Performance, Power, and Area Design Trade-offs in Millimeter-Wave Transmitter Beamforming Architectures

Han Yan and Danijela Cabric

Array Power Consumption & Cost Results

Reference[1] NGMN, “NGMN 5G white paper”, 2015[2] H. Yan et al, “Performance, Power, and AreaDesign Trade-offs in Millimeter-Wave TransmitterBeamforming Architectures”, submitted to IEEECircuits and Systems Magazine, Mar. 2018

Conclusions

• Array size optimization in each architecture: 1) Trade-offbetween output and processing power; 2) Sub-array typicallyrequires much higher number of antennas

• Digital array: 1) Bottleneck is not in digital precoding or DAC;2) Best choice for use cases with high multiplexing regime

• Sub-array: 1) Reasonable option in use cases with lowmultiplexing; 2) Bottleneck is in distribution loss &compensation

• Fully-connected hybrid array: Excessive power andcomplexity in RF signal distribution network

Introduction

The mmWave radio features massive arrays• Beamforming gain in Tx & Rx to compensate for propagation loss• Multiplexing gain for throughput boost

Emerging mmWave arrays architectures: hybrid array• Intends to use analog phase shifters to reduce number of RF-

chain in massive array• Two major designs: sub-array and fully-connected hybrid array

Key innovations and contributions: a comprehensive comparisonof three 5G-NR Tx array candidates that considers• Required performance in typical 5G-NR use cases• Hardware design trade-off including the impact of array size on

hardware specification requirement• Comprehensive hardware block break-down including RF

distribution networks in hybrid arrays

Comparison FrameworkArray candidates

Link budget estimation

Multiuser Beamforming Algorithm and Hardware Design Trade-Off

Beamforming with three Tx array architectures

Methodology for Array Power Consumption & Cost

RF signal distribution budget example in SA

Fig. Total power consumption for three architectures operating in the Dense Urban use case. For each array architecture with varying array size,other design parameters are chosen according the analysis in Section III and throughput demands are guaranteed. Processing power (excludingnon-silicon PA) per array element are listed in text.

Fig. Total power consumption for threearchitectures when the throughput requirements inuse cases are met. Each architecture uses array sizethat reaches lowest power consumption.

Fig. The required power consumption in DAand SA architecture when both are designedto meet the demands of the increasednetwork throughput. DSP efficiencyimprovement in the future is also included.

Fig. IC area breakdown of threearchitectures. Each of architecture usesarray size such that it can meet 5G usecases throughput requirement in anenergy efficient manner.

Signal distribution example in SA

We model the following circuits block in three Tx array architectures• DSP: power scales with required baseband precoding operation

throughput and precision in bits.• Wireline data routing: power scales with throughput of precoded

symbols and required precision in bits• DAC: power scales with required precision in bits• LO/Mixer: fixed power• RF Amp.: power scales with required RF signal distribution budget

PA: power consumption scales with output power

Fig. SINR performance with different hardware impairments. SAand FH have Q = {3,4,5} bits phase shifter (dotted, dashed, andsolid curves). The baseband precoding uses fixed point operationwith precision 2 bits greater than associated DAC quantization.

Figure. Two stage hybrid beamforming illustration in FH architecture. Analogstage and receiver uses maximum ratio transmission and combining while digitalstage is used to control interference.

Fig. Required transmissionpower as function oftransmitter array size toreach SNR target. Results inthree array architectures andthree use cases are shown.

Multiplexing 𝑈 = 8 𝑈 = 2 𝑈 = 1UE SINR (dB) 22.1 6.2 35.5

a. Based on 3GPP model for above-6GHz band..b. Includes shadowing and 25mm/h rain absorbtion.c. Based on 8 receiver antennas and 3dBi antenna gain in firsttwo cases and 256 receiver antennas and 3dBi antenna gain in

backhauling.

ItemsDense Urban

50+Mbps Everywhere

Self-backhauling

BW (MHz) 850 850 850Distance (m) 100 100 707Prop. Lossa

(dB)104.4 125.1 118.3

Other Lossb

(dB)12.7 25.3 17.0

Rx Gainc (dB) 12.0 12.0 27.1Rx NF (dB) 10.0 10.0 10.0

AWGN (dBm) -74.7 -74.7 -74.7SNR w/o Tx Array (dB)

18.7 -14.7 15.5

Typical Use Cases in 5G-NR [1]• Dense Urban: Network throughput

3.75Tbps/km2• 50+Mbps Everywhere: 100Mbps for

2500 connection/km2 NLOS UEs• Self-backhauling: Peak rate 10Gbps

Comparison is based on• Performance requirement survey for

5G-NR use cases• Simulation study of required output

power & array size• Circuits block model of entire

transmission system • Trade-off between transmission power

and processing power

Channel model (𝑳 cluster and 𝑹 intra-cluster rays; independent for UEs)

• Tx beamforming with DAThere is no RF beamformer, i.e.,

Digital beamformer uses regularized zero-forcing, i.e.,

where 𝐆DA 𝑢 = 𝐰𝑢H𝐇𝑢 is post-

combining effective channel

Received signal model (𝑢-th UE)

Assumptions: 1) CSI is perfectly known to both BS and Ues; 2) one data stream for one UE; 3) UEs have the same SNR; 4) Each UE with analog array uses maximum ratio combining.

Digital Array (DA)

Sub-Array (SA)

Fully-connected Hybrid Array (FH)

• Tx beamforming with SARF beamformer is maximum ratio transmission, i.e.,

where 𝐫SA is selected elements in 𝑟-th column of 𝐑SADigital beamformer uses regularized zero-forcing, i.e.,

where 𝐆SA 𝑚 = 𝐰𝑚H𝐇𝑚𝐑SA is the effective channel

with combiner and analog precoders

• Tx beamforming with FHSimilar to SA except full occupation in 𝐑FH due to fully-connection between phase shifters and antennas

Summary of hardware model (see [2] for detailed equations)

Adapted from H-tree based RF signal distribution and compensation network in60GHz 256-element array [Zihir et al, IEEE TMTT, 2016]

Acknowledgment

This work is partially supported by NationalScience Foundation through grant 1718742