Cellular wireless systems have focused, to date, on system capacity, user data rates, latency and coverage as primary design criteria. Energy efficiency has recently received increased attention as another important criterion to be considered. The potential for energy savings in 3GPP LTE systems has been investigated in [1]-[3]. An important design element that determines the potential for energy efficient transmission is the structure of reference signals that need to be transmitted independent of the load in the network, such as the cell-specific reference signals (CRS) in 3GPP LTE systems. This paper discusses modifications to the reference signal structure in LTE that can enable greater energy efficiency. The time and frequency estimation performance implications of such modifications are evaluated and discussed.

Multi-stage interference cancellation (IC) can improve the interference conditions for all users of a cell. In order to exploit the IC gain at system level, it is important to design radio resource management algorithms that account for this gain. For this purpose, two new load control algorithms are proposed in this paper, together with some further enhancements. For Algorithm 1, load control and scheduling are still performed based on the air interface Rise over Thermal (RoT), but with an increased RoT target. The RoT target is determined based on the estimated IC gain. A cell-specific and dynamic RoT target is hence used rather than a constant one. With Algorithm 2, load control and scheduling are performed based on an effective RoT without increasing the RoT target. The load control function is placed after an intermediate IC stage depending on the timing constraints and/or the required IC processing gain of the coverage limiting channels. With the proposed algorithms, system-level simulation results show that the average cell throughput can be considerably improved and at the same time the coverage can be maintained at a level similar to a system without IC.

Uplink co-ordinated multipoint reception (UL CoMP) has received attention recently, especially for LTE Advanced, due to its promise of increasing spectral efficiency. In this paper, UL CoMP for HSPA networks is considered. UL CoMP for HSPA may be deployed without changes to the standard, and may be applied also for legacy UEs. Comparisons and contrasts of CoMP with conventional soft/soft handover are discussed as well as practical receiver structures incorporating antenna selection within the pool of antennas available in a CoMP cell. Radio resource management issues are discussed including mobility, power control, load control, and scheduling. It is concluded that UL CoMP requires only minimal changes to these algorithms. In a practical scenario, gains in cell-edge throughput of at least 118% and 144% are demonstrated for CoMP-3 and CoMP-6, respectively, for the case of full buffer traffic. The gains are even larger for the case of short files uploads where the conventional network suffers from the inability to exploit soft/softer handover due to latencies involved in updating a UE's active set.

In this paper, we study how the capacity and coverage of voice over internet protocol (VoIP) in the WCDMA uplink can be improved by using advanced base station receivers capable of multi-user detection (MUD). To avoid the complexity of joint-detection, a multi-stage successive interference cancellation (SIC) detector or parallel interference cancellation (PIC) detector is used as an implementation of MUD. To further reduce complexity, time-division multiplexing (TDM) is used to separate user groups in time so that MUD can focus on a smaller number of overlapping signals. SIC or PIC cancels a number of temporarily detected user signals from the receive signal before detecting another user signal. A temporarily detected user signal can be regenerated based on demodulated data or decoded data, and based on hard-decision values or soft-decision values. We find that a two-stage SIC or PIC process improves VoIP performance significantly and having more detection stages beyond two does not further improve performance by much. The improved performance allows a VoIP user to reduce the average transmit power, which translates to VoIP capacity and coverage increases. Furthermore, we find that the individual user performance is not sensitive to its detection order. We also quantify the relative performance between the various signal regeneration schemes considered. As expected, post-decoding interference cancellation (IC) offers a significant gain over pre-decoding IC. Furthermore, we find that post-decoding IC is robust to traffic load variations. Also, soft-decision based cancellation achieved the best performance. However, the performance improvement is small compared to hard-decision based cancellation. Finally, we see that MUD can work with G-Rake to suppress one dominant other-cell interfering signal, e.g., due to a high-data-rate user in another cell.

MIMO and higher-order modulation (64-QAM) have recently been introduced in the HSPA downlink (HSDPA) to extend peak data rates. Both techniques demand a high SINR operating point in order to be effective. While traditional macrocell (outdoor) deployments may provide limited high-rate coverage, indoor environments are more likely scenarios for achieving high rates due to higher SINR operating points. This paper evaluates performance for an indoor pico-cell system using a selective-PARC (S-PARC) MIMO scheme in combination with 64-QAM. A typical multi-story office building is considered with pico-cells located on each floor. Within each pico-cell both a single antenna cluster and distributed antenna clusters are considered. Results from system simulations show very large gains in both user and system throughput compared to a receive diversity only system. 90th percentile user throughput over 40 Mbps and system throughput of 30 Mbps are achieved.

Recently a MIMO-HSPA testbed based on the commercial release '99 products has been within Ericsson in order to demonstrate and evaluate enhanced data rates in the downlink for the evolution of HSPA. The testbed employs two antennas at both the base and mobile stations (2 times 2 MIMO) and is able to adapt between single- and dual-stream data transmission. An extensive outdoor measurement campaign has been carried out using dual-polarized antennas at both ends of the link. With such set-up, it is demonstrated that 2 times 2 MIMO double the peak rate compared to conventional HSDPA in areas of the cell experiencing line-of-sight (LOS) conditions with large SNR. More generally, the cell-wide result demonstrate gains in the 90th percentile Layer-1 throughput of 102% and 36% compared to conventional HSPDA with single- and dual-antenna receivers, respectively. Significant gains in the average throughput are also demonstrated.

In this paper we present the main concepts of high speed packet access evolution currently being standardized in 3GPP. In general HSPA evolution consists of introduction of MIMO, higher order modulation, and protocol optimizations and optimizations for voice over IP. We describe these improvements in detail and show that HSPA Evolution can reach performance comparable to those of long term evolution of UMTS terrestrial radio access network in a 5 MHz deployment.

Multiuser detection (MUD) of signals occupying the same time/frequency slot has been shown to provide significant increases in the capacity of a single cell or coverage area. In a cellular system, however, increasing the number of users per cell also produces increased interference from other cells. Compensation by increasing the co-channel cell spacing partially offsets the capacity gain due to MUD, and the net effect on system capacity then rests on the detection method, such as maximum likelihood (ML) or minimum mean-squared error (MMSE). This paper is the first to analyze the net capacity gains due to ML-MUD in time division multiple access (TDMA), and the first to compare ML-MUD with MMSE-MUD on this basis. We demonstrate that ML-MUD with M cochannel users per cell and four base station antennas gives a net system capacity that is M times greater than that of single-user TDMA system with two antennas. In contrast, we show that MMSE-MUD gives only a modest net capacity increase, and that the system capacity may actually decrease with more than one or two users per cell, making it poorer than the original single-user system.

MISO antenna systems employing feedback of the downlink channel state are attractive for achieving diversity in the downlink of cellular systems. In this paper we consider a particular MISO transmit diversity scheme with rich feedback (TDRF), and derive the optimal receiver structure taking into account intersymbol interference, colored noise, and the self-interference from pilot signals used for downlink channel estimation. We then compare the bound on performance established by the optimal receiver with the performance of a low complexity alternative consisting of a fixed receive filter followed by a sampler. Our results show that the application of TDRF to the WCDMA downlink gives substantial gains compared to a single antenna system, even when the extremely simple sampler receiver is used at the mobile terminal.

In this paper, a simple structure is proposed for accommodating reuse-within-cell in the downlink of a time-division multiple-access system using a transmit antenna array at the base station and joint detection of the cochannel signals at the single-antenna mobile receivers. The transmit array is used solely for system capacity increase; diversity is obtained in the traditional way through standard coding and interleaving. Due to the spatial formatting of the coded symbols prior to transmission, a single multiantenna channel usage spans several trellis transitions - an unconventional situation which is handled through a "merged trellis". Using the merged trellis, a combined joint detector/soft-decision decoder is constructed, and an analytical expression is derived for the coded bit-error rate based on the exact computation of pairwise error probabilities. It is shown that while the system suffers a reduction in diversity order as the number of intracell cochannel users increases, the reduction may be countered by increasing the constraint length of the code. Although this increases complexity, it is shown that multiple users may be supported, all enjoying good performance at low signal-to-noise ratios, while keeping the complexity in check.

In this paper, a simple structure is proposed for supporting multiple intracell users in the same time/frequency slot in the downlink using a transmit antenna array at the base station and joint detection at the single-antenna mobile receivers. To support multiple users, the system requires the simultaneous transmission of several successive code symbols from different diversity antennas. As a consequence, a single multi-antenna channel usage spans several trellis transitions - an unconventional situation which is handled with a novel merged trellis. Using the merged trellis, the optimal decoder is identified, and an analytical expression for the average bit-error rate is developed, based on soft-decision joint decoding at the mobiles. Unusual behaviour is demonstrated in terms of diversity order: as the number of transmit antennas increases due to an increasing number of users, the diversity order actually decreases due to the simultaneous transmission of successive code symbols. Even with the loss in diversity order, the method provides a major increase in downlink capacity while maintaining good performance for all users at low signal-to-noise ratios with moderate computational load.

Estimation of the channel impulse responses of multiple cochannel users is a key requirement of all multiuser detection and interference cancellation techniques, though little attention has been paid to the subject in the context of time-division multiple-access (TDMA) systems. This paper addresses a pilot-based MMSE technique for multiuser channel estimation in a TDMA system, and makes two key contributions. Firstly, it allows for time variation of the channels within and between training sequences, an essential feature in a multiuser environment even at moderate fading rates. Secondly, it addresses the design of training sequences for the multiple cochannel users.

We derive a fully analytical expression for the union bound on symbol-error rate for the joint detection of several cochannel fading signals using a diversity antenna array, putting numerical results previously published on a firm analytical base. We prove that with pilot-based multiuser channel estimation, any number of users can enjoy diversity order equal to the number of antennas, and we quantify the performance penalty relative to single-user binary phase-shift keying as a function of number of users, constellation density, and number of antennas. We also demonstrate the interdependence of all users participating in the detection process.

Estimation of the channel impulse responses of multiple cochannel users is a key requirement of all multiuser detection and interference cancellation techniques, though little attention has been paid to subject in the context of TDMA systems. This paper addresses a pilot-based technique for multiuser channel estimation in a TDMA system, and makes several new contributions: it allows for time variation of the channels within and between training sequences, it accounts for colouration of the sampled noise sequence as well as correlation between the channel taps, and it considers users to be asynchronous resulting in a technique whereby explicit timing recovery is unnecessary. In addition, it addresses selection of appropriate training sequences.

Joint detection based on exploiting differences among the channels employed by several users allows a receiver to distinguish cochannel signals without reliance on spectrum spreading. This paper makes a number of new contributions to the topic; it provides an analytical expression for the union bound on the average symbol-error rate for an arbitrary number of users and diversity antennas in a fading environment, for both perfect and imperfect channel state information (CSI), and it compares the performance of joint detection with diversity antennas against classical minimum-mean-square-error (MMSE) combining. The performance is remarkable. With accurate CSI, several users can experience good performance with only a single antenna; moreover, for perfect CSI, only a 2-dB penalty is incurred for each additional user. With several antennas, many more users than the number of antennas may be supported with a slow degradation in performance for each additional user. Furthermore, high accuracy is not required from the channel estimation process. In all cases, the performance of joint detection exceeds that of MMSE combining by orders of magnitude.