*2.5.1 5G NR frame structure*

Since TDD is the main duplexing mode of a 5G NR, we will discuss more detail about TDD. We start with 5G NR frame structure. Just like the TDM system, 5G NR is frame structured. A frame has a fixed duration of 10 ms which consists of 10 subframes of 1 ms duration. Each subframe can have 2*<sup>μ</sup>* slots.

**Figure 7.** *Code division multiple access.*

*Multiplexing Techniques for Applications Based-on 5G Systems DOI: http://dx.doi.org/10.5772/intechopen.101780*

**Figure 8.** *5G NR frame structure.*

**Figure 8** shows the 5G NR frame structure. The number of slots per subframe (i.e., 2*<sup>μ</sup>*), hence the slot duration depends on subcarrier spacing (SCS). 5G NR supports two frequency ranges FR1 (Sub 6GHz) and FR2 (millimeter wave range, 24.25 to 52.6 GHz). 5G NR uses flexible SCS derived from basic 15 KHz used in LTE to values of 30, 60, 120 Khz. For SCS of 15 KHz, a subframe has 1 slot of 1 ms duration. For SCS of 30 KHz, a subframe has 2 slots of 500 μs duration as shown in **Table 3** and **Figure 9** [20].

Each slot is comprised of either 14 OFDM symbols or 12 OFDM symbols based on normal Guard Period (GP) and extended GP respectively. However, mini slots (2, 4, or 7 symbols) can be allocated for shorter transmissions. Slots can also be aggregated for longer transmissions.


#### **Table 3.**

*Number of slots per subframe, slot duration, number of slots in a frame and guard period for reference SCS.*

**Figure 9.** *5G NR scalable slot duration.*

#### *2.5.2 5G NR UL-DL pattern*

Now we know the frame structure. When operating in TDD mode, we have to specify the exact timing for the uplink and downlink transmission. So, how do we define the time slots for uplink and downlink transmission?

Timeslots for uplink and downlink transmission are organized into DL-UL patterns. In LTE TDD, there are 7 predefined patterns for UL and DL allocation in a radio frame. There is no predefined pattern for 5G NR, but we can define a flexible pattern thanks to parameters in TDD UL/DL Common Configuration (*tdd-UL-DLconfigurationCommon*) as shown in **Table 4** below.

You may ask yourself what is the difference between the DL-UL pattern and radio frame? The *dl-UL-TransmissionPeriodicity* parameter, for example 5 ms, defines the periodicity of the DL-UL pattern. So, a radio frame of 10 ms contains 2 DL-UL patterns.

From the above parameters, we can define TDD DL/UL configuration, aka. DL-UL pattern for 5G NR radio transmission as shown in **Figure 10**. In 5G NR, the slot


#### **Table 4.**

*5G NR TDD DL/UL common configuration parameters.*

**Figure 10.**

*5G NR TDD UL/DL common configuration frame structure.*

*Multiplexing Techniques for Applications Based-on 5G Systems DOI: http://dx.doi.org/10.5772/intechopen.101780*

configuration is flexible and can be changed from time to time while maintaining the focus on inter-cell interference aspects [21].

Then, the next question is how to design a transmission pattern? We know that time slots allocation for UL and DL depends on UL and DL traffic. We call that UL/ DL traffic load ratio. To adapt with actual traffic, 5G NR supports 3 different TDD configurations as follows:

**Static TDD configuration:** For static TDD, the UL/DL traffic ratio is usually decided by the statistical UL/DL traffic load ratio among multiple operators in a specific country or region. The slots and symbols are defined over a period of time that are dedicated to either the UL or DL based on the UL/DL traffic ratio.

**Semi-Static TDD configuration:** This configuration is more flexible than the static TDD. We have a certain number of UL and DL slots within a transmission periodicity (defined by *dl-UL-TransmissionPeriodicity*). The remaining slots, which are neither UL nor DL, can be considered 'Flexible' with the help of another IE *TDD-UL-DL-ConfigDedicated*.

**Dynamic TDD configuration:** This is the most flexible configuration for UL/DL transmission for dynamic assignment and reassignment of time-domain resources between the UL and DL transmission. Dynamic TDD is used to adapt to actual traffic but requires coordination to avoid interference between cells, so that there is no fixed UL/DL allocation. With the popularity of video streaming increasing, it is forecast that the proportion of DL content will grow even further in the future, hence it is natural that more resources should be allocated to the DL.

**Example 2. The** DL-UL pattern design. Assuming SCS = 30 kHz and the carrier is FR1 with 100 MHz bandwidth.


Since slot duration for reference SCS of 30 kHz is 0.5 ms, the number of slots in DL-UL periodicity would be

$$\text{NumSlotsDLULPriendicity} = \frac{\text{dl} - \text{UL} - \text{TransmissionPeriodicity}}{\text{Slot length}} = \frac{2.5 \text{ms}}{0.5 \text{ms}} = \text{5 slots, and}$$

NumberOfGuardSymbols

<sup>¼</sup> TotalSymbolsInPattern‐TotalSymbolsWithTypeSpecified

<sup>¼</sup> <sup>14</sup> <sup>∗</sup> NumSlotsDLULPeriodicity

numDLSlots <sup>∗</sup> <sup>14</sup> <sup>þ</sup> numDLSyms <sup>þ</sup> numULSyms <sup>þ</sup> numULSlots <sup>∗</sup> <sup>14</sup>

¼ 2 symbols*:*

This DL-UL pattern is illustrated in **Figure 11**. This pattern repeats itself in the timeline.

### **3. 5G NR MIMO multiplexing operation**

Perhaps the most challenging part of the 5G NR system is the MIMO operation modes. Let us start with SU-MIMO and MU-MIMO. SU-MIMO stands for Single-

**Figure 11.** *Example on design a TDD downlink frame structure.*

User MIMO. In Single User MIMO, both the base station and UE have multiple antennas, and the base station can transmit multiple data streams simultaneously to the UE using the same time/frequency resources. By doing so, it doubles (2 � 2 MIMO), or quadruples (4 � 4 MIMO) the peak throughput of a single user.

MU-MIMO stands for Multi User MIMO. The base station serves more than 2 UEs simultaneously. Since in MU-MIMO, the base station sends multiple data streams, one per UE, using the same time-frequency resources, MU-MIMO mode increases the total cell throughput, i.e., cell capacity. MU-MIMO is not a new concept. We have MU-MIMO in LTE (Transmission Mode 5 - TM5) and WLAN (802.11ad). However, in 5G NR the scale of MU-MIMO will be much larger and deployment will also be more common. 5G NR uses massive MIMO.

Massive MIMO employs a large number of transmit and receive antennas, improves spectral efficiency and increases the transmission data rate through spatial multiplexing to deliver multiple streams of data within the same resource block (time and frequency). Massive MIMO is also called Large Scale MIMO.

By now, you may ask a question: *Why massive MIMO, and how many antenna elements are needed to be called massive MIMO*? In conventional 4G LTE using a normal MIMO, the maximum number of the antenna is 2x2 or 4x4 and even 8x8 is mentioned. We know that the larger the number of antennas, the narrower the beam width. It means the coverage of a specific beam would be smaller. We need a more precise beam control algorithm, but in return, the achievable data rate will be higher. The number of antennas in massive MIMO should be ≫ 8 [22].

### **3.1 Mathematical background**

**Figure 12** shows a typical MIMO system equipped with*NT* transmit antennas and *NR* receive antennas. The data are encoded in both space and time domains and then transmitted by *NT* transmit antennas through a MIMO propagation channel.

The relationship between the input and output of a MIMO system can be written as follows

$$\mathbf{y} = \mathbf{H}\mathbf{x} + \mathbf{n},\tag{1}$$

where.

$$\begin{aligned} \mathbf{x} &= \begin{bmatrix} \boldsymbol{\varkappa}\_1, \boldsymbol{\varkappa}\_2, \dots, \boldsymbol{\varkappa}\_{N\_T} \end{bmatrix}^T \text{ is transmitted signal,} \\ \mathbf{y} &= \begin{bmatrix} \boldsymbol{\jmath}\_1, \boldsymbol{\jmath}\_2, \dots, \boldsymbol{\jmath}\_{N\_R} \end{bmatrix}^T \text{ is received signal,} \\ \mathbf{n} &= \begin{bmatrix} n\_1, n\_2, \dots, n\_{N\_R} \end{bmatrix}^T \text{ is AWGN,} \end{aligned}$$

*Multiplexing Techniques for Applications Based-on 5G Systems DOI: http://dx.doi.org/10.5772/intechopen.101780*

#### **Figure 12.**

*System and channel model for spatial multiplexing.*

$$\mathbf{H} = \begin{bmatrix} h\_{11} & h\_{12} & \cdots & h\_{1N\_T} \\ & h\_{21} & h\_{22} & \cdots & h\_{2N\_T} \\ & \vdots & \vdots & \ddots & \vdots \\ & h\_{N\_R 1} & h\_{N\_R 2} & \cdots & h\_{N\_R N\_T} \\ & & \ddots & \ddots & \ddots & \ddots \end{bmatrix} \text{ is channel matrix},$$

where *hi*,*<sup>j</sup>* is an element of the *i* th row and the *j* th column in the matrix **H**, denotes a channel from the *j* th TX antenna to the *i* th RX antenna.

If the channel matrix **H** is known at both base station (gNB) and UE (i.e., Channel state information - CSI) then we could take *singular value decomposition* (SVD) on channel matrix **H** as

$$\mathbf{H} = \mathbf{U}\mathbf{D}\mathbf{W}^\*,\tag{2}$$

where **U** ∈ *NR*�*NR* and **W**∈ *NT*�*NT* are orthogonal unitary matrices and **D** ∈ ℜ*NR*�*NT* is diagonal matrix, whose diagonal elements are non-negative real numbers and whose off-diagonal elements are zero. The diagonal elements of matrix **D***λ*<sup>1</sup> ≥ *λ*<sup>2</sup> ≥ … ≥*λ<sup>r</sup>* are the ordered singular values of channel matrix **H**, where *r* ¼ min f g *NT*, *NR* is rank of **H**.

Assume the receiver knows the **U** matrix and the transmitter knows the **W** matrix. The transmitted data **x** is precoded by **W** matrix and the received data **y** is equalized by **U** matrix, we have

$$\begin{aligned} \bar{\mathbf{y}} &= \mathbf{U}^\* \mathbf{y} = \mathbf{U}^\* [\mathbf{H} \bar{\mathbf{x}} + \mathbf{n}] = \mathbf{U}^\* [(\mathbf{U} \mathbf{D} \mathbf{W}^\*) \mathbf{W} \mathbf{x} + \mathbf{n}], \\ \bar{\mathbf{y}} &= \underbrace{[\mathbf{U}^\* \mathbf{U}] \mathbf{D} [\mathbf{W}^\* \mathbf{W}] \mathbf{x}}\_{\text{I}} + \mathbf{U}^\* \mathbf{n}, \\ \bar{\mathbf{y}} &= \mathbf{D} \mathbf{x} + \mathbf{U}^\* \mathbf{n}, \\ \bar{\mathbf{y}} &= \mathbf{D} \mathbf{x} + \bar{\mathbf{n}}, \end{aligned} \tag{3}$$

where n~ ∈*CN* 0, *N*0I*NR* ð Þhas the same distribution as **n**. Thus, we have an equivalent representation as a parallel Gaussian channel

$$
\tilde{y}\_i = \lambda\_i \tilde{x}\_i + \tilde{n}\_i, i = \mathbf{1}, \mathbf{2}, \dots, r. \tag{4}
$$

**W** ¼ **w**1j**w**2j, … j**w***NT* ½ � is a precoding matrix. Each symbol *xi* is precoded by precoding vector **w***i*.

From the Eq. (4), we can see that the base station can transmit simultaneously maximum of *r* data streams to the target UE, increasing the channel throughput. This is called spatial multiplexing (SM). MIMO spatial multiplexing takes advantage of multipath effects, where a transmitted signal arrives at the receiver through several different paths. Each path can have a different time delay, and the result is that multiple instances of a single transmitted symbol arrive at the receiver at different times [23].

If SNR is high, the number of data streams and data rate for each stream is chosen by the *waterfilling* algorithm [24]. In the opposite case, with low SNR, the best thing to do is to simply choose one subchannel with the highest singular value. This is called beamforming [25, 26]. We can rewrite the Eq. (3) as

$$
\tilde{\boldsymbol{\gamma}}\_1 = \mathbf{u\_1^\*} \mathbf{H} \mathbf{w\_1} \boldsymbol{\chi}\_1 + \mathbf{u\_1^\*} \mathbf{n}.\tag{5}
$$

Instead of transmitting a vector of symbols, we just transmit a single symbol at a time. The **w**<sup>1</sup> vector defines the beamforming weights and **u**<sup>1</sup> here defines the receive beam.

Now we know how to transmit multiple data streams to a UE. We consider the way 5G NR implement MIMO modes.

Clearly, to implement SM, the network (gNB and UEs) should know the channel matrix **H** then calculate the 3 matrixes **U**, **D**, **W** out of **H**. The transmitter applies **W** as a precoder and the receiver apply **U** for processing of the received signal. For downlink transmission, gNB is a transmitter and UE is receiver and vice versa for uplink transmission.

#### **3.2 Basic terminologies**

The first thing we have to know is the codebook. The **codebook** is a set of predefined precoders (precoding matrices). Why codebook? Consider DL transmission, gNB has to calculate a precoder from a reference signal or selects a predefined precoder with a requested index from UE before transmitting data. The first case is called non-codebook and the second is called codebook-based precoding.

**Codebook type in 5G NR:** There are two types of codebooks specified in 5G NR. Type I is designed for SU-MIMO and selected by UE report and RRC Configuration. Type II is designed mainly for MU-MIMO and is based on a more detailed CSI report. Type I codebook has predefined matrices based on the number of layers and CSI-RS ports. Type II codebooks contain mathematical formula for selecting a set of beams and then specifying relative amplitudes and phases to generate a weighted combination of beams for each layer of transmission.

The requested index into a set of predefined matrices, a so-called codebook is **a precoding matrix indicator** (PMI). PMI is used for DL transmission, conditioned on the number of layers indicated by the RI. In the uplink direction, the PMI is denoted by **Transmit Precoder Matrix Indicator** (TPMI) to differentiate it from the downlink PMI.

Together with the codebook, **the number of layers** is the number of simultaneous data streams. The number of layers is less than or equal to the rank of the channel matrix that we mentioned before. The number of layers depends upon the channel condition between receiver and transmitter antennas. Low correlation propagation paths increase rank and the number of layers and vice versa. **Rank indicator** (RI) defines the number of possible transmission layers for the downlink and uplink transmission under specific channel conditions. However, gNG does not need to transmit RI as requested by the UE.

**Channel state information** (CSI) are parameters related to the state of a channel including the channel quality indicator (CQI), precoding matrix indicator (PMI) and rank indicator (RI). UE reports CSI parameters to gNB as feedback in CSI-RS mode.

**Channel quality indicator** (CQI) is an indicator of channel quality. The CQI index is a scalar value from 0 to 15 representing the highest modulation-and-coding scheme (MCS) to achieve the required block error rate (BLER) for given channel conditions.

**CSI-RS resource indicator** (CRI), used in conjunction with beamformed CSI reference signals. The CRI indicates the beam the device prefers in case the device is configured to monitor multiple beams.

**SRI** is an SRS resource indicator.

### **3.3 Physical antenna configuration versus antenna ports**

It is very important to understand the physical antenna configurations, the antenna port and the relationship between them. The antenna system in 5G NR is an Active Antenna System (AAS). Typical active antennas are made up of a matrix of subarrays. Each subarray consists of individual dual-polarized elements. Each polarization is controlled by a beamforming (BF) coefficient. Therefore, the number of columns is doubled.

For example, **Figure 13a** shows 8T8R configuration with 4 columns, 1 row (4x1) consisting of 4 (1x8) subarrays. **Figure 13b** shows 64T64R configuration which is made up of 8 columns, 4 rows of (1x2) subarrays.

**Figure 14a** shows single panel antenna. 5G NR supports both single panel and uniform (b) and non-uniform multi-panel (c). In 5G NR, logical antenna configuration is described by 3 parameters: *Ng* is the number of panels, *N*<sup>1</sup> is number of columns and *N*<sup>2</sup> is the number of rows in a panel.

In association with *N*<sup>1</sup> and *N*2, 3GPP defines DFT oversampling factors *O*<sup>1</sup> and *O*<sup>2</sup> to determine the sweeping steps of a beam during the beam management (beam tracking). *O*<sup>1</sup> determines the sweeping step in the horizontal direction and *O*<sup>2</sup> determines the sweeping step in the vertical direction.

We have:

• Number of polarizations = 2,

• Number of CSI-RS antenna ports = (2\* *N*1)\**N*2,

**Figure 13.** *Physical antenna configuration.*

**Figure 14.** *Single panel and multi panel antenna configurations.*


**Antenna port:** This is a logical concept and different from the physical one that you see on the antenna tower. You can find the definition of antenna port from the 3GPP specification as "*an antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed*" [27]. We can understand the antenna port like socket and port concept which is used on Internet. For example, port 80 is used for the HTTP protocol, port 20 for FTP, port 22 is for SSH, port 25 is for SMTP. We use "antenna ports" to transmit and receive data. Why the definition of antenna port of 3GPP is related to the channel because we have to estimate the channel model before decoding transmitted data. The channel model has estimated thanks to reference signals. Each antenna port is assigned by a dedicated reference signal. Each antenna port represents a specific and unique channel model. The receiver can use a reference signal transmitted on an antenna port to estimate the channel model for this antenna port and this channel model can subsequently be used for decoding data transmitted on the same antenna port.

Each antenna port carries its own resource grid. One resource grid is transmitted on a given antenna port, subcarrier spacing configuration and transmission direction (downlink or uplink). The resource grid consists of a number of RBs (Resource Blocks) for one subframe.

#### **3.4 Physical channels and signals**

Physical Channels and Signals for DL, UL and corresponding antenna port addresses are as follows (**Table 5**):


*Multiplexing Techniques for Applications Based-on 5G Systems DOI: http://dx.doi.org/10.5772/intechopen.101780*

**Table 5.**

*Physical channels and signals and corresponding antenna port addresses.*

#### **3.5 Mapping antenna ports to physical antennas**

There is no strict mapping of antenna ports to physical antenna ports. **Figure 15** indicates the mapping between antenna ports and physical antennas. One antenna port can be mapped to single or multiple physical antenna(s). Due to each antenna port representing a specific and unique channel model, the number of layers in the physical layer may reach the number of antenna ports. The number of layers may range from a minimum of one layer up to a maximum number of layers equal to the number of antenna ports. The layers are then mapped to the antenna ports.

#### **3.6 Downlink MIMO schemes**

Legacy LTE supports 9 transmission modes (TM). To avoid sophisticated transmission mode handover for different scenarios, 5G NR uses the term *unified transmission mode* [28]. However, according to the channel state information (CSI) acquisition method, downlink MIMO schemes are categorized into (**Figure 16**):

**Figure 15.**

*Mapping antenna ports to physical antennas.*

**Figure 16.** *Downlink MIMO schemes.*

Single User MIMO (SU-MIMO):


Multi User MIMO (MU-MIMO):


DL and UL channels are considered reciprocal. From a channel calculation perspective, in SRS-based Single User MIMO scheme, channel calculation obligation belongs to gNB, the remaining schemes rely on UE's CSI report from its channel calculation. The device's capability and channel condition decide the best MIMO mode among the above schemes.
