With the ever-growing popularity of smartphones, new services are emerging where the local and positioning aspects becomes more important. Additionally, new types of traffic, such as vehicle-to-vehicle communications are being developed. In order to address these new trends, release-12 of LTE has started to evaluate, and possibly standardize, solutions for device-to-device (D2D) communication. This provides the foundations for the deployment of a D2D overlay. However, this should be viewed as a first step, and future LTE releases should encompass more advanced D2D communication techniques and architectures. In this paper, after reviewing the standardization progress for LTE in release-12, several possible improvements are examined for future releases both D2D discovery and communication. It is shown that the future D2D architecture should exploit the network when possible in order to provide an improved user experience and satisfy the need for more local services.

Long-Term Evolution (LTE) will provide substantial enhancements to UMTS 3G systems including improved system capacity and coverage, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. LTE supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes to provide deployment flexibility in accordance with operator’s preference and spectrum allocation. This paper presents an overview of LTE TDD design and highlights key differences with FDD. Design challenges unique to TDD are presented together with adopted technical solutions. Finally, simulation results are provided to demonstrate typical TDD system performance with data applications.

This paper discusses circular buffer rate matching (CBRM) algorithms for the turbo code in the long term evolution (LTE) of the WCDMA-based air interface. To enhance performance at high code rates, systematic bit puncturing is incorporated in conjunction with the CBRM. The RM algorithm is further optimized based on the algebraic properties of the QPP interleavers and the 8-state recursive systematic convolutional code of the LTE turbo code.

This paper describes a scheduling technique for the DFT-SOFDM (SC-FDMA) based uplink of the 3GPP LTE standard. Traditionally, scheduling has been either Frequency- Selective (FS), or Frequency Non Selective (FNS). FS scheduling assigns a user to a portion of the channel bandwidth having high channel-quality. At low mobility, FS typically performs much better than FNS, but at the cost of increased channel-quality feedback overhead. A new class of scheduling methods, called Frequency Semi-Selective (FSS) is introduced, and is shown to perform nearly as well as pure FS scheduling, but with reduced feedback overhead. The technique only requires minimum tracking of the channel and one-bit of additional feedback over FNS techniques.

With the emergence of packet-based wireless broadband systems, work has begun on long term evolution (LTE) of the UMTS Terrestrial Radio Access and Radio Access Network aimed for commercial deployment in 2010. Goals for the evolved system include support for improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. To reach these goals, a new design for the air interface including control channel is envisioned. This paper provides a preliminary look at an efficient downlink control channel design to reduce the overhead required to support data transmission. Initial performance results show that close to optimal system performance can be achieved with downlink control overhead of less than 14%.

The Long Term Evolution (LTE) of the WCDMA standard requires turbo decoding throughputs of up to 100 Mbps. Unfortunately, the existing WCDMA turbo interleaver does not permit an efficient high throughput implementation. The almost regular permutation (ARP) and quadratic polynomial permutation (QPP) interleavers were proposed to rectify this situation with QPP selected for LTE. In this paper, the interleavers are compared and a full suite of LTE designs and performance results for both are presented.

This paper presents a low-complexity interleaver design that facilitates the high throughput turbo decoding required for next generation wireless systems. Specifically, it addresses the interleaver design issues that arise when several Log-MAP processors are used in parallel to improve turbo decoding throughput. In such a parallel decoder, memory access contentions occur when more than one extrinsic value is to be written to or read from the same memory block at the same time. These contentions may be avoided by designing contention- free (CF) interleavers that incorporate hardware constraints into the interleaver description. The paper first derives bounds on the number of CF interleavers, demonstrating that the fraction of interleavers of a given size that are contention-free is quite small. In spite of this, a class of contention-free "inter-window shuffle" (IWS) interleavers are shown via simulation to achieve near-WCDMA performance. Further, the paper shows that the memory requirement of CF IWS interleavers is small compared to an alternate contention-resolving method that uses a modified memory addressing scheme. Finally, we note that the advantages of contention-free interleavers have led to the adoption of a CF quadratic permutation polynomial (QPP) interleaver in the 3 GPP long term evolution (LTE) standard.

Long term evolution (LTE) of the UMTS Terrestrial Radio Access and Radio Access Network is aimed for commercial deployment in 2010. Goals for the evolved system include support for improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. To reach these goals, a new design for the air interface is currently being specified in the 3GPP standards body. The Uplink (UL) for LTE is based on Single Carrier Frequency Division Multiple Access. The UL control channel carries non-data associated control signaling like CQI, ACK/NACK, Scheduling request etc. To maintain the low PA power de-rating, the single carrier property of the UL has to be maintained. As such, special consideration should be given to the UL control channel design. This paper discusses in detail the LTE UL control channel design and its performance.

This work describes a family of low feedback, high performance link adaptation algorithms for multi-carrier systems. The methods include the use of the exponential effective SNR mapping (EESM) method as part of the modulation/coding selection process. The drawbacks of current link adaptation algorithms are reviewed, and means of combating the effect of Doppler and fading gain variation are proposed. In addition, a method enabling conventional SNR feedback reports to be converted to the effective SNR domain is described. This allows the effect of variable power allocation on coded BER to be more accurately estimated. Simulation results show that the proposed methods are effective.

This paper presents a high-speed "turbo-like" decoding algorithm for certain structured LDPC codes such as those adopted in IEEE 802.16e and in the draft 802.11n standards. It is shown that after a key modification, such LDPC codes may be processed as generalized repeat accumulate codes, codes which are known to support "turbo-like" decoding. A GRA-like encoder of structured LDPC codes is derived, which in turn leads to the decoding algorithm. It is also shown that the "structured" properties result in an inherent parallelism, leading to an efficient high speed decoder implementation

With the emergence of packet-based wireless broadband systems such as 802.16e, it is evident that a comprehensive evolution of the universal mobile telecommunications system specifications is required to remain competitive. As a result, work has begun on long term evolution (LTE) of the UMTS terrestrial radio access and radio access network aimed for commercial deployment in 2010. Goals for the evolved system include support for improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. To reach these goals, a new design for the air interface is envisioned. This paper provides a preliminary look at the air interface for Evolved UTRA (E-UTRA) and associated key technologies required to reach its design objectives. Initial E-UTRA system performance results show a 2 to 3x improvement over a reference Rel-6 UMTS system configuration [1, 2] for both uplink and downlink.

Multi-user scheduling and OFDM techniques are well regarded as promising candidates to provide high data- rate and reliable transmissions in future communication systems. To optimally exploit multi-user diversity and frequency selectivity, exhaustive channel information is needed for every user - an impracticality given limited feedback bandwidth. Simple and efficient feedback schemes therefore become a crucial task in designing a next generation standard. In this paper, two near-optimal feedback schemes are proposed that achieve most of the multi-user scheduling benefit with very few feedback bits. It is also shown that the feedback schemes are robust to long feedback intervals. As expected, at higher Doppler spread, the performance gap using the proposed schemes becomes larger but still provides satisfactory performance.

OFDM system-level simulations require a performance estimate for each link, but it is typically infeasible to conduct simultaneous link-level simulations for multiple users within a reasonable amount of time. Therefore, it is critical to have a simple link error probability predictor that accurately models coded OFDM performance. The exponential ESM (EESM) method was shown to be a very good link error prediction method for multi-carrier systems in the case of single transmission. Here, the EESM method is extended to cover hybrid ARQ (HARQ) techniques: Chase combining, incremental redundancy (IR) or an arbitrary combination of both. Further simplifications are introduced to greatly reduce the amount of memory necessary to utilize EESM for modeling OFDM-HARQ transmissions

Interleavers that avoid memory contentions in parallelized log-MAP decoding are analyzed and designed. Bounds are derived demonstrating that the fraction of interleavers that are contention-free is small. Nevertheless, contention-free "inter-window shuffle" interleavers with a simple implementation and reasonable memory requirements are shown to surpass 3GPP performance.

This paper considers techniques to increase the performance of the broadband wireless OFDM system through adaptation in both the time and frequency domains. System throughput estimates are provided for frequency non-selective coding and frequency selective coding for single antenna, Alamouti, and selective closed-loop MIMO configurations. Additional gains from performing multi-user frequency-domain scheduling are achieved and determined using proportional fair and maximum C/I schedulers. The performance gains of multi-dimensional adaptation and scheduling techniques are contrasted with the amount of feedback required. Measured data from a 20 MHz experimental system and turbo coding simulations are used to estimate the size of each feedback report, and an accepted user speed distribution is used to estimate an appropriate feedback rate.

The introduction of adaptive multi-rate (AMR) speech coders in wireless standards, such as GSM, imposes new challenges for forward error correction (FEC) design. Traditional FEC techniques of providing unequal error protection (UEP) may not maximize performance for a given set of design requirements, such as complexity limits. This paper introduces a new FEC technique that improves error correction performance while satisfying complexity constraints. This technique is applied to the new wideband AMR standard for a performance and complexity comparison.

An overview of the hybrid ARQ used by the 3G CDMA evolutions (1xEV-DV for cdma2000 and HSDPA for WCDMA) is provided, along with simulation data supporting several of the key design decisions. Enhancements to AMC and hybrid ARQ operation are also discussed.

In this paper, we investigate the performance of a quasi-static method used to predict link-level error performance in dynamic system simulations. Two flavors of the quasi-static approach are used to model the link characteristics: (a) static curves with fudge factors; and (b) short-term frame error curves. For the fudge factor based method, we examine the various factors (or penalties) required for the method to work properly, their limitations in capturing the various features of the high-speed packet data channel, and their effects on the overall accuracy of the technique. Here, we concentrate on three important penalties: puncturing penalty, Doppler penalty, and demapping penalty. The quasistatic method based on short-term frame error curves is used to generate frame erasures for voice and control channels associated with high-speed packet data channels. From simulation results, it is shown that both variations of the quasi-static method can provide accurate prediction of link-level performance at certain operating ranges, provided that the penalties are correctly obtained. At low frame error rates and moderate to high speeds, however, the quasi-static method often fails to accurately reflect frame error performance at the link level.