 0.52 μsec.
Uplink power output for each UE is defined in terms of the EPRE (Energy Per Resource Element) that is radiated. EPRE is an average measurement of the energy in one subcarrier during one symbol period and excludes the cyclic prefix. The EPRE figure for one UE will be the same across all radiated subcarriers during a power control cycle.
Like WCDMA in R99 UMTS, E-UTRA employs both open and closed loop power control. Open loop power control is used for random access events, when the UE is not receiving power control messages from the eNB. Closed loop power control occurs once a connection has been established and the eNB has begun sending TPC (Transmit Power Control) messages to the UE.
Further Reading: 3GPP TS 36.211, 36.300
The LTE Radio Interface
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