OFDM Scheduling on LTE DL

The eNB advises each UE when and on which resources to transmit its data or informs a UE where it should listen to receive data. The resources are defined by frequency and time units. This procedure of assigning system resource is called scheduling. The system resources are divided into units of RBs. Only integer numbers of RBs can be assigned to one user. Localized and distributed RB allocations are possible. Localized allocations assign adjacent RBs to one UE, distributed allocations distribute the scheduled RBs over the spectrum with gaps, for example, in order to achieve frequency diversity.

A new scheduling assignment is transmitted for each subframe, thus the scheduling period on the time axis is 1 ms. The DL scheduling information is transmitted in the PDCCH. The assignments on the frequency scale vary between one RB (minimal scheduled transmission) and the maximum number of available RBs in respect of the system bandwidth.

Generally, the LTE scheduling algorithm is not defined by the standard; it is a matter for the eNB vendors. This enables the base station vendors to differentiate between each other and use different optimization goals. Various parameters can be used as input for the scheduling decisions: channel quality of different users (measured or reported by the UE with the Channel Quality Indicator, CQI), QoS, congestion/resource situation, fairness, charging policies, and so on. Most schedulers aim to maximize the cell throughput under consideration of fairness metrics between cell edge users and users with very good channel conditions. Figure 1 shows a screenshot of a typical scheduling and cell resource allocation analysis tool. It gives insight into the scheduling process of the eNB and is able to evaluate the scheduler performance and the utilization of cell resources.

 
Figure 1: Scheduling and cell resource allocation analysis

The CQI is reported in the UL direction and can be derived periodically or upon request by the eNB. It gives reception quality feedback to the scheduling algorithm in the eNB in order to schedule data on those frequency regions with the best possible reception characteristics. 

The scheduling information is encoded as Downlink Control Information (DCI). The DCI is then mapped to REGs of the PDCCH. The length of the PDCCH can vary between one and three OFDM symbols depending on the load to be transmitted on the PDCCH. The number of used OFDM symbols is indicated in the PCFICH.

The DCI does not just transmit RB assignments and its assignment type, but also other information needed for the transmission or reception of data. This information is, for example, the Modulation and Coding Scheme (MCS), HARQ feedback information, or power control commands for UL transmission of the Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH) (see below).

The following DCI formats are defined and used for scheduling and UL Transmit Power Control (TPC) commands:

• DCI format 0: UL scheduling grant.
• DCI format 1: Single transport block (code word) scheduling assignment, for example, used for assigning resources to system information, paging, or random access. Further information: MCS, HARQ (New Data Indicator (NDI), redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1A: Compact single transport block scheduling assignment. Further information: MCS, HARQ (NDI, redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1B: A special DCI format for transmission mode 6 (MIMO closed loop rank 1 pre-coding). Further information: precoding vector, MCS, HARQ (NDI, redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1C: Even more compact scheduling format as DCI 1A. For example, used for assigning resources to SIBs, paging, or RARs. This DCI is always transmitted using frequency diversity via distributed virtual resource block assignments using resource allocation type 2 (see below). This is done because channel feedback cannot be derived for such common information, as it is received by multiple users. The modulation is fixed to QPSK. Further information: MCS, HARQ (NDI, redundancy version, HARQ process number), and TPC for PUCCH.
• DCI format 1D: A special DCI format for transmission mode 5 (multi-user MIMO). Further information: MCS, HARQ (NDI, redundancy version, HARQ process number), power offset indicator if two UEs share power resources, and TPC for PUCCH.
• DCI format 2: Scheduling for transmission mode 4 (closed loop MIMO), multiple antenna port transmission operation, addressing multiple transport blocks (code words) to be transmitted on different antenna ports (layers). Further information: MCS for each transport block, HARQ (NDI, redundancy version, HARQ process number), number of transmission layers, precoding, and TPC for PUCCH.
• DCI format 2A: Used with transmission mode 3 (open loop MIMO using Cyclic Delay Diversity (CDD)), multiple antenna port transmission operation, addressing multiple transport blocks (code words) to be transmitted on different antenna ports (layers). Further information: MCS for each transport block, HARQ (NDI, redundancy version, HARQ process number), number of transmission layers, precoding, and TPC for PUCCH.
• DCI format 3: A 2-bit UL TPC command applying for PUSCH and PUCCH. Multiple users are addressed.
• DCI format 3A: A 1-bit UL TPC command applying for PUSCH and PUCCH. Multiple users are addressed.

The resource allocation assignments of the above DCI formats can use different resource allocation types. Table 1 maps the DCI formats to the allowed resource allocation types:

• Resource allocation type 0: With resource allocation type 0 a bit map is transmitted describing Resource Block Groups (RBGs). A RBG is a number of consecutive physical resource blocks (RBs). The number depends on the system bandwidth and has a range between one and four physical RBs. Table 1 maps the size of a RBG to the system bandwidth. The allocated RBGs do not have to be adjacent.

  Table 1: Resource allocation types and the applying DCI formats TS36.213. Reproduced with permission from © 3GPP^™ 

  Resource allocation type	Applying DCI formats
  Type 0	1, 2, 2A, and 2B
  Type 1	1, 2, 2A, and 2B
  Type 2	1A, 1B, 1C, and 1D

  Table 2: Type 0 resource allocation RBG size vs. DL system bandwidth 

  System bandwidth NRD BL	RBG size (P)
  ≤10	1
  11–26	2
  27–63	3
  64–110	4

• Resource allocation type 1: The bit map transmitted with resource allocation type 1 makes use also of RBGs but can address single physical RBs by introducing additional flags. The number of RBGs is smaller than the ones used with resource allocation type 0, thus not reaching the complete bandwidth. The bit map addresses not whole RBGs, but a subset within each RBG which is pointed to by the bit map. A selection flag indicates the position within the RBG regions and a shift flag shows the position of the numbered RBGs within the system bandwidth as the number of RBGs does not address the complete system bandwidth: shift flag = 0 indicates that the RBGs start at the beginning of the system bandwidth leaving an unaddressable region at the end of the system bandwidth; shift flag = 1 indicates that the RBGs are shifted to the end of the system bandwidth leaving the unaddressable region at the beginning of the system bandwidth.
• Resource allocation type 2: This resource allocation type uses virtual RBs as scheduling units. Two types of virtual RB scheduling assignments are used:
  • – A localized type, where the allocated virtual RBs equal a number of consecutive physical RBs addressed with a starting RB and a number of adjacently assigned RBs. This information is encoded into an 11-bit Resource Indication Value (RIV).
  • – A distributed type, where the addressed virtual RBs are distributed over the frequency with one or two gaps (depending on the system bandwidth) hopping at slot boundaries. Virtually distributed RB assignments are always used with DCI format 1C. There is a 1-bit flag indicating whether virtual distributed or virtual localized RB assignment is used in the case of DCI formats 1A, 1B, and 1D.

Instead of addressing a UE with a PDCCH scheduling assignment (DCI) directly by adding a UE ID (e.g., a RNTI) to the DCI, the 16-bit CRC of the PDCCH message is scrambled with the RNTI, introducing common and UE-specific search spaces. This CRC scrambling saves additional resources in DCIs, but increases slightly the chance of decoding a DCI for a different UE which is not intended to be addressed.

Figure 2 depicts an example PDCCH message of DCI format type 1 with all the transmitted information.

 
Figure 2: Example PDCCH message of DCI format 1 (downlink scheduling assignment)

Some special applications require the transmission of small data chunks in equidistant periods of time. An example is a VoIP application. In order to minimize the signaling overhead in such cases, a mode Semi Persistent Scheduling (SRS) is introduced. SPS parameters are configured by the RRC layer, enabling the transmission of data on defined RBs in frequency and time without further scheduling on the PDCCH.

Battery energy saving is always an important topic with mobile handset systems. A potential scheduling assignment could be sent in each PDCCH which occurs every millisecond. Therefore, each attached UE would need to monitor the PDCCH each millisecond for scheduling information. The DRX mode enables the UE just to listen to defined subframes for scheduling assignments and turn off its receiver in between, in order to save battery consumption. Short and long DRX cycle periods are defined. The DRX parameters are set by MAC and RRC. Figure 3 depicts a DRX cycle.

 
Figure 3: DRX cycle.