Brian K. Classon

Performance characteristics of cellular systems with different link adaptation strategies

In this paper, the theoretical performance of cellular systems with different types of link adaptation is analyzed. A general link and system performance analysis framework is developed to enable the system-level performance characteristics of the various link adaptation strategies to be studied and compared. More specifically, this framework is used to compare the downlink performance of fully loaded cellular systems with fixed power and modulation/coding, adaptive modulation/coding (AMC), adaptive power allocation (APA) with system-level AMC, and water-filling (WF). Performance is studied first for idealized methods, and then for cases where some practical constraints are imposed. Finally, a hybrid link adaptation scheme is introduced and studied. The hybrid scheme is shown to overcome most of the performance loss caused by the practical constraints. Moreover, the hybrid scheme, as opposed to WF, enables the system to be tuned to meet the most important performance objective for the system under consideration, such as coverage reliability, capacity, or data rate distribution. The algorithms and the framework presented in this paper can be used to improve the link adaptation performance of modern cellular systems such as HSDPA.

Link error prediction methods for multicarrier systems

Multicarrier modulations such as OFDM with adaptive modulation and coding (AMC) are well suited for high data rate broadband systems that operate in multipath environments and are considered as promising candidates for future generation cellular systems (e.g., 4G). Cellular system performance is normally investigated with system level simulations that are computationally complex. For broadband multicarrier systems, incorporating a detailed physical layer emulator into the system simulator becomes impractical, so there is a need for simplified link performance predictors. However, due to the large variability of the channel in the frequency domain, two links with the same average SNR can experience drastically different performance, thus making it difficult to accurately predict the instantaneous link performance such as the frame error rate. In this paper, the accuracy of two FER prediction methods is studied: Packet error rate indicator (PER-indicator) and exponential effective SIR mapping (Exp-ESM). Both methods are shown to have accuracy within a few tenths of a dB under a wide range of modulation schemes, coding rates and channel types. These methods are then extended to handle more advanced link enhancements such as hybrid ARQ and Alamouti encoding. The Exp-ESM method has slightly better accuracy than the PER-indicator, and is the preferred link error predictor for a system simulator.

Uplink Control Channel Design for 3GPP LTE

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.

Downlink Control Channel Design for 3GPP LTE

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%.

ARP and QPP Interleavers for LTE Turbo Coding

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.

Channel codng for 4G systems with adaptive modulation and codng

Future 4G mobile systems, whether evolutionary (e.g., evolutions of WCDMA and cdma2000) or based on technology such as OFDM applied to a wide-area environment, can achieve very large average user throughputs by using adaptive modulation and coding (AMC) instead of fixed modulation (e.g., QPSK) and power control. This article discusses the impact of the underlying channel coding in 4G systems with AMC. It is shown that 3G-style turbo coding can provide a 0.5-4 dB link gain over 256-state convolutional codes, depending on the frame size, modulation, and channel. The link gains from channel coding (or other techniques) do not directly translate into throughput gains for AMC, but can still be expected to improve throughput significantly. A method of generating soft information for higher order modulations based on reuse of the turbo decoding circuitry is also provided.

High speed downlink packet access performance

The overall throughout and delay performance of the 1XTREME proposal in 3GPP2 and the HSDPA proposal in 3GPP are provided. The performance sensitivity to call model, peak data rate, hybrid ARQ, C/I feedback delay, and multipath interference is examined using a quasistatic system simulator. In all cases, performance is bounded using two simple scheduling approaches, maximum C/I and round robin.

Multi-dimensional adaptation and multi-user scheduling techniques for wireless OFDM systems

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.

Error prediction for adaptive modulation and coding in multiple-antenna OFDM systems

In this paper, the problem of packet error rate (PER) prediction is addressed in the multiple-antenna broadband OFDM context, and its impact on adaptive modulation and coding (AMC) is quantified. The analysis is based on a physical layer comprising various modulation and coding schemes, ranging from robust space-time block coding (STBC) modes to high bit rate spatial division multiplexing (SDM) modes, and also hybrid SDM-STBC schemes. For each mode the expression of several link quality metrics (LQM) enabling PER prediction in the broadband OFDM channel, such as instantaneous signal-to-noise ratio (SNR), capacity, or exponential effective SNR metrics are provided. Their advantages and limitations are investigated. Finally, their performance is benchmarked in the IEEE 802.11a/g/n context. It is shown that the choice of the LQM has a significant impact on the throughput performance of the AMC algorithm.

Performance of coded higher order modulation and hybrid ARQ for next generation cellular CDMA systems

A key requirement for next generation CDMA systems is to provide a high bit rate packet data service and improved sector throughput for both low and high mobility applications. A 1.25 MHz DS-CDMA evolution called 1XTREME (Third Generation Enhanced Modulation and Encoding) has been proposed. 1XTREME uses a forward shared channel (F-SHCH) that is shared by multiple packet data users and is capable of supporting peak rates of up to 5 Mbps, compared to third generation CDMA system indoor peak data rates of 460 kbps and 2 Mbps for IS-2000 and W-CDMA, respectively. In this paper the link performance of the F-SHCH is evaluated for different modulation and coding scheme (MCS) configurations. Based on the link results, the average sector throughput is presented for two simple scheduling algorithms, as well as performance as a function of the maximum allowed FER operating point.