Pål Frenger

Performance comparison of HARQ with Chase combining and incremental redundancy for HSDPA

We compare two hybrid automatic repeat request (HARQ) combining strategies that currently are considered for the high speed downlink packet access (HSDPA) evolution of WCDMA. The two HARQ combining schemes are Chase combining, where the retransmissions are identical copies of the original transmission, and incremental redundancy (IR), where the retransmissions consist of new parity bits from the channel encoder. We show that the link-level performance of a HARQ type-II system can be significantly better with IR compared to Chase combining. The largest gains are obtained for high channel-coding rates and high modulation orders. For low modulation and coding schemes (MCS), the link-level performance gains with IR are less significant. We further show that in a system that uses link adaptation we can not expect any large gains with IR as long as the link adaptation errors are reasonably small. Furthermore, we show that on fading channels there are situations when an IR system actually performs poorer than a Chase combining system.

Reducing Energy Consumption in LTE with Cell DTX

This paper discusses how energy consumption can be significantly reduced in mobile networks by introducing discontinuous transmission (DTX) on the base station side. By introducing DTX on the downlink, or cell DTX, we show that it is possible to achieve significant energy reductions in an LTE network. Cell DTX is most efficient when the traffic load is low in a cell but even when realistic traffic statistics are considered the gains are impressive. The technology potential for a metropolitan area is shown to be 90% reduced energy consumption compared to no use of cell DTX. The paper also discusses different drives for the increased focus on energy efficient network operation and also provides insights on the impact of cell DTX from a life cycle assessment perspective.

Convolutional codes with optimum distance spectrum

New convolutional codes with rates 1/2, 1/3, and 1/4 are presented for constraint lengths ranging from 3 to 15. These new codes are maximum free-distance codes. Furthermore, the codes have optimized information error weights, resulting in a low bit-error rate for binary communication on both additive white Gaussian noise (AWGN) and Rayleigh fading channels. Improved coding gains of as much as 0.6 dB compared to previously published codes have been observed for coherent BPSK over a Rayleigh fading channel and a wide range of signal-to-noise ratios.

The evolution of WCDMA towards higher speed downlink packet data access

The first step of evolving WCDMA towards higher data rates is discussed. It is shown how techniques such as fast link adaptation, fast hybrid ARQ and fast scheduling, among others, can be applied to WCDMA, leading to increased throughput, lower delays, and downlink peak rates in the order of 10 Mbit/s. Through system simulations, the packet-data performance of such an evolved WCDMA system is illustrated.

Code-spread CDMA using maximum free distance low-rate convolutional codes

In code division multiple-access (CDMA) systems, maximum total throughput assuming a matched filter receiver can be obtained by spreading with low-rate error control codes. Previously, orthogonal, bi-orthogonal and super-orthogonal codes have been proposed for this purpose. We present in this paper a family of rate-compatible low-rate convolutional codes with maximum free distance. The performance of these codes for spectrum spreading in a CDMA system is evaluated and shown to outperform that of orthogonal and super-orthogonal codes as well as conventionally coded and spread systems. We also show that the proposed low rate codes will give simple encoder and decoder implementations. With these codes, any rate 1/n, n/spl les/512, are obtained for constraint lengths up to 11, resulting in a more flexible and powerful scheme than those previously proposed.

Neighbor Cell Relation List and Physical Cell Identity Self-Organization in LTE

Automation of radio network management is a key determinant to work reduction for wireless operators. By replacing time consuming and costly tasks with automatic mechanisms, operational expenditure can be reduced. In this paper we present a method for automatic configuration of locally-unique physical cell identities and neighbor cell relation lists in 3G long term evolution (LTE). This method makes use of mobile measurements to update the neighbor cell relation lists in the cells and to detect local cell identity conflicts, report the conflicts to the operation support systems (OSS) and resolve them. The performance of the approach is determined using simulations of realistically deployed macro networks. Conducted simulations illustrate the ability of the method to resolve local cell identity conflicts. In particular, the method is capable of both accommodating new cells and handling a worst case scenario where all cells are initiated with the same local cell identities and where neighbor cell relation lists are empty.

Rate-compatible convolutional codes for multirate DS-CDMA systems

New rate-compatible convolutional (RCC) codes with high constraint lengths and a wide range of code rates are presented. These new codes originate from rate 1/4 optimum distance spectrum (ODS) convolutional parent encoders with constraint lengths 7-10. Low rate encoders (rates 115 down to 1/10) are found by a nested search, and high rate encoders (rates above 1/4) are found by rate-compatible puncturing. The new codes form rate-compatible code families more powerful and flexible than those previously presented. It is shown that these codes are almost as good as the existing optimum convolutional codes of the same fates. The effects of varying the design parameters of the rate-compatible punctured convolutional (RCPC) codes, i.e., the parent encoder rate, the puncturing period, and the constraint length, are also examined. The new codes are then applied to a multicode direct-sequence code-division multiple-access (DS-CDMA) system and are shown to provide good performance and rate-matching capabilities. The results, which are evaluated in terms of the efficiency for Gaussian and Rayleigh fading channels, show that the system efficiency increases with decreasing code rate.

Decision-directed coherent detection in multicarrier systems on Rayleigh fading channels

Novel coherent detectors for DQPSK and /spl pi//4-DQPSK on Rayleigh fading channels are presented. The detectors are based on decision feedback and minimum mean-square error (MMSE) channel estimation. The proposed detectors may be used in both single carrier and multicarrier systems. Analytical results in the form of expressions for channel estimation error variance and bit error probability, both with and without diversity, are derived. The MMSE-based detector has a significantly lower irreducible bit error probability floor compared to a conventional differential detector. For most practical applications, the error floor of the MMSE-based detector is negligible. Simulated results are presented to verify the analysis and justify the approximations made.

The high speed packet data evolution of WCDMA

The first step of evolving WCDMA towards higher data rates through the introduction of HSDPA is discussed. By applying fast link adaptation together with higher order modulation, fast hybrid ARQ and fast scheduling, the best-effort packet data support of WCDMA can be improved in terms of increased throughput, lower delays and increased peak data rates. Simulations illustrate the potential of the concept.

Parallel combinatory OFDM signaling

A parallel combinatory-orthogonal frequency division multiplexing (PC-OFDM) system is proposed and analyzed. The proposed system selects at each symbol interval a subset of the available subcarriers, and the selected subcarriers are modulated by points from an M-PSK signal constellation, PC-OFDM systems can be designed to have lower peak-to-average power ratio (PAPR), higher bandwidth efficiency, and lower bit error probability on Gaussian channels compared to ordinary OFDM systems. A bit mapping procedure using the Johnson (1993) association, together with a position algorithm for the PSK symbols, is proposed. Good analytical approximations of the BER for PC-OFDM systems are derived for AWGN and Ricean fading channels, and extensive simulation results are presented.