Add magma-india blogs by Shubham and Aditya

5g-core/index.md

@@ -7,10 +7,14 @@ pqc/index
 hexaebpf/index
 ebpf/index
 security/index
+standards/index
 ```
 
 ```{toctree}
 :maxdepth: 1
+
+nr-reference-signals
+nr-frame-structure
 5g-upf-architecture
 5gc-operator
 os-5gc-compare

5g-core/nr-frame-structure.md

@@ -0,0 +1,89 @@
+# NR Frame Structure
+
+**Author:** [Shubham Kumar](https://www.linkedin.com/in/chmodshubham/)
+
+**Published:** July 10, 2022
+
+![NR frame structure overview](images/nr-frame-structure/image-1.png)
+
+Data(UL/DL) is transmitted in the form of radio frames in the air. **Radio Frames** are of a duration of **10ms** which consists of **10** **subframes** each having a duration of **1ms**. Subframes inside a radio frame are serialized as SF0, SF1, SF2, SF3, …., and SF9. A subframe is made up of a **Resource Grid** which is a (m x n) matrix of Resource Elements where 'm' defines the number of Sub-carriers and 'n' defines the number of OFDM Symbols.
+
+This arrangement of frames and subframes is similar to what is present in the LTE frame structure.
+
+Subframes are further divided into slots. LTE has fixed 2 slots per subframe as per numerology, 15KHz. But in NR, the number of slots varies, depending on the **SCS**(*SubCarrier Spacing*). Slot length gets shorter as Subcarrier Spacing gets wider.
+
+> Note: SCS is equivalent to numerology, mu(μ).
+
+![NR subframe and slot structure](images/nr-frame-structure/image-2.png)
+
+LTE supports **numerology** of 15KHz only but in 5G, the supported numerologies are 15KHz, 30KHz, 60KHz, 120KHz, and 240KHz. Multiple Subcarrier Spacing provides flexibility for multiple services on the same carrier frequency but it also introduces interference with other services having different numerology.
+
+Formula to calculate, **SCS = 15 x 2μ kHz**(where μ = 0, 1, 2, 3, 4).
+
+A **Slot** is another matrix of 12 subcarriers along with a variable number of symbols in the time domain or simply a **Resource Block**. It can further be classified based on the number of OFDM symbols. The number of OFDM Symbols varies with **slot configuration**, and the type of **Cyclic Prefix** used.
+
+There are 2 types of slot configuration:
+
+- **Slot Configuration 0** – This configuration is newly introduced in the NR System. The number of symbols in a slot is always 14 for Normal Cyclic Prefix and 12 for Extended Cyclic Prefix.
+
+- **Slot Configuration 1** – In this configuration, the number of symbols in a slot is always 7 for Normal Cyclic Prefix and 6 for Extended Cyclic Prefix. This is the old Slot configuration mechanism used in the 4G System for resource allocation.
+
+![Slot configuration](images/nr-frame-structure/image-3.png)
+
+**OFDM**(*Orthogonal Frequency Division Multiplexing*) is an efficient **modulation technique** in which a wide frequency band is split into many small frequencies, known as subcarriers, and transmit in such a way that they overlap each other but do not influence other subcarriers. These subcarriers are **orthogonal** to each other which means the peak point of a sub-carrier occurs at the NULL point of others such that the resources can be used with maximum efficiency.
+
+On a technical basis, it is the combination of **QAM**(*Quadrature Amplitude Modulation*) and **FDM**(*Frequency Division Modulation*) techniques to increase the channel efficiency and reduce bandwidth consumption, ultimately producing a high data rate communication system. OFDM symbols can be classified as **D**(*Downlink*), **U**(*Uplink*), and **X**(*Flexible*) based on the **slot format**.
+
+Propagation of signals takes multiple diversions before reaching their destination due to which signals get distorted by fading and the doppler effect. At the frame structure level, if one symbol gets delayed a bit, then it coincides with the next symbol and causes interference between them called **ISI**(*Inter Symbol Interference*) which ultimately affects the transmission quality of digital signals.
+
+So, to overcome this problem, a **time gap** is introduced between every 2 symbols. But leaving the space empty like turning off the transmission, would cause problems for the amplifier. So, to encounter this, a **CP**(Cyclic Prefix) is introduced in the space.
+
+![Cyclic prefix](images/nr-frame-structure/image-4.png)
+
+The **Cyclic Prefix** in OFDM refers to copying the end part of the signal and adding it at the beginning of it. This Cyclic Prefix is discarded at the receiver end.
+
+Cyclic Prefix is of 2 types:
+
+- **Normal CP** – In Normal CP, the slot is divided into **14/7 symbols** based on slot configuration. The normal CP length is designed to support propagation conditions with a delay spread up to 4.7 μs.
+
+- **Extended CP** – The slot is divided into **12/6 OFDM symbols** based on slot configuration in the case of extended CP. This is intended to support deployments where the delay spread is up to 16.7 μs. This is only supported for the μ value 2 i.e. 60KHz SCS.
+
+With the **increase** in the value of **μ**, the **OFDM symbols** occupying space in the **time domain** start to **decrease** with a simultaneous **increase** in the size of the **frequency domain**.
+
+## Different Numerologies
+
+1. For **μ = 0**, that means **15 kHz Subcarrier Spacing**. In this, a subframe contains only 1 slot which means a radio frame contains **10 slots** in it. For slot configuration 0, the number of OFDM symbols within each slot is 14. This configuration is for OFDM symbols having **Normal Cyclic Prefix**.
+
+![μ=0 numerology](images/nr-frame-structure/image-5.png)
+
+For **μ = 1**, that means **30 kHz Subcarrier Spacing**. In this, a subframe is divided into 2 slots which means a radio frame contains **20 slots** in it. For slot configuration 0, the number of OFDM symbols is 14 within each slot. This configuration is for OFDM symbols having **Normal Cyclic Prefix**.
+
+![μ=1 numerology](images/nr-frame-structure/image-6.png)
+
+3. For **μ = 2**, that means **60 kHz Subcarrier Spacing**. In this, a subframe is divided into 4 slots which means a radio frame contains **40 slots** in it. This is further **categorized** based on the **type of Cyclic Prefix**.
+
+- For OFDM symbols having **Normal Cyclic Prefix**, the number of OFDM symbols is **14** within each slot for slot configuration 0.
+
+![μ=2 Normal CP](images/nr-frame-structure/image-7.png)
+
+- For OFDM symbols having **Extended** **Cyclic** **Prefix**, the number of OFDM symbols is **12** within each slot configuration 0.
+
+![μ=2 Extended CP](images/nr-frame-structure/image-8.png)
+
+4. For **μ = 3**, that means **120 kHz Subcarrier Spacing**. In this, a subframe is divided into 8 slots which means a radio frame contains **80 slots** in it. For slot configuration 0, the number of OFDM symbols is 14 within each slot. This configuration is for OFDM symbols having **Normal Cyclic Prefix**.
+
+![μ=3 numerology](images/nr-frame-structure/image-9.png)
+
+5. For **μ = 4**, that means **240 kHz Subcarrier Spacing**. In this, a subframe is divided into 16 slots which means a radio frame contains **160 slots** in it. For slot configuration 0, the number of OFDM symbols is 14 within each slot. This configuration is for OFDM symbols having **Normal Cyclic Prefix**.
+
+![μ=4 numerology](images/nr-frame-structure/image-10.png)
+
+**Resource Element** is the **smallest physical time-frequency resource** consisting of **1 subcarrier** in **1 OFDM symbol**. **Subcarrier** is defined in the **frequency domain** and **OFDM Symbol** is defined in the time domain. To **identify the position** of each resource element, 2 parameters **(k,l)** are used where **'k'** and **'l'** are the indexes in the **frequency** and **time domain** respectively.
+
+![Resource element](images/nr-frame-structure/image-11.png)
+
+In **LTE**, a **Resource Block** is defined in the **time domain** of **0.5 ms** and **12 subcarriers** in the **frequency domain** but in **NR**, a **resource block** is **only defined** in the **frequency domain**. Unlike LTE, 5G has more flexibility in the time duration for different transmissions. The time domain can be altered based on the need. **E.g.** if a particular activity needs high throughput, it can be scheduled for multiple symbols and if it requires low latency, only fewer symbols will be allocated.
+
+The number of **Resource Blocks** **varies** with **numerology**. **Resource Block** is defined as **12 consecutive subcarriers** in the **frequency domain**.
+
+![Resource blocks and numerology](images/nr-frame-structure/image-12.png)

5g-core/nr-reference-signals.md

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+# NR Reference Signals
+
+**Author:** [Shubham Kumar](https://www.linkedin.com/in/chmodshubham/)
+
+**Published:** July 19, 2022
+
+**Reference signals** are unique signals only **present** in **Physical channels** and are used to deliver a **reference point** for **resource scheduling** during **Uplink** and **downlink** **transmission**. It occupies specific resource elements within the grid.
+
+NR constitutes 4 reference signals:
+
+- **DMRS**(*Demodulation Reference Signal*)
+- **PTRS**(*Phase Tracking Reference Signal*)
+- **CSI-RS**(*Channel State Information Reference Signal*)
+- **SRS**(*Sounding Reference Signal*)
+
+![NR reference signals overview](images/nr-reference-signals/image-1.png)
+
+## NR RF vs LTE RF
+
+![NR RF vs LTE RF comparison](images/nr-reference-signals/image-2.png)
+
+- In LTE, there is a reference signal called **C-RS**(*Cell-specific Reference Signal*) which is mainly used for **downlink purposes** such as **demodulation**, and **channel quality estimation**. But in NR, this reference signal is **removed** and instead, uses different downlink signals for different purposes.
+
+- A **new reference signal**, **PTRS**, is **introduced** which tracks the phase of a local oscillator at the transmitter and receiver end. This is used to counter phase noise at higher frequencies.
+
+- In LTE, DMRS is introduced only in Downlink transmission, but this limitation is no more available in NR. **NR** introduces **DMRS** for **both downlink** and **uplink channels**.
+
+- In LTE, reference signals are always enabled for maintaining the link between the device and the network but in NR, **reference signals are transmitted only when they are required** which ultimately optimizes their performance.
+
+## Types of Reference Signals
+
+### DMRS
+
+![DMRS](images/nr-reference-signals/image-3.png)
+
+- **DMRS**(*Demodulation Reference Signal*) is the reference signal which is used for **demodulation** i.e. extracting the original signals from the received one(*modulated*) by altering its frequency and amplitude.
+
+- DMRS is designed **specifically** for **each UE**, i.e. no 2 UE's use the same DMRS for the physical channels demodulation.
+
+- As DMRS is used particularly for demodulation and **RRM**(*Radio Resource Management*) **measurement**, so this is transmitted only when it is needed.
+
+- DMRS is **mapped** to different physical channels for both **Downlink** and **Uplink**. The physical channels that are associated with DMRS are **PDSCH, PDCCH PUSCH,** and **PUCCH**.
+
+Apart from this, it is also found in **association with PBCH** inside the **SSB**(**Synchronization Signal Block**). **PBCH DMRS** occupies **25%** of **REs**(*Resource Elements*) allocated to PBCH. The **REs occupied** by the **PBCH** **DMRS** are **dependent** on the **PCI**(*Physical Cell Id*) value and its **location** is determined by the formula '**PCI mod4'**.
+
+- Multiple **orthogonal** DMRSs i.e. *isolated from each other's effects*, can be allocated to **support** **MIMO**(*Multiple Input and Multiple Output*) transmission for higher throughput. It supports up to about 12 orthogonal layers.
+
+- The network controls the rate of transmission of DMRS signals based on rate change. In high mobility scenarios, tracking fast changes in the channel increases the rate of transmission of DMRS signal whereas, in low-speed scenarios where the channel shows little change, it sends this information occasionally.
+
+- **DMRS** is also found in **association** with **PTRS** only **once** **per** **transmission**. DMRS can also be **beamformed**.
+
+### PTRS
+
+![PTRS](images/nr-reference-signals/image-4.png)
+
+- **PTRS**(*Phase Tracking Reference Signal*) is used to **track** the **phase** of the local **oscillator** present at the transmission and receiver end.
+
+- The initial angle made by a sinusoidal function of a waveform generated by an oscillator is known as a **phase**. Any kind of **fluctuations** that occur in the phase of a **waveform** is called **phase noise**. The **orthogonality** of the **subcarriers** gets **destroyed** due to **ICI**(*Inter-Carrier Interference*) and this phase noise causes a **common phase rotation** to all the **subcarriers** known as **CPE**(*Common Phase Error*).
+
+PTRS is responsible for **minimizing** the effect of the oscillator **phase noise** on system performance, especially at **mmWave frequencies**. The **phase noise increases** with an **increase** in the **frequency** of waves.
+
+- PTRS has a **low density** in the **frequency domain** and **high density** in the **time domain** as phase noise tends to change across time but remains the same across the frequency domain. Or in other words, there are **low correlation** characteristics among the **consecutive OFDM symbols**.
+
+- PTRS **occurs** **only** in **combination** with **DMRS** in physical channels. It is present in both uplink and downlink with **PUSCH** and **PDSCH** channels respectively.
+
+- PTRS allocation within subcarriers is carried out depending on the quality of the oscillators, carrier frequency, subcarrier spacing, modulation, and coding schemes that are used.
+
+### SRS
+
+![SRS](images/nr-reference-signals/image-5.png)
+
+- **SRS**(*Sound Reference Signal*) is an **uplink reference signal** **transmitted** by **UE** which is used by the gNodeB to **estimate** the **uplink channel quality** over a wider bandwidth.
+
+- Unlike DMRS and PTRS, **SRS** is **not associated** with any **uplink physical channels** but **supports** uplink **resource scheduling** and link adaption(*selecting an appropriate modulation and coding scheme to maximize the transmission of user bit rate*).
+
+- SRS resources **span** over **1, 2, or 4 consecutive symbols** in the **time domain**. It is always **transmitted** in the **last 6 symbols** of the **slot**.
+
+- SRS **provides information** about the **combined effect** of **multipath fading**, **scattering**, **Doppler**, and **power loss** of the transmitted signal. This information is used by the base station for **beam management** and **power control** of the signal.
+
+- Max of 12 UEs can transmit SRS simultaneously using 1 antenna port.
+
+### CSI-RS
+
+![CSI-RS](images/nr-reference-signals/image-6.png)
+
+- **CSI-RS**(*Channel State Information Reference Signal*) is a **downlink reference signal** used by UE to **measure** the **quality** of the **downlink** **channels** and **report** this to the base station through the **CQI**(*Channel Quality Indicator*) report. This information is used by gNodeB to implement appropriate modulation schemes, code rates, beamforming, etc.
+
+- It is **used** for the **calculation** of **RSRP**(*Reference Signal Received Power*), **RSRQ**(*Reference Signal RecivedQuality*), and **SINR**(*Signal Interference + Noise Ratio*) during **mobility** and **beam management** in connected mode.
+
+- It is also used in **frequency and time tracking**, and **UL reciprocity-based precoding**(*channel estimation in uplink so that it can be directly used for link adaption in the downlink*). For time and frequency tracking, CSI-RS transmission can either be **periodic** or **aperiodic**.
+
+- 5GS(*5G System*) allows a **high** level of **flexibility** in **CSI-RS** **configurations**, its resources can be configured up to 32 ports.
+
+- CSI-RS resources can be scheduled on any OFDM symbols within the slot but it **usually occupies 1, 2, or 4 OFDM symbols** based on configured number of ports.
+
+- CSI-RS is uniquely configured for each UE but multiple UEs can share the same resources as they all are served by the same gNB.