Resource Blocks | Radio Interface Basics

This section describes how LTE defines and divides the bandwidth from physical subcarriers of the OFDM symbol in the logical abstract sense used for scheduling shared channel data.

The smallest division of the LTE spectrum carrying data is a subcarrier, as described in more detail in the sections above. OFDM systems modulate all data in the frequency domain on the subcarrier of the OFDM spectrum. A modulated subcarrier is defined as a Resource Element (RE) and is the smallest logical unit of the LTE spectrum. One subcarrier or RE has a bandwidth of 15 kHz in normal and extended CP mode, but a special 7.5 kHz subcarrier spacing mode is defined with extended CP transmission. All physical LTE channels use REs to modulate the data. Each RE of the shared channel is modulated using a variable modulation scheme from Quadrature Phase Shift Keying (QPSK) to 64QAM as assigned by the scheduling process of the eNB. REs carry a variable number of data bits, due to the variable number of bits mapped to a RE because of the modulation order, but also because of applied CSs. CSs are used to make the data transmission more robust against transmission errors. They add redundancy to the transmitted information, which increases the probability of the receiver to retrieve the information error-free. LTE defines an Adaptive Modulation and Coding (AMC) process.

A physical Resource Block (RB) defines the smallest unit used by the scheduling algorithm. Therefore, the minimal scheduled user transmission on the shared channels is one RB. A RB consists of 12 adjacent REs on the frequency axis. Consequently, it has a bandwidth of 180 kHz, since one RE is 15 kHz wide in normal and extended CP mode (additionally a mode with 7.5 kHz is defined for extended CP). The possible configurations of CPs are given in Table 1.

Table 1: Configuration of cyclic prefix. Reproduced with permission from © 3GPP™ 

Configuration		NRBsc	NDLsymb
Normal cyclic prefix	Δf = 15 kHz	12	7
Extended cyclic prefix	Δf = 15 kHz	24	6
	Δf = 7.5 kHz		3

From a time perspective, a RB spans one scheduling period which is defined as one subframe. One subframe has a duration of 1 ms. A subframe is divided into two slots of 0.5 ms. Within a subframe 14 OFDM symbols are transmitted in the case of normal CP length and 12 OFDM symbols in the case of extended CP length, hence a RB covers an area of, respectively, 12 × 14 and 12 × 12 REs. 

A third dimension is introduced by using multiple antenna ports with MIMO. The MIMO transport layers depend on and correlate with the number of used transmit antenna ports. Each antenna port layer adds additional RB elements to the antenna port dimension. Figure 1 shows the two-dimensional (frequency and time) area of a RB.

 
Figure 1: Downlink resource grid. Reproduced with permission from © 3GPP^™

RBs have a primary role in the scheduling process, but are also used for describing the LTE overall cell bandwidth. The cell bandwidth is announced in the data transmitted in the Physical Broadcast Channel (PBCH) in number of resource blocks. Table 2 maps the number of RBs to the LTE spectrum bandwidth in megahertz.

Table 2: Commonly used number of resource blocks 

Bandwidth (MHz)	Resource blocks
1.4	6
3	13
5	25
10	50
15	75
20	100

The scheduling procedure defines virtual resource blocks. Virtual resource blocks and physical resource blocks are of equal size. The scheduler always uses virtual resource blocks for defining user allocations. There are two different types of virtual resource blocks:

• Localized virtual resource blocks.
• Distributed virtual resource blocks.

Localized virtual RBs are equal to physical RBs. Therefore, localized virtual RBs address physical RBs directly. Distributed RB mapping enables the usage of frequency diversity without scheduling distributed RBs directly. Distributed virtual RBs split a physical RB at the slot boundary into two halves. The first half of the scheduled distributed virtual RB directly equals the physical RBs. The second slot is hopped to another second slot of another UE which is virtually scheduled in the distributed way. There is one hopping gap between the scheduled RBs at system bandwidths smaller 50 RBs and two gaps at systems with a larger number of RBs. Virtual RBs are used with resource allocation type 2.

Resource Element Groups (REGs) are defined to map physical channels to OFDM symbols, this is done especially in the first OFDM symbols used by the PDCCH. The maximum length of the PDCCH is three OFDM symbols, thus other PHY channels are mapped into the resources of the PDCCH. A REG is defined in such a way that it spreads the information over a bigger frequency range to gain frequency diversity