Wan Choi

What Will 5G Be

What will 5G be? What it will not be is an incremental advance on 4G. The previous four generations of cellular technology have each been a major paradigm shift that has broken backward compatibility. Indeed, 5G will need to be a paradigm shift that includes very high carrier frequencies with massive bandwidths, extreme base station and device densities, and unprecedented numbers of antennas. However, unlike the previous four generations, it will also be highly integrative: tying any new 5G air interface and spectrum together with LTE and WiFi to provide universal high-rate coverage and a seamless user experience. To support this, the core network will also have to reach unprecedented levels of flexibility and intelligence, spectrum regulation will need to be rethought and improved, and energy and cost efficiencies will become even more critical considerations. This paper discusses all of these topics, identifying key challenges for future research and preliminary 5G standardization activities, while providing a comprehensive overview of the current literature, and in particular of the papers appearing in this special issue.

Overcoming interference in spatial multiplexing MIMO cellular networks

Multi-antenna transmission and reception (known as MIMO) is widely touted as the key technology for enabling wireless broadband services, whose widespread success will require 10 times higher spectral efficiency than current cellular systems, at 10 times lower cost per bit. Spectrally efficient, inexpensive cellular systems are by definition densely populated and interference-limited. But spatial multiplexing MIMO systems- whose principal merit is a supposed dramatic increase in spectral efficiency- lose much of their effectiveness in high levels of interference. This article overviews several approaches to handling interference in multicell MIMO systems. The discussion is applicable to any multi-antenna cellular network, including 802.16e/WiMAX, 3GPP (HSDPA and 3GPP LTE), and 3GPP2 (lxEVDO). We argue that many of the traditional interference management techniques have limited usefulness (or are even counterproductive) when viewed in concert with MIMO. The problem of interference in MIMO systems is too large in scope to be handled with a single technique: in practice a combination of complementary countermeasures will be needed. We overview emerging system-level interference-reducing strategies based on cooperation, which will be important for overcoming interference in future spatial multiplexing cellular systems.

Opportunistic Space-Division Multiple Access With Beam Selection

In this paper, a novel transmission technique for the multiple-input multiple-output (MIMO) broadcast channel is proposed that allows simultaneous transmission to multiple users under a limited feedback requirement. During a training phase, the base station modulates a training sequence on multiple sets of randomly generated orthogonal beamforming vectors. Then, based on the users feedback, the base station opportunistically selects the users and corresponding orthogonal vectors that maximize the sum capacity. From theoretical analysis, the optimal amount of training to maximize the sum capacity is derived as a function of the system parameters. The main advantage of the proposed system is that it provides throughput gains for the MIMO broadcast channel with a small feedback overhead, and provides these gains even with a small number of active users. Numerical simulations show that a 20% gain in sum capacity is achieved (for a small number of users) over conventional opportunistic space division multiple access, and a 100% gain (for a large number of users) over conventional opportunistic beamforming.

The Capacity Gain from Base Station Cooperative Scheduling in a MIMO DPC Cellular System

As an alternative to traditional static frequency reuse patterns, this paper investigates cooperatively scheduling among neighboring base stations in a cellular multiple antenna system, where each cell adopts dirty paper coding. It is shown that cooperatively scheduled transmission can achieve almost the same amount of interference reduction as conventional frequency reuse and achieve an extra capacity gain. We analytically quantify the capacity gain of cooperatively scheduled transmission over conventional frequency reuse in an Mt times Mr dirty paper coded MIMO system. The theoretical analysis of this paper also provides an altered view of multiuser diversity in the context of a multi-cell system. Because the positions of the users are important in a multi-cell system, we find that the gain is O(radiclog K), from selecting the maximum of a compound lognormal-exponential distribution, whereas multiuser diversity capacity gain has been previously known to grow as O(log log K), from selecting the maximum of K exponentially-distributed powers

Multiuser Antenna Partitioning for Cellular MIMO–CDMA Systems

Improving the capacity of code-division-multiple-access (CDMA) systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multiantenna technologies called multiple-input multiple-output (MIMO) are an obvious candidate to increase, particularly, downlink capacity. Nearly all research on MIMO-CDMA, however, has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still the most important design goal in QoS-constrained voice systems. In this paper, we consider the downlink of a heavily loaded multicell CDMA system with multiple transmit and receive antennas. Straightforward application of the known MIMO techniques to such a system does not substantially increase the number of supportable users. To overcome this, a novel MIMO-CDMA system design based on user partitioning is developed, in which each user is assigned to a single transmit antenna either without regard to channel knowledge (static) or based on antenna-selection feedback bits (dynamic). These proposed multiuser-antenna-partitioning techniques have a minimal increase in complexity and would require only small changes to existing CDMA standards. The outage probability and capacity of the proposed systems are derived, and it is shown that, particularly, the dynamic partitioning scheme has a large gain over both the conventional CDMA and the static MIMO-CDMA scheme. This gain can be credited to multiuser antenna selection diversity. Unlike prior research, the multiuser antenna selection diversity gain is achieved without the typical expense of a loss in spatial multiplexing gain.Improving the capacity of code-division-multiple-access (CDMA) systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multiantenna technologies called multiple-input multiple-output (MIMO) are an obvious candidate to increase, particularly, downlink capacity. Nearly all research on MIMO-CDMA, however, has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still the most important design goal in QoS-constrained voice systems. In this paper, we consider the downlink of a heavily loaded multicell CDMA system with multiple transmit and receive antennas. Straightforward application of the known MIMO techniques to such a system does not substantially increase the number of supportable users. To overcome this, a novel MIMO-CDMA system design based on user partitioning is developed, in which each user is assigned to a single transmit antenna either without regard to channel knowledge (static) or based on antenna-selection feedback bits (dynamic). These proposed multiuser-antenna-partitioning techniques have a minimal increase in complexity and would require only small changes to existing CDMA standards. The outage probability and capacity of the proposed systems are derived, and it is shown that, particularly, the dynamic partitioning scheme has a large gain over both the conventional CDMA and the static MIMO-CDMA scheme. This gain can be credited to multiuser antenna selection diversity. Unlike prior research, the multiuser antenna selection diversity gain is achieved without the typical expense of a loss in spatial multiplexing gain.

On spatial multiplexing in cellular MIMO-CDMA systems with linear receivers

In this paper, the effectiveness of spatial multiplexing in the forward link of cellular CDMA systems with linear (i.e. low complexity) receivers is investigated. General MIMO systems without spreading are a special case of our analysis when the spreading gain is unity. Through the development of new closed-form results on outage probability and capacity for MIMO-CDMA, we show that cellular MIMO outage capacity is severely degraded by the enhancement of other-cell interference by the linear spatial receiver, and that a large number of transmit and receive antennas is required to simply break even with a SISO system. The results indicate that for practical cellular MIMO systems, which will be interference-limited and have low complexity receivers, future research is required on feasible methods for reducing the impact of other-cell interference. The framework presented in this paper can be used for future analysis of multicell MIMO systems.

Throughput of the 1x EV-DO system with various scheduling algorithms

The average sector throughput of the popular 1x EV-DO system with three different scheduling algorithms is mathematically analyzed for a multicell system with lognormal shadowing. The scheduling algorithms considered are round robin, equal latency, and relative fairness. The trade off between throughput and latency among the three scheduling algorithms is illuminated by the analysis, since the key design parameters are identified. From numerical examples, it is shown that the average throughput in a typical urban area is around 600-800 kbit/s. The results of this paper will help engineers do cellular planning and reliably predict throughput for 1x EV-DO and other data-based cellular systems.

Outage probability for maximal ratio combining receivers in asynchronous CDMA channels

Direct sequence code division multiple access (DS-CDMA) receivers typically use maximal ratio combining (MRC) to favorably combine the energies of distinct multipath components from diversity branches. There have been a large number of studies on DS-CDMA and MRC receivers, respectively, but there have been a comparatively small number of papers analyzing MRC for DS-CDMA. Prior work does not accurately analyze the outage probability using the actual random characteristics of the cross correlations, instead, equality has been assumed. The contribution of this paper is to provide a general and accurate analysis of the outage probability of the diversity receiver in CDMA systems without compromising approximations. From the analytical and numerical results, it is shown that the analysis provides an accurate closed-form solution.

Generalized performance analysis of a delay diversity receiver in asynchronous CDMA channels

In this letter, the performance for the delay diversity receiver is analyzed in asynchronous code division multiple access (CDMA) channels. The outage probability and the bit error probability of the delay diversity receiver are accurately derived and compared with those of the conventional diversity receiver. From the analytical and numerical results, it is confirmed that the delay diversity receiver achieves a remarkable diversity gain with reasonable cost and complexity in asynchronous CDMA channels. Specifically, for roughly the same hardware complexity, the delay diversity receiver achieves nearly twice the diversity order of the conventional receiver.

Improved bit error probability analysis for maximal ratio combining in asynchronous CDMA channels

The paper provides a general framework for accurately analyzing the bit error probability of the maximal ratio combining (MRC) diversity receiver in CDMA systems. In previous research on MRC receivers for direct sequence code division multiple access (DS-CDMA), compromising assumptions have been made to simplify the analysis, including a Gaussian approximation for interference or constant equal cross correlations. These assumptions can produce inaccurate results, especially when the number of users is small or when the diversity order is high, as will increasingly be the case in high-rate CDMA systems. We derive a novel closed-form bit error probability expression without these assumptions and verify through simulations that this new framework is highly accurate.