Rise time hara teristi s

RSRQ – Reference Signal Received Quality

LTE Radio Measurement Types

The basic air interface measurements applicable to E-UTRA are RSRP (Reference Signal Received Power), RSSI (Received Signal Strength Indicator) and RSRQ (Reference Signal Received Quality).

The RSRP measurement provides an average of power levels received across all reference signal symbols within a given time period and across a nominated bandwidth. On the uplink this means that the eNB only needs to take into account the reference signals received from the channel assigned to a particular UE when taking measurements in relation to that UE. On the downlink a UE only needs to take measurements from the cell-specific reference signal elements related to the cell it is currently being served by. If MIMO or some other form of receiver diversity is in use by the UE, the

measurements taken will be an average of the RS power received on all active diversity branches.

Received signal strength, as the name suggests, is a measurement of everything received on a given channel regardless of source – so wanted channel noise, co- and adjacent-channel interference and even thermal noise all contribute to the overall RSSI measurement. Current versions of the relevant 3GPP specification (TR 36.214) do not state when RSSI measurements should be taken during the E-UTRA frame cycle.

RSRQ is derived from the previous two measurements. The measured RSRP is divided by the current RSSI value and the result is multiplied by the number of resource blocks contained within the bandwidth measured to obtain the RSSI figure.

Quality and signal strength feedback are required to allow link adaptation to take place. The eNB schedules regular opportunities for each active UE to send CQI and other measurements. If the UE has data to transmit when a feedback opportunity is scheduled it is piggybacked onto the PUSCH (Physical Uplink Shared Channel) transmission. If not, the UE instead sends the feedback using the defined PUCCH (Physical Uplink Control Channel) resource elements.

Further Reading: 3GPP TS 36.211, 36.300, 36.801

LTE/SAE Engineering Overview

BCCH PCCH CCCH DCCH DTCH MCCH MTCH

PDCCH PCFICH PHICH

Physical Signals

Downlink Physical Channels

The PDSCH (Physical Downlink Shared Channel) is the only channel available for carrying data to individual terminals – E-UTRA has no equivalent of the dedicated channels employed by other GSM and UMTS variants. The PDSCH carries a multiplexed data stream for some or all active users in a cell or on a MIMO stream in a cell.

The PMCH (Physical Multicast Channel) is designed to carry multicast traffic, such as MBMS services and other traffic flows destined for multiple users. Traffic carried by the PDSCH and PMCH is ultimately mapped into the available space in downlink resource grids.

The PDCCH (Physical Downlink Control Channel) occupies up to the first three symbol periods in every subframe. The main function of the PDCCH is to carry resource assignment messages for downlink capacity allocations and scheduling grants for uplink allocations. A PDCCH notification specifies which connections have been allocated capacity, which RBs have been assigned and the number of symbols for which the allocation lasts. Due to the limited complexity of the allocation method, capacity allocations always consist of sets of contiguous RBs and symbols, leading to a two-dimensional assignment within a resource grid. The notification will also specify the format of the allocation – the modulation and error coding scheme employed plus any MIMO or advanced antenna parameters applicable to the

transmission.

The PCFICH (Physical Control Format Indicator Channel) carries details of the formatting of a subframe’s control fields and the PHICH (Physical HARQ Indicator Channel) carries ARQ (Automatic Repeat Request) ACK/NACK messages from the eNB back to the UEs.

The PDCCH, PCFICH and PHICH all share capacity in the first three symbol periods of each subframe.

The PBCH (Physical Broadcast Channel) carries the Master Information Block, which directs UEs to the system information carried in the PDSCH.

In addition to the physical channels specified above, the E-UTRA downlink also carries several ‘physical signals’. These are the reference and synchronization signals.

Further Reading: 3GPP TS 36.211, 36.321, 36.300

The LTE Radio Interface

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Downlink Transmission Structure

Resource Blocks carrying

PBCH, PSS and SSS Resource Blocks carrying only PSS and SSS

Resource Block 0

Downlink Transmission Structure

An example of a populated downlink subframe (using frame Type 1 and the normal CP) is shown in the diagram. To allow for flexible, low latency scheduling and also to allow the capacity scheduler to focus its functions on groups of users with similar requirements, multiple PDCCHs can operate in parallel during the same subframe periods. Each separate PDCCH manages a different CCE (Control Channel Element), which has responsibility for allocating the capacity of a subset of resource elements to a subset of active users. Segmentation of the PDCCH into smaller CCEs also allows UEs to decode and process only the channel control information that relates to their own CCE, rather than expending processing time and power decoding all PDCCH messages.

The PBCH logical channel is transmitted during subframe 0 of each 10 ms frame and is spread over slots 0 (symbols 3 and 4) and 1 (symbols 0 and 1). The subcarriers chosen to carry the PBCH consist of those in the six RBs clustered either side of the DC carrier (the centre frequency of the radio channel), which provides a total of 72 subcarriers. Slots assigned to reference signals are excluded from this, however, meaning that in slot 0 symbol 4 and slot 1 symbol 0 a reduced number of subcarriers are assigned to carry the PBCH.

PSS and SSS synchronization signals are transmitted twice in each 10 ms frame period, in subframe 0 and subframe 5. They again occupy the six resource blocks either side of the DC carrier during symbols 5 and 6.

Also included in the diagram is an example of the configuration of a downlink channel employing 2x2 MIMO. Separate subframe maps are created for each antenna port, which themselves then map onto different MIMO streams. To reduce the potential for inter-stream interference, resource elements in one stream that correspond to resource elements carrying reference signals in the other stream are left unassigned.

Further Reading: 3GPP TS 36.211, 36.300

LTE/SAE Engineering Overview

Uplink Physical Channels

The PUSCH (Physical Uplink Shared Channel) is the shared uplink data channel. Again, as with the downlink, the E-UTRA uplink does not support the concept of a dedicated channel. Traffic bursts from multiple UEs are scheduled by the eNB and are interleaved to share the same uplink resources.

The PUCCH (Physical Uplink Control Channel) provides an uplink control path for each UE to send HARQ (Hybrid Automatic Repeat Request) ACK/NACK indications, uplink scheduling requests, CQIs and MIMO feedback. If control traffic is sent at the same time as uplink data is being transmitted, the UE time-multiplexes the two streams of data together to preserve the ‘single carrier’ nature of the SC-FDMA service. If control data is to be sent when there is no uplink capacity grant scheduled, the UE will

transmit the message using a set of specially set aside resource elements at the extreme edges of the overall channel. The size of this reserved region is configurable depending on the expected amount of PUCCH traffic and its location, at the top and bottom ends of the radio band. It allows UEs to transmit on these subcarriers at slightly higher power levels than would be desirable on subcarriers with a traffic-bearing channel either side of them.

The PRACH (Physical Random Access Channel) carries random access attempts from the UEs to the eNB. The location of the resource elements set aside for PRACH use are flagged in the PDCCH. The location of the resource elements set aside for PRACH use is based on a fixed mapping determined by the PRACH configuration option in force in a cell, which itself is flagged on the BCCH (Broadcast Control Channel).

The Type 1 frame structure allows a maximum of 64 orthogonal preambles per cell and the list of allowed preambles is carried on the PBCH, as is the required length of the preamble to be transmitted in that cell. The guard gap after the preamble is designed to prevent PRACH signals transmitted by a distant UE overlapping into a subsequent PRACH period.

Further Reading: 3GPP TS 36.211, 36.321, 36.300

The LTE Radio Interface

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D

Uplink Transmission Structure

An example of a populated uplink subframe (using frame Type 1 and the normal CP) is shown in the diagram. The uplink frame structure is much simpler than that employed by the downlink. Symbol 3 in each slot carries the uplink demodulation reference signal, leaving the other six symbols available to carry traffic.

A configurable number of outer subcarriers, as shown in the diagram, can be set aside to carry contention-based PUCCH messages.

The nature of SC-FDMA means that UEs are assigned capacity in terms of a number of RBs, but only transmit across a subset of subcarriers within each RB.

Further Reading: 3GPP TS 36.211, 36.300

LTE/SAE Engineering Overview

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DL MIMO

UL MU- MIMO

Stream 1

Stream 2

User 2 on RB User 1 on RB

Advanced Antenna Options

The higher-order data rates potentially achievable through the E-UTRA – rates above 50 Mbit/s on the downlink – are only possible if advanced antenna techniques are employed.

MIMO transmission uses a diverse set of transmit and, optionally, receive antennas to create multiple signal ‘streams’ over the same physical radio channel. The physical separation of the transmit antennas – either spatially or in terms of polarization – allows a receiving station to perceive the separate streams as multipaths, which can then be recovered using a rake receiver.

The higher data rates available through E-UTRA systems only become possible with 2x2 MIMO – two transmit and two receive antennas. This system requires that each eNB transmits using two antennas per sector and each UE is equipped with spatially or polarity separated antennas. The very high downlink data rates, 300 Mbit/s and above, that are mathematically possible with E-UTRA, are only possible using more extreme versions of MIMO – up to 4x4.

MU-MIMO (Multi-User MIMO) allows an eNB to schedule two UEs to use the same RBs simultaneously.

As the UEs are assumed to be physically distant from each other, the resulting combined transmissions arrive at the eNB as multipaths and can be processed in the same way as separate MIMO streams.

MU-MIMO effectively doubles the capacity of the uplink, but cannot be used in all circumstances.

Further Reading: 3GPP TS 36.211, 36.300

LTE/SAE Engineering Overview

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Section 4