10 PHYSICAL CHANNELS

Further Reading: 3GPP TS 36.211; 36.321; 36.300

PBCH

Logical

Transport

Physical

PDSCH PMCH

BCH PCH DL-SCH MCH

PDCCH PCFICH PHICH

Physical Signals

Figure 15

Downlink Channel Structure

PBCH

The PBCH carries the type of information traditionally associated with BCCH functions: cell identities, random access parameters, cell status, allowed services, etc.

PBCH information is transmitted once every 10 ms frame period.

Physical Signals

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

Further Reading: 3GPP TS 36.211, 36.321, 36.300

PBCH

Logical

Transport

Physical

PDSCH PMCH

BCH PCH DL-SCH MCH

PDCCH PCFICH PHICH

Physical Signals

Figure 15 (repeated) Downlink Channel Structure

10.2 Downlink Subframe

An example of a populated downlink subframe (using frame Type 1 and the normal CP) is shown in the diagram.

10.2.1 PDCCH Mapping

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 Control Channel Element (CCE), 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.

10.2.2 MIMO Mapping

Also included in the diagram is an example of the likely 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.

10.2.3 Reference Signal Mapping

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.

10.2.4 PBCH Mapping

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.

10.2.5 Synchronization Signal Mapping

P-SCH and S-SCH 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.

Further Reading: 3GPP TS 36.211, 36.300

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

Resource Blocks carrying PBCH, P-SCH and S-SCH

Resource Blocks carrying only P-SCH and S-SCH

Resource Block 0

10.3 Uplink Physical Channels

The uplink configuration is far simpler than the downlink, with only three channels defined: the Physical Uplink Shared Channel (PUSCH), the Physical Uplink Control Channel (PUCCH) and the Physical Random Access Channel (PRACH).

PUSCH

The PUSCH 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.

PUCCH

The PUCCH provides an uplink control path for each UE to send H-ARQ 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.

PRACH

The PRACH 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.

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

PRACH

Logical

Transport

Physical

PUSCH

UL-SCH DCCH DTCH CCCH

PUCCH RACH

RLC

MAC

Physical

Figure 17

Uplink Channel Structure

10.4 Uplink Subframe Example

An example of a populated uplink subframe (using frame Type 1 and the normal CP) is shown in the diagram opposite.

The uplink frame structure is much simpler than that employed by the downlink.

10.4.1 Demodulation Reference Signal Mapping

Symbol 3 in each slot carries the uplink demodulation reference signal, leaving the other six symbols available to carry traffic.

10.4.2 PUCCH Mapping

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

10.4.3 PUSCH Mapping

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.

10.4.4 PRACH Mapping

The location of the resources made available for random access procedures is not made clear in the current versions of the 3GPP specifications.

Further Reading: 3GPP TS 36, 36.300

D

11.1 Downlink MIMO

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.

A true MIMO service requires diverse transmit and receive antennas, although other variations exist. Single Input Multiple Output (SIMO) uses one transmit antenna to multiple receive antennas and is an option for use on the uplink in WiMAX. Multiple Input Single Output (MISO) uses diverse transmit antennas to a single receive antenna and would more commonly be described as transmit diversity.

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 Mbits plus, that are mathematically possible with E-UTRA, are only possible using more extreme versions of MIMO – up to 4x4.

The current levels of antenna technology would struggle to equip a small, handheld mobile device with four diverse antennas sufficiently physically separated to make MIMO reception possible, so for the foreseeable future 2x2 systems are likely to provide the upper limit of what the E-UTRA can offer.

11.2 Uplink Multi-User MIMO (MU-MIMO)

Multi-User MIMO (MU-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.