- Open Access
Design and experimental evaluation of a low-complexity spatial combiner for LTE distributed antenna systems
© Soler-Garrido et al.; licensee Springer. 2013
- Received: 29 September 2012
- Accepted: 18 February 2013
- Published: 14 March 2013
This article presents a distributed antenna system (DAS) architecture for small-cell base stations (BTSs), whereby cooperation between the DAS infrastructure and the BTS allows for an increase in performance compared to conventional systems, while at the same time keeping complexity and cost at low levels. Specifically, the article investigates the improvements in uplink physical layer performance achieved by adding an initial antenna combining step in the DAS system before conventional combining and equalization at the BTS. This initial step can be implemented in a very low-complexity fashion by performing all operations in the time domain and using channel state information calculated at the BTS itself. The article presents this technique in the context of an LTE DAS system. Results from both a software simulator and a custom-made hardware prototype are presented, establishing the feasibility of the proposed architecture.
- Channel Estimate
- User Equipment
- Maximal Ratio Combiner
- Resource Block
- Distribute Antenna System
In recent times, multiple antenna systems have been one of the main technological drivers for the evolution of wireless communications. In particular, in cellular systems multiple-input multiple-output (MIMO) techniques have successfully been used to increase cell throughput, link capacity, and/or quality [1–3]. Current network deployments typically resort to the use of distributed radio units (RUs) in order to improve the service provided. The so-called remote radio heads containing antennas and RF front-ends but very little processing power [4, 5] are strategically placed to optimize coverage. These are typically connected to a central unit or base station (BTS) via fibre–optical links, and can significantly increase system capacity and coverage . These distributed antenna systems (DASs) have been proposed as a cheaper alternative to deploying multiple BTSs, for example, in an indoor space to improve coverage [7, 8].
In conventional deployments, BTS and DAS are separate subsystems in the overall network. The DAS can be seen as a fixed infrastructure which routes the signal to and from a third party BTS. Its cost is typically related to the area that has to be covered. The BTS itself can have different capabilities which depend on the requirements, e.g., total capacity, of the particular deployment. In general, larger deployments in terms of area do not necessarily require a more powerful BTS, especially if the number of users, or their throughput requirements, are low.
Specifically, we present a two-stage coherent combining scheme where the two combiners serve different purposes and jointly exploit the gain offered by a potentially very large number of RUs. Furthermore, in order to keep complexity and bill of materials low on the DAS infrastructure side, we explore the possibility to perform the combining stage at the HU only in the spatial domain. Thereafter, the second stage at the BTS performs a conventional frequency selective spatial equalization. Moreover, the combining weight calculation at the HU is assisted by existing channel estimation capabilities at the BTS, reducing complexity further. The HU only performs a simple linear combining operation which does not require the provision of expensive FPGAs or digital signal processors.
Our two-stage DAS concept is applied to a 3GPP LTE Release 8  uplink communication setup as an example. The main deployment scenarios we envisage for our architecture are those typical for DAS systems, i.e., indoor office or residential building, and up to small urban cells, where coverage extension is sought without deploying expensive and heavy-maintenance BTS equipments but rather by using inexpensive DAS infrastructure.
The main contributions of this article are
We provide a description of a DAS for uplink LTE. A (potentially) large number of remote antennas are connected to an HU which processes the signals before passing them on to a BTS. Despite the reduced number of BTS antenna ports, the system is able to exploit the spatial gain provided by the multiple RUs.
As a further simplification, we present the calculation of space-only combiner weights, such that the amount of synchronization, and domain conversions required at the DAS hardware are kept to a minimum.
A hardware test-bed has been developed in order to prove the concept. Measurements using a hardware channel emulator allow for a direct comparison between different system architectures, and it is shown that the proposed DAS modifications offer a significant benefit over conventional systems.
The remainder of the article is organized as follows. A system overview of our DAS architecture and motivation for the choice of the two-stage combiner are given in Section 2. The problem of computing the combiner weights is addressed in Section 3 and simulation results are shown in Section 4. The hardware setup and measurement results are presented in Section 5, and conclusions are drawn in Section 6.
where γ k is the (instantaneous) SNR on the k th subcarrier and is the noise variance.
from the IDFT. Hence, the weights can be computed as a simple average directly from the frequency response hn,k without the need for IDFTs or eigenvalue decomposition.
In this section, a hardware prototype of the described uplink LTE DAS system is presented, along with some experiments aimed at quantifying, in practical scenarios and under realistic propagation conditions, the performance gains that can be attained by the HU processing described in previous section, and linking them with the simulation results provided.
5.1 Hardware prototype
The eNodeB is a picoChip PC9609 small-cell LTE development system, with a fully featured Rel. 8 PHY. This system contains two antenna ports and operates in 3GPP band 13, i.e., with uplink centred at 782 MHz and downlink at 751 MHz. The bandwidth employed is 10 MHz. In this case, as we are interested in measuring performance at the physical layer, no protocol stack is employed, using instead a simple PHY driver collecting low level measurements.
At the other end of the system lies an Aeroflex TM500 test UE, which again is operated in HARQ only mode, i.e., it establishes only low level communication with the eNodeB. Most of the settings are manually configured and remain static during the experiments, including uplink and downlink resource allocation, MCS values and transmitter power.
The weight calculation modules updates the weights every millisecond, but CSI for only one of the four branches is updated on every subframe period. In the prototype, this CSI for the antenna port used for training is sampled at the HU from the digital GPIO outputs at the eNodeB baseband chip. The communication is performed in serial mode, with a single coefficient being sent for each RB in 32-bit format (I and Q), followed by a single scaler applied at the BTS across the entire bandwidth.
In the evaluation setup, the BTS records values of postequalizer SNR. It is always using MRC on its two input ports, regardless of whether these come from two antennas or the HU. In both cases, the SNR is calculated by the BTS in a conventional manner by using the MIMO channel estimate, the MRC coefficients and the noise power estimate. The measured results are averaged, and captured at a rate of approximately 3 per second. The results presented compare the distribution of measured SNR for the cases of either a two fixed antenna case or a space-only MRC combiner at the HU. The channel emulator is programmed with the channel taps corresponding to scenario B1 NLOS of the Winner II model . Parameters such as transmit power and uplink MCS are selected in order to achieve a midrange average SNR. Variations on SNR are entirely due to random fading, which in this case is uncorrelated for all the antennas.
In this article, we propose a DAS architecture suitable for small-cell BTSs. It provides a low-complexity solution able to make better use of the spatial gain in the uplink of DAS while still employing low-cost BTS hardware. This is achieved by introducing an initial combining stage at the HU, where signals from different RUs are coherently combined. The article explores the performance of a space-only combiner where the uplink SNR is enhanced by applying a single set of weights across all frequencies at the HU. These results are backed by measurements from a hardware prototype making use of a precommercial Rel. 8 eNodeB, a custom-made RF and digital baseband combiner setup, a test LTE UE and a hardware channel emulator. The experimental results show clear performance gains for the combiner case even when the average received power is the same for all RUs. Moreover, the measurements are in line with simulated results, establishing the feasibility of the proposed architecture in practical real-life deployment in the presence of practical hardware impairments and realistic propagation conditions.
The authors would like to thank the directors at Toshiba’s Telecommunication Research Laboratory in Bristol for their constant support. They would also like to acknowledge the support of the Wireless Systems Laboratory at Toshiba’s Corporate R&D Center, and especially the assistance provided by Dr. Tsuguhide Aoki throughout the entire project. They would also like to acknowledge the work done by Yue Tian during his MSc internship at Toshiba TRL supporting this project.
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