Traversing through the Wormhole to 5G

People have high expectations for 5G mobile communication, which provides full connection and a thousandfold increase in capacity. 5G will be commercially deployed in 2020. Dr. Xiang Jiying, CTO of ZTE wireless products, proposed first the pre5G concept at the 5G World Summit 2014 in Amsterdam. He also proposed using some 5G technology on 4G terminals to meet user requirements. This has drawn great attention in the industry. Reporter Zheng Hong recently interviewed Dr. Xiang Jiying about pre5G and 5G.

Q: What do you think of 5G and how will it develop?

A: I think the 3G key feature is CDMA and the 4G key feature is OFDM+MIMO. But so far there has been no single technology that identifies 5G. Therefore, we believe that unlike 3G and 4G, 5G will no longer have an identifying technology as its key feature. Instead, 5G will incorporate a family of technologies, including massive MIMO, ultra-dense network (UDN), high-frequency communication, multiuser shared access (MUSA), collaborated networking and so on. 5G may also include technologies for limited application scenarios like faster than Nyquist (FTN), filter-bank multi-carrier (FBMC), and multi-domain coding. Software defined network (SDN) and network function virtualization (NFV) will also be a part of 5G.
From 3G to 4G, users have experienced overnight a significant improvement in network performance. However, the evolution from 4G to 5G will possibly be a gradual improvement process. One important thing for 5G is its compatibility with 4G. Some 5G technologies starting with R14 will be accepted as standards.

Q: Will 5G be revolutionary or evolutionary? What technologies will belong to 5G and what belong to 4G? How do you define them?

A: There are different opinions on whether 5G is revolutionary or evolutionary because 5G does not have a single identifying technology, and most of 5G technologies lie somewhere in between evolutionary and revolutionary processes. Moreover, it is subjective to define whether 5G is evolutionary or revolutionary. For example, if a student got 99 points and ranked first in the class, the 99 points were undoubtedly a good score. If the student got 40 points and ranked last in the class, the 40 points were undoubtedly a bad score. But if the student got 65 points, it would be difficult to define this score as either good or bad.
Anyway, 5G needs to be defined. We believe 5G can be defined by enumeration, a method to artificially list what technologies belong to 5G and what belong to 4G. Of course, there must be an organization such as 3GPP that is fully authorized to release the result of the enumeration.

Q: In what circumstances did ZTE propose the pre5G concept at the 5G World Summit held in Netherlands in June 2014? How do we understand the pre5G concept?

A: Pre5G is in fact an operator-driven concept. Operators generally agree that there is not yet a clear definition for 5G. Moreover, 2020 is far from now, and most operators think it is unnecessary to pay much attention to 5G at present.
ZTE proposed the pre5G concept that can deploy 5G on 4G terminals. Users get familiar with pre5G, and pre5G will no longer be a lab technology implemented in the long term. In addition to the only two options: evolution and revolution, pre5G introduces a third state that makes easy definition of 5G.
Put simply, pre5G contains four elements: adopt 5G technologies; provide user experience (throughput and delay) that is far better than 4G and close to 5G; be available far earlier than 2020; and be deployed over existing air interfaces or even 4G terminals.
Some may question how to define pre5G without a clear definition of 5G. ZTE believes that 5G has been well defined in terms of demand and key technologies although 5G has not been defined from a standard perspective. So there is no problem with defining pre5G based on some accepted 5G technologies and demands.

Q: ZTE has completed pre-commercial field testing of the world’s first pre5G massive MIMO base stations. Could you give us some more details?

A: Massive MIMO is one of the most important 5G technologies and can even be regarded as the only technology that offers multi-fold spectrum efficiency. UDN and high-frequency communication can only improve space utilization without greatly changing spectrum efficiency.
Massive MIMO has two main features. The first is to provide semi-dynamic coverage for broadcast channels, including CRS and PBCH, which is more reasonable and effective than static coverage by traditional antennas. The second is forming full-dynamic digital waveform for PDSCH to greatly increase cell capacity. Compared with an eight-antenna, the massive MIMO brings about a four-to-sixfold increase in capacity. This is very close to the final target for 5G spectrum efficiency improvement, an eight-to-tenfold increase in spectrum efficiency.
A traditional base station has a maximum of eight antennas, but a massive MIMO base station has 100 or more antennas. Therefore, the traditional 4G channel feedback system needs large overheads, and even reference signals can consume 80% of resources. To address this issue, 5G must greatly alter the 4G channel feedback system. In this way, it will become a long-term technology. Further research, however, shows that the symmetry of upstream and downstream channels in TDD mode can be used to estimate the downstream channel based on the upstream sounding channel. This removes overheads, even when the number of channels is more than 100. Since the codebook quantization error and feed-back latency are removed, the performance is much better.
In November 2014, ZTE cooperated with China Mobile to finish pre-commercial testing of the world’s first pre5G massive (3D) MIMO base stations. ZTE’s latest 64-port/128-antenna massive (3D) MIMO base stations were used. Mature 4G terminals were used and the base station contained everything: the antenna, the RF, and the baseband in one box. Such high integration benefited from ZTE’s high-performance vector processor SOC chipset.
In the tall building coverage test, the massive (3D) MIMO on the 35th floor supports 3.66 times the data throughput of an eight-antenna base station. In the indoor coverage test, with similar reference signal receiving power, the throughput of the massive (3D) MIMO is 1.52 times that of the eight-antenna base station. This indicates that PDSCH channels have high gain and ZTE’s MIMO algorithm is applicable to both line-of-sight and non-line-of-sight propagation scenarios.

Q: What achievements has ZTE made regarding other pre5G core technologies?

A: ZTE has a unique understanding of 4G and 5G and has made outstanding progress in most pre5G technologies.
UDN is a key 5G technology. In a 5G UDN, micro-micro interference is a major challenge that is different from macro-macro and macro-micro interference (Hetnet) in 4G. In principle, ZTE’s cloud radio for 4G macro networks can also be used in 5G.
For cloud radio in the 5G UDN, ZTE has proposed the virtual cell technology that enables a network to dynamically create UE IDs for specific users. From the user perspective, a virtual cell is equal to a virtual logical site that moves with users. Virtual cell can greatly improve service smoothness and performance.
MUSA is another 5G technology proposed by ZTE. Traditional telecom technologies adopt orthogonal codes to distinguish users. Different users are assigned with different time, subcarriers, or spaces to avoid interference. However, in MUSA, users are assigned with the same time, subcarrier, or space. Unlike 3G CDMA, MUSA may assign non-orthogonal codes to users for spectrum spreading only. So, how can we distinguish the users? The successive interference cancelation (SIC) technology helps in this case. For example, there are two users: local and remote. Traditionally, system capacity can only be maximized at the expense of the remote user. However, MUSA can guarantee maximum system capacity with reasonable balance between the two users. MUSA can offer performance improvement with reasonable complexity. In principle, MUSA does not need any synchronization, which helps prolong battery life. Hence, the upstream MUSA technology is quite applicable to machine-type communications (MTC).
ZTE has also invested a lot into developing 4G vector processor chipset. This chipset has flexible software extensibility and powerful processing capability and can meet pre5G or even 5G needs by simply adjusting instruction set without any change to hardware.

Q: What are true 5G technologies following the pre5G? How is ZTE progressing with them?

A: ZTE has invested a lot into both pre5G and 5G. The two technologies are inheritable. Almost each pre5G technology corresponds to a respective stage in 5G. Massive MIMO in the pre5G stage is based on TDD sounding channels that use a non-feedback mode. In the 5G stage, massive MIMO will also include a feedback mode that uses compressive sensing to minimize the overhead. Compressive sensing is to exploit channel sparsity in the time, frequency, and space domains to compress and restore information like RS and PMI.
The definition of air interfaces needs to be modified for high-frequency communication with different fading and multipath features. To adapt to large Doppler frequency shifts, wider sub-carriers must be used. Accordingly, the number of sub-carriers, symbols, and CPs must also be reduced.
ZTE has also invested a lot in the development and research of FBMC, FTN, multi-domain coding, MTC and D2D, and has gained a large number of achievements.