Kevin L. Baum

Cyclic-prefix CDMA: an improved transmission method for broadband DS-CDMA cellular systems

This paper describes a novel type of single-carrier DS-CDMA system that combines the use of OFDM-style cyclic prefixes with frequency-domain equalization, for use on the forward link of broadband CDMA cellular systems. The proposed method is referred to as CP-CDMA. The combination of cyclic prefixes and MMSE frequency-domain equalization solves many of the performance and complexity problems that arise in CDMA receivers when techniques such as multi-level modulation and full code usage are employed. Simulation results are presented for a 20 MHz system with multi-level modulation and these results are compared to the performance of a conventional CDMA system with either a RAKE receiver or a time-domain equalizer. Results indicate that CP-CDMA provides significantly improved performance, yet has lower complexity than the time-domain equalized system.

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.

Adaptive antennas for OFDM

This paper explores the combination of adaptive antennas and orthogonal frequency division multiplexing (OFDM) for operation in a faded delay-spread channel. We consider the performance of the well-known sample matrix inversion (SMI) algorithm for controlling an adaptive antenna in a pilot-symbol-assisted OFDM system. Several strategies are considered for deploying SMI with different levels of tracking capability over an OFDM time-frequency slot. We propose a modification to the SMI method which incorporates the concept of pilot-interpolation into the computation of the sample covariance matrix. This modified SMI algorithm is shown to provide performance superior to the other variations of SMI under consideration.

Obtaining channel knowledge for closed-loop multi-stream broadband MIMO-OFDM communications using direct channel feedback

This paper introduces direct channel feedback in broadband multi-stream MIMO-OFDM communications as an efficient means for providing the base station with complete broadband channel information. The mobile station first estimates the complete (frequency selective) downlink channel responses between all transmit and receive antenna combinations and then transmits these responses back to the base in the frequency domain in place of the usual data symbols. The base station equalizes the received feedback signal and recovers the channel information by using standard channel estimation techniques. Using some signal processing tricks, multiple channel responses can be encoded together and sent back in a single feedback message, making the feedback very bandwidth efficient. Additional gains in bandwidth efficiency are provided by allowing multiple mobile stations to transmit their feedback on the same time-frequency resource in a spatial division multiple access (SDMA) fashion. Simulation results demonstrate that in IEEE 802.16 type systems, up to 16 complete broadband channels can be sent in one OFDM symbol period using the direct channel feedback method

Cyclic-prefix CDMA with antenna diversity

This paper explores the addition of antenna diversity schemes to the recently-proposed cyclic-prefix CDMA (CP-CDMA) system concept CP-CDMA is a novel type of single carrier DS-CDMA that employs OFDM-style cyclic prefixes for the purposes of facilitating efficient frequency-domain equalization in broadband CDMA cellular systems. CP-CDMA is intended for high-bandwidth, high-delay-spread cellular channels (especially in the forward link), such as those envisioned for systems that evolve from current 3G CDMA standards. This paper proposes transmit and receive antenna diversity techniques for CP-CDMA systems that operate with high user loading and multi-level modulation in broadband high delay-spread channels. Simulation results show that the proposed transmit diversity technique and receive frequency domain equalization provide vastly superior performance than previous Walsh-code-based transmit diversity methods using RAKE reception.

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.

A comparison of differential and coherent reception for a coded OFDM system in a low C/I environment

This paper compares the performance of differential and coherent QPSK for a high data rate slow frequency hopping (SFH) OFDM system in a channel with strong co-channel interference. Two-branch diversity reception and channel coding are included in the comparison. The coherent system uses a rate 1/2, 64-state convolutional code and includes a relatively high amount of pilot symbol overhead to facilitate the measurement of channel state information (CSI) including the instantaneous co-channel interference level. The CSI is incorporated in the combining and decoding process to improve the decoded bit error rate performance. Although the differential system does not support the measurement of the co-channel interference level, the savings in pilot symbol overhead enables the use of a more powerful rate 1/3, 64-state convolutional code. By using a rate 1/2 code in the CSI-coherent system and a rate 1/3 code in the differential system, the information data rates of the two systems are similar. Results indicate that CSI-coherent system performs better than the differential system due to the inclusion of accurate CSI in the combining and decoding process.

Overview of UMTS Air-Interface Evolution

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.

Efficient OFDM-HARQ system evaluation using a recursive EESM link error prediction

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