Abstract:

Small cell networks have seen a rapid rise in popularity due to their ability to enhance coverage and capacity in densely populated areas. As small cells become more prevalent, understanding the physical (PHY) layer and its differences from macro cells and Open Radio Access Networks (ORAN) is crucial for network architects and engineers. In this article, we’ll delve into the PHY layer of small cells, discuss the Tx and Rx chains, and compare their features to those found in macro cells and ORAN.

Physical Uplink and Downlink Channels in Small Cells

Small cells operate in the same frequency bands as macro cells but with lower transmission power, enabling increased network capacity and improved indoor coverage. This section will discuss the differences in physical uplink and downlink channels in small cells compared to macro cells.

Physical Uplink Channels (PUCCH)

The Physical Uplink Control Channel (PUCCH) is responsible for transmitting control information from the user equipment (UE) to the small cell base station (eNodeB). In small cells, the PUCCH is configured to have lower transmission power and shorter transmission distances due to the small coverage area.

Physical Uplink Shared Channel (PUSCH)

The Physical Uplink Shared Channel (PUSCH) carries user data from the UE to the eNodeB. In small cells, the PUSCH can be configured with a lower modulation and coding scheme (MCS) to ensure a robust link, given the reduced coverage area and increased interference potential.

Physical Downlink Control Channel (PDCCH)

The Physical Downlink Control Channel (PDCCH) carries control information from the eNodeB to the UE. In small cells, the PDCCH can be configured with a smaller aggregation level, as the increased cell density allows for a more granular allocation of resources.

Physical Downlink Shared Channel (PDSCH)

The Physical Downlink Shared Channel (PDSCH) transmits user data from the eNodeB to the UE. In small cells, the PDSCH can utilize a lower MCS to maintain a robust link, given the reduced coverage area and potential for increased interference.

Physical Broadcast Channel (PBCH)

The Physical Broadcast Channel (PBCH) is responsible for broadcasting system information in both small cells and macro cells. In small cells, the PBCH may use a lower transmission power and a narrower frequency band to conserve resources and limit interference with neighboring cells.

Physical Layer Processing in Small Cells

The physical layer processing in small cells is responsible for preparing data for transmission over the air interface and receiving data from the air interface.

The main components of the Tx (transmission) and Rx (reception) chains include channel coding, rate matching, HARQ, interleaving, modulation, and FFT/IFFT. These components and processes are responsible for encoding, modulating, and transmitting data on the uplink and downlink channels.

 

Figure 1: Tx/Rx chain

Channel Coding: CRC and LDPC

Channel coding is a crucial step in the transmission process to protect data against errors caused by noise and interference. In small cells, two main channel coding techniques are employed: Cyclic Redundancy Check (CRC) and Low-Density Parity-Check (LDPC).

CRC is an error-detecting code used to detect accidental changes to raw data. It generates a fixed-size checksum for each data block, which is then transmitted alongside the data. Upon reception, the receiver calculates the CRC again to verify if the received data has been corrupted.

LDPC is an advanced error-correcting code that offers a high coding gain with low complexity. It is used to recover transmitted data that has been corrupted due to noise and interference. LDPC codes have a sparse bipartite graph structure, which enables efficient iterative decoding algorithms.

Rate Matching and HARQ

Rate matching is the process of adapting the coded data rate to the available transmission resources. This is achieved by either puncturing (discarding) some coded bits or repeating (duplicating) others. The process ensures that the number of coded bits transmitted is consistent with the available resources.

Hybrid Automatic Repeat reQuest (HARQ) is a technique that combines error detection and error correction. When the receiver detects an error, it sends a negative acknowledgment (NACK) to the transmitter, which retransmits the data with additional redundancy information. The receiver then combines the original data and the retransmitted data to improve error correction performance.

Interleaving and Deinterleaving

Interleaving is the process of rearranging the bits within a codeword to protect against burst errors. This is particularly important in wireless communication systems, where fading and interference can cause consecutive bits to be lost or corrupted. Deinterleaving is the reverse process that restores the original order of bits at the receiver side.

Modulation and Demodulation

Modulation is the process of mapping the coded bits onto complex symbols that can be transmitted over the air interface. Small cells support various modulation schemes, such as Quadrature Phase-Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16-QAM), and 64-QAM. The choice of modulation scheme affects the spectral efficiency and robustness of the transmission. Demodulation is the process of recovering the transmitted bits from the received complex symbols.

FFT and IFFT

Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) are essential components in the physical layer processing of small cells. These operations enable efficient implementation of Orthogonal Frequency Division Multiplexing (OFDM), a widely used multicarrier modulation technique in wireless communication systems. The IFFT operation generates the time-domain OFDM signal at the transmitter side, while the FFT operation recovers the frequency-domain symbols at the receiver side.

Comparing small cell PHY layer with macro cell and ORAN

 

Parameter	Small Cell	Macro Cell	ORAN
Transmission Power	Lower	Higher	Configurable
Coverage Area	Smaller	Larger	Configurable
Modulation and Coding Scheme	Lower (more robust link)	Higher (higher data rate)	Configurable
Resource Allocation	More granular	Less granular	Configurable
Interference Management	Higher density	Lower density	Advanced interference management
FFT/IFFT Domains	Smaller (lower bandwidth)	Larger (higher bandwidth)	Configurable
Channel Configurations	Adapted for smaller coverage areas	Optimized for larger coverage	Configurable
PHY Processing Complexity	Lower	Higher	Configurable

 

Small cells, macrocells, and ORAN systems differ in their transmission power, coverage areas, modulation schemes, resource allocation, FFT/IFFT domains, channel configurations, PHY processing complexity, and RF bandwidths due to their distinct coverage areas, capacity requirements, and deployment scenarios.

1. Small Cells: Designed for densely populated areas and indoor environments, small cells have low transmission power, limited coverage, and typically use lower modulation schemes. They have less resource allocation complexity, shorter FFT/IFFT domains, simpler channel configurations, lower PHY processing complexity, and smaller RF bandwidths, usually ranging between a few MHz and tens of MHz (e.g., 10 MHz, 20 MHz), depending on the specific deployment and capacity requirements.
2. Macrocells: Providing larger coverage areas, macrocells have high transmission power and employ higher modulation schemes. They require more advanced resource allocation techniques, have longer FFT/IFFT domains, more complex channel configurations, higher PHY processing complexity, and larger RF bandwidths to support higher data rates and more simultaneous users. The RF bandwidth for macrocells can range from a few tens of MHz to hundreds of MHz (e.g., 20 MHz, 40 MHz, 100 MHz), depending on the specific deployment, spectrum availability, and network capacity requirements.
3. ORAN (Open RAN): With configurable transmission power and coverage, ORAN systems are adaptable to different deployment scenarios. They support a range of modulation schemes, advanced resource allocation algorithms, interference management techniques, and configurable FFT/IFFT domains. ORAN systems offer flexible channel configurations and can have varying levels of PHY processing complexity, depending on the specific deployment and requirements. As a disaggregated, virtualized, and open radio access network architecture, ORAN allows for flexible and configurable RF bandwidths, dynamically allocating and adjusting them based on factors like user density, coverage requirements, and spectrum availability. The RF bandwidth for ORAN systems can range from a few MHz to hundreds of MHz, similar to macrocells, but with added flexibility to adapt to different deployment scenarios.

Advanced PHY Layer Techniques for Small Cell Networks:

As the telecommunications industry continues to evolve, emerging and advanced techniques are being researched and implemented to enhance the performance of the physical layer in wireless communication systems.

These advanced PHY layer techniques are particularly relevant for small cell networks, offering potential improvements in network capacity, coverage, and interference management.

1. Full-Duplex Communication: Full-duplex communication enables simultaneous transmission and reception in the same frequency band, improving spectral efficiency. Small cells, with their limited coverage areas and reduced interference, are well-suited for full-duplex communication. However, implementing full-duplex in small cells presents challenges, such as managing self-interference and adapting existing network architectures. In contrast, macro cells and ORAN systems may face more significant interference challenges due to their larger coverage areas and more complex network topologies.
2. Massive MIMO and Multi-User MIMO: Massive MIMO and Multi-User MIMO techniques enhance network capacity, coverage, and interference management in small cell networks. In small cells, these techniques can be employed to improve the spatial multiplexing of users and maximize the available spectrum. Macro cells can also benefit from Massive MIMO and Multi-User MIMO, but their larger coverage areas may pose challenges in terms of antenna deployment and signal processing. ORAN systems, with their flexible and configurable architectures, can adapt to different deployment scenarios and requirements, making them well-suited for implementing MIMO techniques.
3. Non-Orthogonal Multiple Access (NOMA): NOMA techniques allow multiple users to share the same time and frequency resources, improving spectral efficiency and user experience in small cell networks. With their high user density and limited coverage areas, small cells are ideal for implementing NOMA. In comparison, macro cells may face challenges in managing interference and maintaining user quality of experience due to their larger coverage areas. ORAN systems, with their advanced resource allocation algorithms and configurable architectures, can support NOMA techniques while dynamically adapting to different deployment scenarios and user densities.

Small cells can benefit significantly from these advanced techniques due to their high density and potential for increased interference.

However, these techniques can still play a role in enhancing macro cell network performance in certain scenarios.

ORAN systems, with their advanced interference management capabilities and flexible architectures, can dynamically adapt to various deployment scenarios and requirements, making them well-suited for implementing advanced interference management techniques.

Conclusion:

In conclusion, the physical layer in small cells is essential for efficient communication in dense areas. A deep understanding of the PHY layer components and their differences across small cells, macro cells, and ORAN systems is essential for network architects and engineers to design, optimize, and maintain efficient and robust wireless networks.

As technology continues to advance and the demand for high-quality wireless communication grows, the exploration and implementation of innovative PHY layer techniques will remain crucial for meeting the ever-increasing capacity and performance requirements in the telecommunications industry.