|Issue:||Europe I 2015|
|Topic:||Millimeter wave capable small cells with modular antenna arrays|
for Next-Generation wireless systems
|Author:||Dr. Ali Sadri|
Dr. Ali Sadri is a Sr. Director of mmWave Standards and Advanced Technology at Intel Corporation. Sadri has more than 25 years of experience in academia and Industry. Beginning with visiting professor at Duke University and working at IBM in NC and later joining Intel Corporation in CA. His expertise is in Wireless Communications theory, channel modeling, power control, beamforming, and adaptive modulation techniques. He holds more than 100 issued and pending patent applications in communications and wireless technologies. After successfully leading the WiGig Alliance and creating the first commercialized 60 GHz wireless standards, Ali is extending the mmWave technology beyond WPAN. His current responsibilities include advanced technology development and prototyping of the future mmWave systems. Those systems include the next generation WiGig technology for access and backhaul as well as the next generation mmWave capable cellular systems (5G).
The use of mmWave frequency bands as part of the next generation cellular systems is gaining increased momentum in the industry and academia. The mmWave frequencies offer larger spectrum availability and much faster data rates, thus being considered as an important component in enabling high capacity small cells.
The projected and remarkable growth in data traffic led by the widespread availability and use of digital multimedia content is creating a significant challenge for future cellular networks. Availability of licensed spectrum, especially in lower bands, continues to be scarce and quite expensive worldwide. As such, when coupled with the expected, at least, ten-fold increase in the density of the worldwide connected mobile devices, a rethinking of today’s cellular network architecture is required in order to meet the future demand on network capacity, device density and, most importantly, improved user experience. The distribution of traffic demand is highly unbalanced, both in terms of number of users per cell and the areas served within the cells. If the demand is not adequately met, user experience across the multiple cells may significantly degrade. Therefore, mobile operators have been contemplating a combination of few approaches to serve the increased user demand including the deployment of denser cells, which allows the same capacity to be served to a smaller region or a fewer number of users. These cells are commonly known as ‘small cells’, since they have a much smaller cell coverage than the existing macro cells. With the deployment of small cells, mobile operators can rely on macro cells to offer mobility while small cells provide for targeted infill of capacity. Additionally, to meet the growing data traffic demand, mobile operators are likely to deploy small cells in much larger numbers than macro cells. Such large scale and dense deployment require that the technologies applied within the small cells provide their seamless integration with existing macro cell architecture without causing undue interference to other cells. A promising approach is the use of millimeter wave (mmWave) frequency bands (30 GHz – 300 GHz) for small cells, which is expected to provide the necessary scalability, capacity and density required for a seamless integration of these cells into the cellular network infrastructure. The mmWave frequency bands offer much larger spectrum availability than the lower bands, increased network capacity, and greater potential for network densification while reducing the need for complicated high spectral efficiency modulation techniques. These benefits may come at the expense of potentially greater system complexity in terms of radio frequency (RF) front end and antenna design. However, recent advances on product development for the unlicensed 60 GHz band, such as WiGig/IEEE 802.11ad, have proven cost effective and low power solutions that can be leveraged to overcome these challenges. This article introduces the novel concept of mmWave capable small cells (MCSC) using modular antenna arrays (MAAs) architecture as a key ingredient of the next generation 5G cellular system.
Millimeter wave RF and antenna
The major technical challenge at the physical layer for the implementation of MCSCs is the availability of a cost effective, low power and small form factor RF and antenna solution. A viable MCSC requires an adaptive antenna with high directivity. This is in contrast with modern communication systems, which require a station to be capable of covering a relatively wide sector around it to communicate with other stations regardless of their locations. Traditional antenna architectures used in mmWave band are, generally, not capable of combining wide angle coverage with high directivity. Existing reflective, parabolic dishes and lens antennas can create narrow beam, thus delivering the needed 30-40 dB antenna gain, but they lack the flexibility to cover wide angle coverage and are relatively bulky. Phased patch antenna arrays allows steering the beam to a desired direction. However, to achieve the necessary directivity, the array must consist of a large number of elements (several hundred to thousands). Antenna array architectures currently used for mass production and intended for personal devices employ a single module; containing an RFIC chip that includes controlled analogue phase shifters capable of providing several discrete phase shifting levels. The antenna elements are connected to the RFIC chip via feed lines. However, due to the loss inherent in the feed lines, this approach reduces antenna gain and efficiency, and becomes a severe problem when the number of antenna elements and radio frequency increase as in MCSCs. To overcome this limitation, we propose a novel MAA architecture for MCSCs that provides flexibility in form factor choice, beam steering capability and array gain that is adaptive and cost effective. The architecture is shown in Figure 1 and is essentially a type of massive multiple-input multiple-output (MIMO) system. However, instead of utilizing a single antenna module, MAA is constructed by utilizing a modular, composite mmWave antenna arrays. Each module is implemented with a dedicated RFIC chip serving several antenna elements and an RF beamforming (RF-BF) unit with discrete phase shifters. Given its modularity, the length of the feed lines in the MAA architecture can be kept much shorter and confined within a single module. Hence incurring much lower feed line loss compared to traditional approach for mmWave massive MIMO systems.
Figure 1 – High level block diagram of the proposed large antenna array (left) and example of layout for the case of planar sub-array modules (right)
MIMO access using MAA
There are several challenges that affect applicability of traditional MIMO in the mmWave band. Traditional MIMO implementations in low frequency bands assume each antenna element can make use of its own transceiver RF chain. In practice, this may be too complex and challenging for mmWave systems due to the sheer number of elements involved in any degree of antenna array. Furthermore, such tightly located components will be prone to cross-coupling which affect the MIMO system performance. In that case, it is possible to use the MAA architecture as depicted in Figure , where each phased array module (sub-array) has a dedicated transceiver with coarse RF beamforming unit and, therefore, may create a dedicated RF beam. Furthermore, beams of individual sub-arrays may be steered in various directions to achieve flexible behaviour of the system.
Figure 2 – Implementing MIMO with MAA
In the case of Figure 2, each antenna sub-array module may be seen as a single antenna port in the context of a MIMO system. The beamforming procedure of the entire antenna system may be provided in hybrid manner by the coarse phasing of each antenna element within a module and fine signal weighting in a MIMO Base Band (BB) processor. For example, at the transmitter side, fine beamforming in BB may be used to create spatially orthogonal signals for different antenna sub-arrays for both single user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) modes. In addition, different MIMO schemes may be implemented in the receiver, such as maximum ratio combining and inter-stream interference cancellation.
Multiple accesses using MAA
The existing cellular systems adopt Orthogonal Frequency Division Multiple Access (OFDMA) as the DL access scheme and Single Carrier (SC) FDMA as the uplink access scheme. Consequently, each User Equipment (UE) is allocated part of the frequency band for each individual resource block. In mmWave, the coarse analog beamforming may be applied in RF, which implies that the entire frequency band will be beamformed in one or several directions at one time providing coverage for the number of simultaneous UE within the same coverage area. Figure 3 illustrates an example of a multiple access in an MCSC system when the MAA architecture is used by the small cell. Depending on factors such as the number of antenna modules at the small cell, geographical location of the UEs, channel propagation, etc., small cell may cover more than one UE at once. Therefore, in addition to providing a multiple access scheme that services UEs covered by different beams, the multiple access mechanism used in an MCSC is also required to support multiple accesses at the beam coverage level.
Figure 3 – Multiple access in an MCSC
The use of mmWave frequency bands as part of the next generation cellular systems is gaining increased momentum in the industry and academia. The mmWave frequencies offer larger spectrum availability and much faster data rates, thus being considered as an important component in enabling high capacity small cells. We presented viable system architecture of mmWave bands through the introduction of MCSCs with MAAs. This practical architecture can significantly increase capacity and density for 5G cellular systems. Research on the use of mmWave bands for cellular communication is at its early stages. Consequently other system architectures should also be explored to find the most viable architecture for the next generation cellular systems.