Tag: 5G NR PHY

Bandwidth Part (BWP) in 5G NR

Key focus: Bandwidth Part (BWP): Allocates segments of spectrum for flexible resource allocation in 5G NR networks, enhancing efficiency and adaptability. Know the difference between bandwidth part and transmission bandwidth

Introduction

The 3rd Generation Partnership Project (3GPP), in its Release 15, specified the new radio-access technology called 5G New Radio (5G NR). The 5G NR continues to evolve in the subsequent releases that addresses performance improvements and new use cases.

In the release 15, new features for 5G NR were introduced so as to support a varied range of devices, services and deployments. Once such new basic feature in 5G NR is the Bandwidth Part (BWP).

5G NR Frequency ranges

3GPP defines the frequency ranges for 5G NR operation, as specified in Table 1 (illustrated in Figure 1).

Definition of frequency ranges as given in 5G NR 3GPP TS 38.104
Table 1: Definition of frequency ranges as given in TS 38.104 [1]
Figure 1: 5G NR frequency ranges

Operating bands

Within each Frequency Range (FR), the 3GPP defines operational bands. 3GPP 5G NR defines 60 operating bands in FR1 and 7 bands in FR2 (refer section 5.2 in [1]). These operating bands represent specific frequency ranges that come with their own unique radio frequency requirements (for example: some of these bands are designated for use in Frequency Division Duplexing (FDD), Time Division Duplexing (TDD), Supplemental Up Link (SUL), Supplemental Down Link (SDL) communication). The width of these operating bands can vary significantly, ranging from just a few megahertz to several gigahertz. Different mobile network operators might be allocated (typically by respective governments through spectrum auction) different portions of the available spectrum within a given operational band.

To cater to the diverse spectrum allocation scenarios while keeping implementation complexities in check, the New Radio (NR) technology supports a wide range of channel bandwidths (a.k.a Base station (BS) channel bandwidth) spanning from 5 MHz to 2 GHz. In this context, “channel bandwidth” refers to the width of an NR carrier, the fundamental component of the 5G system.

Channel bandwidth

The base station’s channel bandwidth enables the use of a single NR RF carrier in either the uplink or downlink. The user equipment (UE) may support different channel bandwidth than the BS.

The UE connects to the BS. The UE receives information about the cell’s channel bandwidth from the network. The network also provides the UE with details regarding the location and extent of a Bandwidth Part (BWP). The core concept is that a User Equipment (UE) can employ a wide-bandwidth when a significantly large data is required to be transmitted/received, but operate with a limited bandwidth during other periods of limited activity.

Bandwidth Part (BWP)

As we know, 5G NR defines scalable Orthogonal Frequency Division Multiplexing (OFDM) using numerology \(\mu = \left\{ 0,1,2,3,4 \right\}\) that defines the subcarrier spacings (SCS) as \( \Delta f = 2^{\mu} \times 15 \; kHz\). A resource element is the smallest time-frequency resource over one subcarrier of a single OFDM symbol. A resource block (RB) is a block of \( N_{sc}^{RB} = 12 \) subcarriers over which the transmissions are scheduled (Figure 2).

Figure 2: Resource element, resource block and resource grid in 5G NR

In a broad sense, a BWP can be described as a collection of contiguous Resource Blocks (RBs) configured within a given channel bandwidth. The BWP provides a flexible mechanism for adapting to varied bandwidth ranges in order to aid in reduction of power consumption.

Another reason is to cater to devices with varying bandwidth capabilities by setting up different BWPs for each device. A Base Station (BS) might have the ability to accommodate an exceptionally broad channel bandwidth, even though certain User Equipment (UEs) may not have the capability to handle it. The concept of Bandwidth Part (BWP) offers a way to dynamically allocate radio resources, ensuring that a UE’s signals are limited to a portion of the BS’s channel bandwidth that aligns with the UE’s capabilities.

Point A, illustrated in Figure 3, functions as a standardized reference across all resource grids within the frequency. It acts as the central marker for subcarrier 0 within a common resource block 0 of the lowest resource grid and it can reside outside the carrier’s bandwidth. Each individual resource grid (RG) adheres to a distinct 5G NR numerology and commences at a frequency offset from point A.

As shown in Figure 3, the bandwidth part consists of contiguous set of RBs for a particular numerology on a given carrier and starts at an offset RB_{start} from the reference Point A.

Figure 3: Spectrum configuration in 5G NR illustrating channel bandwidth and bandwidth part (BWP)

Transmission bandwidth

5G NR defines another concept called transmission bandwidth that is closely related with bandwidth part. While BWP refers to a portion of the overall channel bandwidth that is allocated for specific communication purposes, transmission bandwidth refers to the range of frequencies that are used to transmit signals. It encompasses the total frequency range used for communication, including both the data-carrying portion and any guard bands or additional spectrum used for various purposes.

In short, the BWP refers to a specific allocated portion of the transmission bandwidth, while the transmission bandwidth encompasses the entire range of frequencies used for communication.

Figure 4: Transmission bandwidth and Channel bandwidth in 5G NR
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References

[1] 3GPP Specification TS 38.104 (version 17.7.0 Release 17) – 5G NR Base Station (BS) radio transmission and reception

Volterra series – RF power amplifier model

Key focus: Volterra series is an useful tool for modeling nonlinear time invariant systems in general, and they are commonly used to model RF power amplifiers with memory effects

Introduction

Power amplifier are nonlinear devices responsible for compensating for the signal power loss between a transmitter and receiver. The input power levels to the power amplifiers can be increased to compensate for the power loss during transmission. At low input power regime, an ideal power amplifier provides constant gain to the output signal.

However, in the high input power regime, nonlinear effects of the power amplifier come to play, the power of the output signal shows diminished gain (compressed) and starts to saturate.

1-dB compression point is the most important measure of quantifying the beginning of non-linearity of the power amplifier (Figure 1). It is the point where the RF output power level decreases 1 dB from the constant value. Therefore, 1-dB compression point is a measure of the beginning of distortion, where a perfect sine wave fed at the input of the power amplifier do not maintain the perfect shape at the output of the power amplifier.

Figure 1: 1-dB compression point

It is therefore critical to model and understand the power amplifier in order to correct for the non-linear distortion it creates and not break the requirements defined by the agreed-upon standards like 3GPP.

Memoryless models: Taylor series

Memoryless models place emphasis on the power amplifier with memoryless nonlinearity, which means that the current output is determined solely by the current input via a nonlinear method. A nonlinear memoryless system can be modeled using Taylor series. For example, a discrete time system with input x[n] and output y[n], the nonlinearity is incorporated using nonlinear terms with corresponding coefficients/weights (ɑi) for each term.

\[ y[n] = \alpha_1 x[n] + \alpha_2 x^2[n] + \alpha_3 x^3[n]+ \cdots + \alpha_ x^n[n] + \cdots \quad \quad (1) \]

Memory models : Volterra series

The bandwidth of the signals, such as in 3GPP LTE and 3GPP 5G NR, gets wider. When the wideband signals are input to high power amplifiers (such as in base stations), the power amplifiers start to exhibit memory effects. As a consequence, the power amplifier’s current output is affected not only by the current input, but also by previous input values. Volterra series and its derivatives are commonly used to model power amplifiers with memory.

Volterra series is an useful tool for modeling nonlinear time invariant systems in general, and they are commonly used to model RF power amplifiers with memory effects. The Volterra series is a polynomial based mathematical model that captures the nonlinear terms and memory effects present in these devices.

For a discrete time system with input x[n] and output y[n], the Volterra series with nonlinearity order K and memory depth Q is given by

\[y[n] = \sum_{k=1}^K y_k[n] \quad \quad (2)\]

where,

\[ \begin{align} y_k[n] &= \sum_{i_1=0}^{Q-1} \cdots \sum_{i_k=0}^{Q-1} h_k \left(i_1, \cdots, i_k \right) x[n-i_1]x[n-i_2]\cdots x[n-i_k] \\ & = \sum_{i_1=0}^{Q-1} \cdots \sum_{i_k=0}^{Q-1} h_k \left(i_1, \cdots, i_k \right) \prod_{r=1}^{k} x[n-i_r] \\ \end{align}\]

hk(i1, …, ik) are the nth order Volterra Kernels. It can be understood that it models any system by using the system’s impulse responses. Because of the high complexity and high resource requirements, Volterra series cannot be used in practical applications.

For practical applications, Volterra series has to be truncated in such a way that it has the least impact on the models performance. This can be accomplished by omitting some of the terms in equation (2) for each application. Special versions of Volterra series allow for use in power amplifier behavioral modeling. Memory polynomial model is one of the derivative of Volterra series that is easier to implement. We will discuss this model with an example simulation model in the next post.

5G NR Resource block

Key focus: 5G NR resource block : a block of 14 OFDM symbols (1 slot across time domain) with the corresponding 12 subcarriers for those symbols

5G NR protocol stack

In the 5G New Radio (NR), the protocol architecture can be separated into two categories: user plane protocol stack and control plane protocol stack. The user plane protocol stack architecture is responsible for delivering user data and the control plane architecture is responsible for setting up the connection, maintaining mobility and providing end-to-end security.

The user plane protocol stack for 5G New Radio (NR) is shown in Figure 1. We see that at the physical layer interface (air interface) between the gNB and UE, the transmission occurs in the form of radio frames.

Figure 1: 5G NR user plane protocol stack architecture

Radio frame structure

Looking at the 5G New Radio (NR) frame structure from the time domain perspective, the radio transmissions are categorized into radio frames, subframes, slots and mini-slots (Figure 2).

A radio frame is for a duration of 10 ms and it comprises of 10 subframes of duration 1 ms each. Each subframe may consist of one or multiple adjacent slots with each slot having 14 OFDM (Orthogonal Frequency Division Multiplexing) symbols. The possibility of transmission over a fraction of a slot is referred to as mini-slot.

Figure 1 shows the radio frame structure for the supported transmission numerologies (μ = 0, 1, 2, 3, 4) as per Table 1.

Figure 2: 5G NR frame structure

Because the duration of an OFDM signal is inversely proportional to its subcarrier spacing, the time duration of a slot scales with the selected numerology (Table 1).

Table 1: 5G NR – supported transmission numerologies

From the frequency domain perspective, an OFDM symbol is comprised of 12 subcarriers, each subcarrier may be spaced according to the scalable numerology as per Table 1.

Time-Frequency resource

A resource element is the smallest time-frequency resource over one subcarrier of a single OFDM symbol. It is identified as (k,l)p,μ where k is the index of the subcarrier in the frequency domain, l is the OFDM symbol position in time domain, p is the antenna port and μ is the subcarrier spacing configuration defined in Table 1 above.

A resource block (a.k.a physical resource block – PRB) is a block of N RBsc = 12 subcarriers over which the transmissions are scheduled. 5G NR physical layer uses time-frequency resource (physical resource block) for transmission.

A resource grid consists of N size,μgrid,x  subcarriers and N subframe,μ symb number of OFDM symbols (Table 2 and 3).

Figure 3: Resource element, Resource block, Resource grid in 5G NR
Table 2: Number of OFDM symbols per slot, slots per subframe and OFDM symbols per subframe for normal cyclic prefix configuration
Table 3: Number of OFDM symbols per slot, slots per subframe and OFDM symbols per subframe for extended cyclic prefix configuration

π/2 BPSK (pi/2 BPSK): 5G NR PHY modulation

The 5G New Radio (NR) supports quadrature phase shift keying (QPSK), 16- quadrature amplitude modulation (16-QAM), 64 QAM and 256 QAM modulation schemes for both uplink and downlink [1][2]. This is same as in LTE.

Additionally, 5G NR supports π/2-BPSK in uplink (to be combined with OFDM with CP or DFT-s OFDM with CP)[1][2]. Utilization of π/2-BPSK in the uplink is aimed at providing further reduction of peak-to-average power ratio (PAPR) and boosting RF amplifier power efficiency at lower data-rates.

π/2 BPSK

π/2 BPSK uses two sets of BPSK constellations that are shifted by 90°. The constellation sets are selected depending on the position of the bits in the input sequence. Figure (1) depicts the two constellation sets for π/2 BPSK that are defined as per equation (1)

\[d[i] = \frac{e^{j \frac{\pi}{2} \left( i \; mod \; 2\right) }}{ \sqrt{2}} \left[ \left(1 – 2b[i] \right) + j \left(1 – 2b[i] \right)\right] \quad \quad (1) \]

b[i] = input bits; i = position or index of input bits; d[i] = mapped bits (constellation points)

Figure 1: Two rotated constellation sets for use in π/2 BPSK

Equation (2) is for conventional BPSK – given for comparison. Figure (2) and Figure (3) depicts the ideal constellations and waveforms for BPSK and π/2 BPSK, when a long sequence of random input bits are input to the BPSK and π/2 BPSK modulators respectively. From the waveform, you may note that π/2 BPSK has more phase transitions than BPSK. Therefore π/2 BPSK also helps in better synchronization, especially for cases with long runs of 1s and 0s in the input sequence.

\[d[i] = \frac{1}{ \sqrt{2}} \left[ \left(1 – 2b[i] \right) + j \left(1 – 2b[i] \right)\right] \quad \quad (2)\]
Figure 2: Ideal BPSK and π/2 BPSK constellations
Figure 3: Waveforms of BPSK and π/2 BPSK for same sequence of input bits

Figure 4, illustrates the constellations for BPSK and π/2 BPSK when the sequence of mapped bits are corrupted by noise.

Figure 4: BPSK and π/2 BPSK constellation for Eb/N0=50dB

Note: Though the π/2 BPSK constellation looks like a QPSK constellation, they are not the same. Give it a thought !!!

References

[1] 3GPP TS 38.201: Physical layer; General description (Release 16)
[2] 3GPP TS 38.211: Physical channels and modulation (Release 16)
[3] Gustav Gerald Vos, ‘Two-tone in-phase pi/2 binary phase-shift keying communication’, US patent number 10,931,492