Wireless Communications

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity. >

On the capacity of a cellular CDMA system

In this paper we review the most peculiar and interesting information-theoretic and communications features of fading channels. We first describe the statistical models of fading channels which are frequently used in the analysis and design of communication systems. Next, we focus on the information theory of fading channels, by emphasizing capacity as the most important performance measure. Both single-user and multiuser transmission are examined. Further, we describe how the structure of fading channels impacts code design, and finally overview equalization of fading multipath channels.

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Fading channels: information-theoretic and communications aspects

Wireless location refers to the geographic coordinates of a mobile subscriber in cellular or wireless local area network (WLAN) environments. Wireless location finding has emerged as an essential public safety feature of cellular systems in response to an order issued by the Federal Communications Commission (FCC) in 1996. The FCC mandate aims to solve a serious public safety problem caused by the fact that, at present, a large proportion of all 911 calls originate from mobile phones, the location of which cannot be determined with the existing technology. However, many difficulties intrinsic to the wireless environment make meeting the FCC objective challenging. These challenges include channel fading, low signal-to-noise ratios (SNRs), multiuser interference, and multipath conditions. In addition to emergency services, there are many other applications for wireless location technology, including monitoring and tracking for security reasons, location sensitive billing, fraud protection, asset tracking, fleet management, intelligent transportation systems, mobile yellow pages, and even cellular system design and management. This article provides an overview of wireless location challenges and techniques with a special focus on network-based technologies and applications.

Network-based wireless location: challenges faced in developing techniques for accurate wireless location information

Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied. >

Analysis of a simple successive interference cancellation scheme in a DS/CDMA system

The performance of a DS/CDMA system using M-ary orthogonal modulation with noncoherent demodulation is evaluated. The system operates in a multipath fading channel. A RAKE receiver structure with equal gain combining is used for demodulation. An approximation to the bit error probability is given which depends only on the first- and second-order moments of the multipath energies. The analysis results are compared with the results from computer simulations. It is seen that the approximation is accurate for multipath energies with realistic coefficient of variation. The system performance is also evaluated in terms of the capacity, which is defined as the number of users that can be supported at a given bit error probability. The approximation is used to evaluate the capacity reductions due to power variations caused by multipath fading. >

Performance analysis of DS/CDMA with noncoherent M-ary orthogonal modulation in multipath fading channels

A generic structure of Hybrid ARQ using Turbo Codes is provided which requires the function of channel coding, redundancy selection, buffering and maximum-ratio diversity combining, channel decoding, error detection, and sending back an acknowledgement to the transmitter (Fig.3). The functions of channel coding and redundancy selection (Fig.1) are performed at the transmitter while the remaining functions are performed at the receiver. The initial code rate can be explicitly communicated to the receiver or blindly detected.

Adaptive hybrid arq using turbo code structure

In a wireless communication system using a power control command to control power of a transmitted traffic channel (58), a signal from a transmitter (64) is received (104) via a channel. A channel quality is measured (70, 108) using the signal from the transmitter. In response to the measured channel quality (70) exceeding (80, 110) a channel quality threshold (82), a decrease traffic channel power command is sent (112) to the transmitter, and a channel quality threshold is increased (114). In response to the channel quality threshold (82) exceeding (80, 110) the measured channel quality (70), an increase traffic channel power command is sent (116) to the transmitter, and the channel quality threshold is decreased (118). A traffic signal quality may be measured (72) and compared (88) to a traffic signal quality threshold (86). The result of the comparison (132) may be used to adjust (134) the channel quality threshold (82). The traffic signal quality threshold may be adjusted (90, 126, 128) in response to whether or not a frame error is detected (74, 124).

Method and system for generating a power control command in a wireless communication system

A method of combining soft-handoff with a hybrid ARQ scheme to maximize throughput and gain in a communications system. After receiving a frame from the MS ( 110 ), the BTSs ( 104 and 106 ) will process the frame and communicate to the MS over a forward control channel whether the frame contained any errors. If all BTSs communicate that the frame contains errors, the MS will retransmit the same frame to all BTSs with a flush bit set to instruct the BTSs 104 and 106 to combine the retransmitted frame with the original frame. If only some BTSs communicate that the frame contains errors, the MS will transmit the next frame to all BTSs that successfully decoded the frame with the flush bit set to instruct the BTSs to erase the previous frame from memory and not to combine the previous frame with the current frame. The MS will retransmit the frame to the BTSs that did not successfully decode the frame with the flush bit set to instruct the BTSs to combine the previous frame with the retransmitted frame.

Closed loop method for reverse link soft handoff hybrid automatic repeat request

Separate forward dedicated and shared control channels (17, 18) are provided in a spread-spectrum communication. The forward dedicated control channel (18) is used to communicate persistent control information and point to the shared control channel (17) when further intermittent control information concerning transmission of data to a mobile station (14) needs to be communicated. The use of a dedicated control channel for only necessary persistent control information, while only pointing to a shared control channel when it is needed, affords more efficient utilization of system resources.

Louay Jalloul

Papers

Performance analysis of DS/CDMA with noncoherent M-ary orthogonal modulation in multipath fading channels

Adaptive hybrid arq using turbo code structure

Method and system for generating a power control command in a wireless communication system

Closed loop method for reverse link soft handoff hybrid automatic repeat request

Apparatus and method for providing separate forward dedicated and shared control channels in a communications system