Now that the initial 5G New Radio (NR) standard has been defined, wireless engineers are turning from research to rapid development of products that can deliver higher data rates, lower-latency, and more power-efficient implementations. Along the way, they will face a new set of technical challenges in designing a physical layer architecture that can handle more complex multiuser environment and channel conditions at higher frequencies.
Understanding how requirements and technologies in 5G differ from LTE can help design engineers prepare for working with emerging architectures.
New Design Architectures and Algorithms for 5G
The leap in 5G broadband speeds will be enabled by massive MIMO communication in the millimeter wave (mmWave) frequency range and by new radio algorithms that achieve more efficient use of spectrum. New design architectures and algorithms will affect every aspect of 5G systems, from antennas to RF electronics to baseband algorithms. The performance of these subsystems is so tightly coupled that they must be designed and evaluated together.
New 5G mmWave designs will also require massive MIMO antenna arrays with hundreds of antenna elements on base stations (eNodeB). Having many antenna elements in a small area makes it practical to achieve a high beamforming gain for mmWave frequencies that can be up to 100x smaller than an array for microwave frequencies. The highly directional beams also help offset the increased path loss at the higher operating frequencies because the beams steer power in a specific direction. Behavioral simulation of the RF and digital elements of massive MIMO systems can accelerate development and optimization of beamforming designs.
Behavioral Simulation for Massive MIMO
Achieving an optimal design for today’s wireless systems requires combined models of the antenna arrays and beamforming algorithms to simulate their interaction and impact on system performance. This puts a strain on current 3G and 4G design tools, which typically separate antenna design from system architecture and signal processing algorithms. MIMO simulation times are also typically 10 times longer than 3G and 4G simulations.
Behavioral-level simulation of the antenna array system addresses these challenges and will have increased impact as more 5G simulations come online. Simulating at the behavioral level reduces the simulation time. This enables engineers to experiment with different array architectures and algorithms, simulate the performance of the array and associated algorithms, iteratively adjust parameters to mitigate the effect of antenna coupling, and achieve better beamforming control.
While smaller wavelengths enable massive MIMO implementation within small form factors, engineers will begin to find that signal path and propagation challenges associated with mmWave frequencies will increase as wireless communication systems trend toward 5G. To achieve better beamforming control and flexibility for the future’s systems, it would be ideal to have independent weighting control over each antenna array element, with a transmit/receive (T/R) module dedicated to each element. However, this is generally not practical due to cost, space, and power limitations.
Hybrid beamforming is a technique for partitioning beamforming between the digital and RF domains to reduce the cost associated with the number of RF signal chains. Hybrid beamforming combines multiple array elements into subarray modules, with one T/R module dedicated to a subarray in the array.
For engineers looking to implement this strategy for 5G designs, a key challenge to keep in mind is how to meet required performance parameters, while meeting implementation cost constraints. Software platforms like Simulink can enable unified, multi-domain modeling and simulation of the RF domain and digital domain components. Circuit envelope simulation of the RF components ensures fast simulation of the hybrid system.
Figure 2 shows a section of a multi-domain model containing digital beamforming weights used to shape the signals feeding the RF subarrays, where phase shifts are applied. The resulting hybrid weights produce the desired array pattern and help to prepare systems for modeling.
Modeling and Linearizing Power Amplifiers for 5G Systems
The linearity of power amplifiers (PAs) is, and will be a critical specification of every future 5G transmitter. Backing off power amplifiers to operate in the highly linear region is simply not a viable commercial solution, especially when applied to the higher frequencies and larger bandwidths associated with 5G. For this reason, digital predistortion (DPD) techniques are typically applied to increase the efficiency of the transmitter and at the same time limit spectral regrowth and interchannel interference.
Developing a quality DPD algorithm is challenging because it requires a deep understanding of the effects introduced by the power amplifier and adjacent subsystems, such as the antenna. Because power amplifiers are nonlinear and are affected by finite memory, the characterization of power amplifiers strongly depends on the type of signal used to drive it.
Because of this complexity, DPD algorithms are often developed in the lab, using rapid prototyping platforms that enable the testing algorithms together with the actual PA. While this approach is useful to validate and fine-tune the algorithms, it is harder to apply when the actual PA is not yet available, or to explore the algorithmic DPD design space.
Adapting to New 5G RF Algorithms
The 5G wireless communication standard will provide significantly higher mobile broadband throughput with its enhanced mobile broadband (eMBB) mode. Key elements of the new 5G standard include:
- Shorter slot durations, corresponding to increased subcarrier spacing, for increased signal bandwidth and shorter latency.
- New coding schemes such as LDPC for data and polar codes for control information, for error correction and improved data rates.
- Enhanced waveforms with improved out-of-band emissions (OOBE), enabling more efficient use of bandwidth resources.
- Spatial channel models for operation at current (<6 GHz) and mmWave (>28 GHz) frequencies.
While these elements have the potential to improve system efficiency, they can add complexity and delay to your design schedule. When implementing the above techniques for 5G designs, engineers will find that simulation can help them overcome the complexities at a much earlier stage.
Design tools simplify the exploration of different algorithm and architecture design trade-offs at an early stage of development. By simulating the various components of a 5G system in the same environment with realistic propagation channels, engineers will be able to validate end-to-end system performance before heading to the test lab. As a result, the introduction and integration of 5G wireless systems may not be such a daunting task after all.