In my last blog, I touched on several aspects of using mmWave spectrum for 5G mobile access that present formidable challenges still under investigation. During the recent RAN1 meeting in Prague, the issue of beams surfaced and progress was made to define the 5G physical layer to address the control and management of these beams.
For those unfamiliar with the concept, the 3GPP has officially adopted mmWave spectrum as an option for 5G deployments. Because of the propagation loss in free space for transmission at these frequencies, Phased Array Antennas (PAA) will be used. PAAs differ from conventional antenna technology used in today’s 4G LTE systems in that they employ multiple-elements to form “beams” to capture and transmit 5G signals.
Thus, the 3GPP has adopted beamforming but with a slight twist. For mmWave, 5G systems will use a combination of analog and digital techniques referred to as hybrid beamforming for both the UE and the basestation or gNB. Analog beam steering using phase shifters is integrated into the PAA assembly close each element, and the baseband subsystem will steer the beam in azimuth and elevation. The baseband subsystem will also employ digital beamforming to fine tune the beam within a coarse direction set by the analog control.
Both the UE and the gNB must implement beam control and management, and consistently monitor beam states. Because of the highly directional nature of these beams, both UE and gNB manufacturers must implement a variety of beam shapes and the control of these beams becomes critically important. In other words, what beam to use and when, and then when to change the beams to enhance communication and throughput.
As an initial step, the gNB transmits a wide beam to scan for UEs in an area. The UE receives the weaker scanning beam and then transmits a beam to the gNB to begin to establish a link. Once a UE has been identified the gNB switches to a narrower beam with enhanced gain for communication to the UE. After a link has been established, the gNB continues to monitor each attached UE’s beam characteristics and constantly rank them. The beam characteristics change continuously, and the gNB determines when to switch beams and/or to command the UE switch, or some combination of the two. In a mobile use case where the UE moves, the beam switching must occur very quickly and continuously.
Finally, there are also system conditions to consider. The directional nature of the beams produce the high gain necessary to overcome the free space path loss but this also means that objects can block the beams. When a beam is blocked, how does the gNB determine this condition and how do the actors (gNB and the UE) recover from such an event?
While 4G systems are certainly complex, the combination of 5G on mmWave frequencies increases the complexity of these systems and networks by an order of magnitude. The 3GPP continues to work through these issues and this work will blaze new trails in communication system design, and will likely lead to continuous innovation over the next several years.