5G wireless networks are expected to usher in massive Machine Type Communications (mMTC) pertaining to the Internet of Things (IoT). A large fraction of the devices participating in IoT are likely to be low-power devices that generate sporadic traffic. It is anticipated that autonomous grant-free communications would be needed to accommodate these devices and the traffic they generate. Preamble transmission is a key part of such grant-free communications, and system performance including the desired power savings for the devices depends critically on the preamble detection-false alarm characteristics. In this context, low-power devices in extended coverage scenarios present a stiff challenge to system designers. In this study, we show that with narrowband resource allocations, use of preamble acknowledgements and judiciously set power control targets one can achieve the desired performance even for low-power devices in challenging scenarios.

Throughput performance of WiFi networks, which employ the Carrier Sense Multiple Access (CSMA) technique, can be estimated fairly accurately using Bianchi's analytic solution provided all elements of the network are within the carrier sensing range of one another. However, this solution cannot be applied to networks that are spread over a wide area. A paper by Liew, et al proposed an interesting approximate solution for such networks. While this solution was shown to work well in a variety of conditions, it suffers from a limitation in that it requires all transmitter-receiver pairs to have independent and identically distributed (i.i.d.) transmission times. In this paper, we generalize some of the key results of Liew, et al by removing the restriction on transmission times, which leads to a solution technique that can be used to analyze CSMA networks where different transmitter-receiver pairs operate at different bit rates. We also provide a few examples to illustrate how well this technique works in different settings and compare it with solutions based on other methods.

In this paper, we present a practical method to construct dirty paper coding (DPC) schemes using sum codes. Unlike the commonly used approach to DPC where the coding scheme involves concatenation of a channel code and a quantization code, the proposed method embodies a unified approach that emulates the binning method used in the proof of the DPC result. Auxiliary bits are used to create the desired number of code vectors in each bin. Sum codes are obtained when information sequences augmented with auxiliary bits are encoded using linear block codes. Sum-code-based DPC schemes can be implemented using any linear block code, and entail a relatively small increase in decoder complexity when compared to standard communication schemes. They can also lead to significant reduction in transmit power in comparison to standard schemes.

Base station cooperation has been shown to significantly mitigate the interference perceived in cellular networks and improve the quality of user experience. On the uplink, joint processing (JP) across antennas within a cluster of base stations yields significant gain, but requires significant backhaul to transport complex I & Q signals. The Network Interference Cancellation Engine (NICE) can yield throughput gains similar to that of JP, but with one to two orders of magnitude less backhaul. While NICE demonstrates significant potential for cellular networks, its performance has been evaluated under somewhat idealistic conditions. For instance, full buffer traffic has generally been assumed, thereby leading to a fully loaded system with strong interference. Bursty traffic may result in a partially loaded system with less interference and impact the gains of interference-mitigating techniques such as NICE. In addition, an ideal backhaul with negligible delay has generally been assumed. Delays in information exchanges between base stations may decrease the efficacy of base station cooperation. In this paper, the performance of NICE is evaluated under non full buffer traffic and non-ideal backhaul.

In this paper, we present a practical Dirty Paper Coding (DPC) scheme using sum codes based on LDPC codes. While a typical sum-code-based DPC scheme uses a constrained decoder as part of the encoding operation, such an approach fails in the case of LDPC codes since the constrained decoder based on a standard LDPC decoder tends to get stuck at highly suboptimal points in typical scenarios. We get around this difficulty via a simple code construction method based on bit-mapping or permutation. Examples included in this paper show that compared to standard communication schemes the proposed DPC scheme yields substantial savings in terms of the transmit energy required to attain desired error performance.

While the information theoretic result commonly referred to as “dirty paper coding” [1] has been known for long, it has not led to practical schemes that can be implemented easily in real-life communication systems. In this paper, we present the concept of sum codes and show how they provide a convenient platform for dirty paper coding. Comb-combined sum codes, which provide a convenient method of sum-code construction, are particularly useful in this respect. One can build comb-combined sum codes based on any linear block code to achieve reduction in the transmit energy required to attain a desired performance. Using an example based on tail-biting convolutional codes, we show how practical dirty paper coding may be implemented using any convenient linear block code.

Although the information theoretic result referred to as “dirty paper coding” [1] has been known for quite a while, it has not led to schemes that look attractive enough for real-life communication systems. In a companion paper [2], we presented the concept of sum codes and showed how they provide a convenient platform for interference suppression via dirty paper coding. In this paper, we present a dirty paper coding scheme based on trellis-coded modulation, which could be looked upon as an alternative to the sum-codes-based approach. Since trellis-coded modulation is designed for multi-level signal constellations, it is expected to work well as a platform for the proposed dirty paper coding scheme in which multi-level signal constellations play a critical role.

It is widely believed that cooperation among base stations can be employed to enhance the interference mitigation capability of base station receivers in cellular networks. Among the base station cooperation schemes that have been proposed to address the uplink interference problem, two promising ones are 1) Joint Multi-Cell MMSE Processing also referred to as Network MIMO; and 2) Multi-Cell Successive Interference Cancellation (MC-SIC). While both of these methods can substantially improve the performance of cellular networks, they have complementary strengths and limitations when implemented in a practical setting. In this paper, we propose a hybrid scheme called Generalized Network Interference Cancellation Engine, which combines the strengths of these two approaches while working within the constraints imposed by practical implementations. This view is borne out by the performance results presented in this paper.

The demand for high-rate wireless services grows unabated as new mobile devices and data-hungry applications continue to emerge. However, despite many advanced physical and link layer techniques that are employed in today's cellular networks, a large number of mobile users, especially at the cell edges, receive inadequate data rates due to high interference and path loss. Several techniques that exploit inter-base station cooperation have been proposed to address the inter-cell interference problem. While the initial studies show promising gains in idealized environments, it is important to analyze these techniques in a more realistic setting. This study provides detailed analyses of the benefits provided by uplink base station cooperation with several practical constraints and issues that would be encountered in a realistic cellular network deployment. Our results indicate that the cooperative signal processing techniques are robust to channel estimation errors given the pilot structure available in contemporary wireless systems. Furthermore, uplink cooperation can substantially improve the interference management capability of the cellular network, providing significant rate improvement over current non-cooperative networks.

Heterogeneous networks comprising overlapping layers of macro cells and small cells are expected to be widely deployed to cope with the rapidly increasing demand for wireless bandwidth. While judicious resource sharing and interference management mechanisms allow these networks to provide adequate performance for static or pedestrian users, they often fail to meet the needs of highly mobile users. In this paper, we propose a cooperative data transmission scheme called "Seamless Macro-Cell-Anchored Radio Transmission" that enables even highly mobile users to experience performance levels similar to those enjoyed by static users. The performance improvements effected by this scheme occur mainly at low percentiles; in that sense it can be said to deliver sustained end-user experience even to highly mobile users. Further improvement in edge performance is possible with the use of rate thresholds in conjunction with the proposed scheme.

Recently, there has been considerable interest in the potential of inter-base station cooperation techniques to manage excessive out-of-cell interference in evolving cellular networks. It has been observed that on the uplink, significant improvements in interference rejection can be attained by exchanging complex-valued received signal samples across a cluster of cells and employing joint multi-antenna receiver processing. However, such processing incurs substantial increases in backhaul overhead, exceeding the actual information delivered by at least an order of magnitude. In this paper, we propose and describe a signal processing approach based on a novel Network Interference Cancellation Engine (NICE), which opportunistically cancels out-of-cell interference by exchanging decoded dominant interferer data among cooperating cells and reconstructing interfering signals. Simulation results with NICE processing in cellular networks show that substantial improvements (5-7 dB) in signal-to-interference-plus-noise ratio may be achieved over current non-cooperative multi-antenna receiver processing techniques while allowing backhaul overhead to be reduced by factors on the order of 40 to 50 relative to joint processing techniques employing the exchange of complex-valued received signal samples.

Single frequency networks (SFN) in combination with a multi-carrier transmission scheme such as OFDM are often proposed as a means to provide continuous coverage of broadcast applications over a large area. In this paper, we propose a distributed beam-forming algorithm that makes use of channel state information fed back by receivers to improve the performance of the weaker connections. The distributed nature of the proposed algorithm makes it easier to implement when compared to its centralized counterparts. Simulation studies show that the proposed algorithm results in a significant improvement in the performance of SFNs supporting broadcast or multicast applications.

In this paper, the feasibility and capacity benefits of deploying low-cost relay nodes with in-band wireless backhaul capability in cellular networks are studied. Our proposed scheme is similar to a cell-splitting architecture where the base station density is increased, thus reducing the effective cell size. Cell-splitting in an interference-limited deployment provides capacity gains that are proportional to the number of additional base stations deployed, but at significant cost driven primarily by wired backhaul requirements. The use of in-band wireless backhaul may significantly reduce the cost, but sharing bandwidth with the backhaul results in reduced access rates relative to that of cell-splitting with wired backhaul. Further cost savings may be achieved with the use of low power, low complexity relays in lieu of the additional base stations. In this paper, we show that a cellular architecture with base stations supplemented by simple decode-and-forward relays can provide significant capacity improvements despite the overhead associated with in-band backhaul.

Single frequency networks (SFN) in combination with a multi-carrier transmission scheme such as OFDM are often proposed as a means to provide continuous coverage of broadcast applications over a large area. While this combination does succeed in providing continuous coverage, it has certain limitations. Specifically, at points where signals from multiple base-stations are received, the overall signal-to-noise ratio can be poor because of relative phase differences. In this paper, we show how, through the use of orthogonal space-time codes, this limitation of SFNs can be overcome to improve their overall performance. This scheme could be viewed as an ldquoopen-looprdquo implementation of multi-cell cooperative transmission.

In this paper, we investigate joint resource allocation and routing for a wireless mesh network consisting of fixed access points inter-connected through multi-channel wireless links. Some of the mesh routers are assumed to function as gateways with high bandwidth connections to a wired network and the mesh network is assumed to provide multi-hop capability where the traffic entering the mesh via gateways may be carried over multiple wireless hops towards the destination mesh routers. The main objective of this work is to optimize resource allocation and routing in such networks; in this regard, we study a cross- layer optimization problem that includes power control, channel allocation, link scheduling and routing. We provide results highlighting the capacity benefits of our proposed approach; these results indicate that significant throughput improvements can be achieved with cross-layer system design.

Streaming media over wireless networks typically rely on end-to-end feedback from client devices to help media servers adjust to changing channel conditions. In rapidly fluctuating channel conditions, the client device may not be able to effectively track changes and report them to the server. Furthermore, bandwidth constraints often limit the frequency of end-to-end feedback. In order to overcome these constraints, we propose Clearmedia, a signaling proxy-based architecture for streaming media services over wireless networks. This architecture enables media servers to receive frequent and reliable feedback on network conditions, which leads to enhanced service quality. Our performance studies show that the Clearmedia architecture achieves improvement in several key performance metrics that are representative of end-user perceived video quality.

In this paper, we propose a cross layer optimization framework for multi-hop routing and resource allocation design in an orthogonal frequency division multiple access (OFDMA) based wireless mesh network. The network under consideration is assumed to consist of fixed mesh routers (or base station routers) inter-connected using OFDMA wireless links with some of the mesh routers functioning as gateways to a wired network. The objective of our cross-layer formulation is to allow joint determination of power control, frequency-selective OFDMA scheduling and multi-hop routing in order to maximize the minimum throughput that can be supported to all mesh routers. Results of our investigations under typical cellular deployment, propagation and channel model assumptions show that this approach achieves significant mesh throughput improvements primarily due to the following: (a) frequency selective scheduling with OFDMA which provides improved tone diversity thus allowing more efficient bandwidth utilization relative to single carrier methods; and (b) multi-hop routing which provides improved path diversity relative to single hop transmissions.

This paper investigates the performance benefits of different resource allocation strategies that take advantage of permutation zones specified in IEEE 802.16e-based OFDMA systems. In particular, we propose and investigate the benefits of a sub-zone scheduling approach where each permutation zone is partitioned into sub-zones that are used for scheduling subsets of eligible users according to their prevailing link quality. In the scenarios considered, the proposed sub-zone scheduling approach allows improvements in sector throughputs as high as 80% relative to a baseline IEEE 802.16e-based OFDMA system while maintaining the system outage at acceptable levels.

In this paper, different neighbor discovery methods are considered for adhoc networks without fixed spectrum allocations where discovery of neighboring nodes may need to occur over a large frequency range (e.g. tens of MHz to a few GHz). In such systems, co-existence policies or dynamic spectrum access protocols may also introduce temporal variability in the availability of frequencies for neighbor discovery thus making it challenging to discover neighbors in a short period of time and with low energy consumption. In this paper, we analyze the performance of a baseline scheme (representative of current techniques) where nodes perform discovery by transmitting and receiving beacons randomly across a large frequency range. We also propose and analyze a family of fast, energy-efficient neighbor discovery techniques that use pseudo-random frequency hopping sequences for beacon transmission and reception. For the cases considered, these techniques provide substantial improvements in discovery time and energy consumption relative to the baseline

An adaptive sleeping and awakening protocol (ASAP) is proposed for nodes in a synchronous adhoc sensor network. In order to increase energy efficiency, nodes within the network enter into a sleep mode and awaken at pre-determined time slot(s) to listen for transmissions from its immediate neighbors. Implicit knowledge of awakening slots for neighboring nodes is used to schedule transmissions within the neighborhood. Finally, nodes adapt their sleeping cycles based on neighbor topology and remaining battery life in order to maximize the network lifetime while satisfying the latency requirements of the underlying sensor application. Simulation results show that with delay constrained routing, ASAP can achieve twice the energy efficiency (or battery life) of a synchronous awakening approach.

Radio link control (RLC) protocols are typically employed for reliable in-sequence delivery of service data units (SDUs) in wireless packet data systems. The RLC layer segments packets obtained from the upper layer (referred to as SDUs) into smaller RLC transmission units (or blocks) and uses selective-repeat automatic repeat request (SR-ARQ) for error recovery of RLC blocks. In earlier work, SR-ARQ performance is typically characterized in terms of the long-term throughput or in-sequence delivery delay of RLC blocks. The SDU delivery delay which is a more meaningful measure of RLC performance (in terms of the service provided to a higher layer, e.g., transmission control protocol) has not been quantified. In this paper, we analyze the SDU delivery delay of SR-ARQ as a function of the SDU size and the channel coding scheme employed. Closed-form delay expressions as well as approximations are provided. The analysis is verified through enhanced general packet radio service RLC simulations. Based on the analysis, we propose that link adaptation be backlog dependent in order to reduce the SDU delivery delay at the RLC layer.

A new class of channel assignment methods are investigated in a power controlled, time slotted wireless communications system. In practical systems, there are constraints on the transmit power. Channel assignment algorithms that do not consider these constraints achieve smaller power control gains and result in increased co-channel interference and reduced system capacity. It is shown that assignment algorithms that employ users' received signal strength provide significant gains in the presence of these constraints.

An expanded effort is underway to support the evolution of the UMTS standard to meet the rapidly developing needs associated with wireless Internet applications. The support of high speed downlink packet access (HSDPA) is provided by means of a new, shared channel called high speed downlink shared channel (HS-DSCH) which is enabled by a number of performance enhancing technologies such as adaptive modulation and coding (AMC), hybrid ARQ (HARQ), fat-pipe scheduling, fast cell selection (FCS) and multiple input multiple output (MIMO) antenna techniques. In this paper, the advantage of FCS is investigated. To avoid the difficulty in HARQ transmission-state synchronization, FCS is allowed only among the cells (sectors) belonging to the same base station. Two FCS approaches are compared in this paper. The user controlled cell selection (UCCS) selects the serving cell based only on the channel quality seen by the user. While a new-network controlled cell selection (NCCS) approach jointly considers channel quality and network loading. It is shown that FCS significantly improves the HSDPA performance at low mobility. Among the FCS schemes, NCCS is superior to UCCS when the load among the cells in the active set is unbalanced.

In some communications systems, uncoded symbol error rate (SER) estimation is required to be performed "on-the-fly" at the receiver, in order to provide a measure of the quality of service or to aid in the power control process required for CDMA systems. An example is the 3GPP WCDMA UMTS standard, where transport channel and physical channel bit error rates have been required as metrics to be estimated by the receiver. In this paper, we analyze the performance of a number of uncoded SER estimation techniques, including a novel technique in which we use the soft information of the received symbols to obtain a fast and accurate estimate of the uncoded SER. The technique we introduce outperforms the commonly used decode/re-encode/compare technique, in terms of accuracy and required estimation interval. Our technique also outperforms the brute-force technique, which is to send a known test-pattern, demodulate it at the receiver, and count the observed discrepancies.

In wireless packet data systems such as the Enhanced General Packet Radio Service (EGPRS), selective ARQ is used for error recovery over a radio link. In earlier work, the radio link control (RLC) performance is typically characterized in terms of the throughput and the size of the higher layer protocol data units (PDUs) or the packets to be transferred is ignored. We propose that the link adaptation be based on user perceived delay instead of the throughput.

The General Packet Radio Service (GPRS) was introduced by ETSI to meet the ever-increasing demand for wireless data over the GSM radio interface. Enhanced GPRS (EGPRS Phase 1) was later introduced in order to increase the spectrum efficiency and data throughput by using 8-PSK modulation and link adaptation techniques. While (E)GPRS uses packet-switched technology to efficiently transmit best-effort data, real-time services (e.g. voice) are currently supported only via circuit-switched systems. This service separation reduces the potential spectrum efficiency gain resulting from multiplexing different services on the same radio channel. This paper describes the key new concepts needed to support statistical multiplexing of different radio access bearer classes over the GSM/EDGE radio access network (GERAN) air interface. The focus is on the definition of new traffic and control channels to support statistical multiplexing of speech, real-time data, and non-real-time data, and the corresponding new MAC procedures that are needed to guarantee quality of service (QoS).

Cellular packet data systems require medium access control (MAC) protocols that allow flexible resource allocation on both the uplink and the downlink. The overhead introduced for managing MAC functions should be minimized, and control fields should be well protected in order to provide robustness to channel errors. The IS-136 TDMA digital control channel specifies shared channel feedback (SCF) procedures and fields for managing uplink resources. However, the SCF mechanism is not well suited to long packet data transactions. We describe a simple and robust packet channel feedback (PCF) mechanism for an IS-136 based TDMA packet data channel by identifying the minimal set of functions and fields required to manage both contention access and reserved access. The proposed scheme has unambiguous state transitions and better performance under error conditions. The PCF scheme provides considerable flexibility in assigning bandwidth to multiple users, and allows the efficient scheduling of contention and reservation opportunities on each subchannel without wasted slots.

Optimum demodulation of intersymbol interference may be performed using maximum-likelihood sequence estimation receivers. Standard performance analysis of these receivers assume that the receiver has perfect knowledge of the channel. We assume instead a mismatched receiver that uses an imperfect estimate of the channel to decode the information sequence. A free-distance expression for a mismatched ML receiver is derived and applied to the case of linear-coded modulation. A truncated union bound approximation accurately predicts the error probability.

We propose a technique to measure channel quality in terms of signal-to-interference plus noise ratio (SINR) for the transmission of signals over fading channels. The Euclidean distance (ED) metric, associated with the decoded information sequence or a suitable modification thereof, is used as a channel quality measure. Simulations show that the filtered or averaged metric is a reliable channel quality measure which remains consistent across different coded modulation schemes and at different mobile speeds. The average scaled ED metric can be mapped to the SINR per symbol. We propose the use of this SINR estimate for data rate adaptation, in addition to mobile assisted handoff (MAHO) and power control. We particularly focus on data rate adaptation and propose a set of coded modulation schemes which utilize the SINR estimate to adapt between modulations, thus improving the data throughput. Simulation results show that the proposed metric works well across the entire range of Dopplers to provide near-optimal rate adaptation to average SINR. This method of adaptation averages out short-term variations due to Rayleigh fading and adapts to the long-term effects such as shadowing. At low Dopplers, the metric can track Rayleigh fading and match the rate to a short-term average of the SINR, thus further increasing throughput.

A turbo-type scheme that uses parallel concatenation and soft-output iterative detection is proposed for transmission over linear intersymbol interference channels. When the receiver has perfect knowledge of the channel, the scheme offers only marginal improvement in performance over optimum linear coded modulation with Viterbi or reduced search detection. However, the improvement is significantly higher when the detector is mismatched, i.e., if it uses an imperfect channel estimate. The delay is low since the block sizes are small, and the performance improvement is achieved with few iterations. The technique may be applied as a retransmission strategy in order to save power for systems that suffer from losses due to ISI and channel mismatch.

Controlled intersymbol interference may be used to design bandwidth efficient coded modulation schemes. The performance of the optimum receiver depends only on the free distance of the code and is independent of its phase. Large reductions in receiver complexity are obtained by using reduced search detection techniques like the M-algorithm (MA). However, we show that MA detector performance depends critically on phase, and is characterized by the partial energy of the encoder or channel. The minimum phase encoder needs by far the least detector searching. We propose modifications to the receiver, so that the reduced search can operate on all nonminimum phase channels at this same low complexity.