Yifan Liang

Generalizing Capacity: New Definitions and Capacity Theorems for Composite Channels

We consider three capacity definitions for composite channels with channel side information at the receiver. A composite channel consists of a collection of different channels with a distribution characterizing the probability that each channel is in operation. The Shannon capacity of a channel is the highest rate asymptotically achievable with arbitrarily small error probability. Under this definition, the transmission strategy used to achieve the capacity must achieve arbitrarily small error probability for all channels in the collection comprising the composite channel. The resulting capacity is dominated by the worst channel in its collection, no matter how unlikely that channel is. We, therefore, broaden the definition of capacity to allow for some outage. The capacity versus outage is the highest rate asymptotically achievable with a given probability of decoder-recognized outage. The expected capacity is the highest average rate asymptotically achievable with a single encoder and multiple decoders, where channel side information determines the channel in use. The expected capacity is a generalization of capacity versus outage since codes designed for capacity versus outage decode at one of two rates (rate zero when the channel is in outage and the target rate otherwise) while codes designed for expected capacity can decode at many rates. Expected capacity equals Shannon capacity for channels governed by a stationary ergodic random process but is typically greater for general channels. The capacity versus outage and expected capacity definitions relax the constraint that all transmitted information must be decoded at the receiver. We derive channel coding theorems for these capacity definitions through information density and provide numerical examples to highlight their connections and differences. We also discuss the implications of these alternative capacity definitions for end-to-end distortion, source-channel coding, and separation.

Evolution of Base Stations in Cellular Networks: Denser Deployment versus Coordination

It has been demonstrated that base station cooperation can reduce co-channel interference (CCI) and increase cellular system capacity. In this work we consider another approach by dividing the system into microcells through denser base station deployment. We adopt the criterion to maximize the minimum spectral efficiency of served users with a certain user outage constraint. In a two-dimensional hexagon array with homogeneous microcell structure, under the proposed propagation model denser base station deployment outperforms suboptimal cooperation schemes (zero-forcing) when the density increases beyond 3 - 12 base stations per km2, the exact value depending on the rules of outage user selection. However, close- to-optimal cooperation schemes (zero-forcing with dirty-paper- coding) are always superior to denser deployment. Performance of a hierarchial cellular structure mixed with both macrocells and microcells is also evaluated.

CTH13-3: Symmetric Rate Capacity of Cellular Systems with Cooperative Base Stations

Cooperation among base stations has demonstrated substantial capacity gain in cellular systems. The optimal transmission scheme under full base station cooperation requires all users to transmit simultaneously and a central joint receiver for multi-user detection. We consider some sub-optimal but more practical schemes of orthogonal channel access either within a cell (intra-cell TDMA) or among cells (inter-cell time sharing), which correspond to a partitioning of overall channel resources. The effects of various schemes on the uplink capacity of a cellular system are then compared for a modified Wyner model.

Capacity Definitions of General Channels with Receiver Side Information

We consider three capacity definitions for general channels with channel side information at the receiver, where the channel is modeled as a sequence of finite dimensional conditional distributions not necessarily stationary, ergodic, or information stable. The Shannon capacity is the highest rate asymptotically achievable with arbitrarily small error probability. The outage capacity is the highest rate asymptotically achievable with a given probability of decoder-recognized outage. The expected capacity is the highest expected rate asymptotically achievable with a single encoder and multiple decoders, where the channel side information determines the decoder in use. Expected capacity equals Shannon capacity for channels governed by a stationary ergodic random process but is typically greater for general channels. These alternative definitions essentially relax the constraint that all transmitted information must be decoded at the receiver. We derive equations for these capacity definitions through information density. Examples are also provided to demonstrate their implications.

CTH13-5: Coverage Spectral Efficiency of Cellular Systems with Cooperative Base Stations

Coverage spectral efficiency (CSE) characterizes the tradeoff between efficient channel reuse and the achievable rates per cell, under the assumption of detection by a single base station and intra-cell FDMA. It is well known that intra-cell FDMA is not in general optimal. In this paper we study an alternative intra- cell wide-band scheme as well as the base station cooperation in detection, which has demonstrated potential capacity gain. The effect on CSE of different schemes are then compared and the optimal reuse distance is determined for each scheme.

Adaptive Channel Reuse in Cellular Systems

In cellular systems a large reuse distance reduces co-channel interference while a small reuse distance increases bandwidth allocated to each cell. The optimal reuse distance is chosen to balance these two factors. Instead of applying a fixed reuse distance to the entire system, in this paper we study the effect of adaptive channel reuse based on channel strength. Assuming the traditional single base station transmission, adaptive channel reuse under different propagation models, with or without fading, is analyzed for the Wyner linear cellular model. A new approach where base stations collaborate in transmission is also considered. We observe that adjacent base cooperation does not show much advantage over traditional single base transmission under intra-cell orthogonal schemes and AWGN channel models. In order to fully exploit the benefit of base station cooperation, more sophisticated transmission schemes need to be investigated.

Coverage Spectral Efficiency of Cellular Systems with Cooperative Base Stations

Coverage spectral efficiency (CSE) characterizes the tradeoff between efficient channel reuse and the achievable rates per cell, under the assumption of detection by a single base station and intra-cell FDMA. It is well known that intra-cell FDMA is not in general optimal. In this paper we study an alternative intra- cell wide-band scheme as well as the base station cooperation in detection, which has demonstrated potential capacity gain. The effect on CSE of different schemes are then compared and the optimal reuse distance is determined for each scheme.

Interference Suppression in Wireless Cellular Networks through Picocells

It has been demonstrated that base station cooperation can reduce co-channel interference (CCI) and increase cellular system capacity. In this work, we consider another approach by dividing the system into picocells through denser base station deployment. For a two-dimensional hexagon cellular array and the propagation model under consideration, we observe that the operating regime shifts from interference-limited to noise-limited when the density increases to about 20 base stations per km2. To compare the performance of both approaches, we adopt a criterion to maximize the minimum served spectral efficiency with a certain user outage constraint. Simulations show that denser base station deployment outperforms suboptimal cooperation schemes (zero-forcing) when the density increases beyond 3~12 base stations per km2, the exact value depending on the rules of outage user selection. However, close-to-optimal cooperation schemes (zero-forcing with dirty-paper-coding) are always superior to denser base station deployment.

Generalized Capacity and Source-Channel Coding for Packet Erasure Channels

We study the transmission of a stationary ergodic Gaussian source over a packet erasure channel, which is a composite channel with degraded states. A broadcast channel code can be applied to a composite channel to obtain different rates in the different channel states. However, we show that a non-broadcast direct transmission strategy achieves a higher expected rate than the broadcast code, although it does not meet Shannons definition of reliable communication since it does not guarantee which bits will be received. Each channel code has a matching source code: the multiresolution source code allows the broadcast channel code to transmit prioritized information, and the symmetric multiple description source code enables direct transmission of unprioritized information. The end-to-end expected distortions of these schemes are also compared.