We consider a time-division duplex (TDD) two-tier network, comprising macro- and femtocells coexisting in a co-channel operation mode, in which inter- and intra-tier interference are incurred. The proposed cell selection accounts for both uplink and downlink capacities. Comparison is made with path loss based and received signal strength based cell selections. Through simulation, we evaluate the performance of indoor and outdoor users, in terms of cdf, average capacity and 5%-outage capacity under different parameters. Our results indicate appealing performance of the proposed capacity based cell selection under TDD two-tier cellular networks.

We develop new transmission schemes for multipoint-to-multipoint (M2M) networks which, unlike the existing approaches, address duplexing/multiplexing and multiple access jointly and possess true M2M characteristics in the physical layer. In the proposed schemes, all forward and reverse links of the M2M network share the overall spectrum concurrently in an orthogonal frequency division manner. This provides increased degrees of freedom and enhanced diversity in scheduling and resource allocation, thus leading to performance enhancement. We first discuss practical implementation issues. Next, we develop our proposed schemes for both centralized and distributed access scenarios. Under the centralized access, we illustrate advantages of the proposed approach for M2M networks using a scheduling algorithm and provide a closed-form analytical throughput upper-bound. Next, we extend the proposed approach to multicasting and present a new scheduling strategy and its closed-form analytical approximate throughput expression. Under the distributed access, we develop a generalized multi-channel carrier sensing multiple access/collision avoidance scheme for M2M networks and propose a new scheme which exploits local channel information. Closed-form analytical throughput expressions for a specific scenario are also presented. Simulation results corroborate substantial performance gains of the proposed schemes over the conventional schemes in both centralized and random access scenarios.

Orthogonal multicarrier division duplexing (MDD) is a new duplexing scheme that gains the advantages of both time division duplexing and frequency division duplexing. Resource allocation for multipoint-to-multipoint (M2M) MDD is considered in this paper. We propose joint subcarrier and power allocation for M2M MDD that maximizes the sum rate of the network under either total or individual power constraint. Simulation results show that MDD outperforms conventional TDD and TDD-OFDMA systems. Some insights on M2M MDD under different channel conditions are given and convergence of the proposed algorithm is shown through simulation results.

Transmit power of a femtocell access point (FAP) is a key design parameter in two-tier networks. When the FAP is near the macrocell edge, existing transmit power design based on the strongest interference from the macrocell base station (MBS) may not be appropriate. This paper proposes three criteria for FAP transmit power control, two of which taking into account the interference from neighbor MBSs. Particularly, the proposed transmit power control based on the sum interference criterion is suitable for an FAP located near the macrocell edge and outperforms an existing criterion under the target FAP coverage.

While femtocell network enhances throughput and coverage, it also introduces cross-tier interference. In the uplink, to protect existing macrocell users (MUs), the interference accumulated at the macrocell base station (MBS) by femtocell users (FUs) should be limited. Due to variable distances between a femtocell base station (FBS) and the MBS, FU associated with the FBS closer to the MBS will impose more interference to the MBS due to smaller path loss. To meet the interference constraint at the MBS, the FU closer to the MBS has a stricter transmission power limit than the FU further away from the MBS. In this situation, fairness issue arises among the FUs. In this paper, we consider an uplink power allocation problem for orthogonal frequency division multiplexing (OFDM) transmissions under the rise-over-thermal constraint at the MBS and evaluate throughput and fairness of the FUs. Our simulation results indicate that the instantaneous channel knowledge between the FUs and the MBS increases throughput and fairness of the FUs compared to the average channel knowledge. In addition, wall loss is beneficial in improving the throughput and fairness as it suppresses interference from FUs to the MBS.

In the uplink, while macrocell mobile stations (mMSs) are synchronized to the macrocell base station (mBS), their signals will arrive asynchronously at the femtocell base station (fBS). For OFDMA, if the timing misalignment among the mMSs is greater than the cyclic prefix duration, intersymbol interference (ISI) and intercarrier interference (ICI) will occur in the received signal at the femtocell. In this paper, we show that the average performance of closed access OFDMA femtocell under timing misalignment depends on its distance to the mBS and on the number of mMSs. Simulation results show performance degradations in ergodic capacity and symbol error rate due to ISI and ICI induced by timing misalignment of the mMSs.

Opportunistic scheduling for wireless network coding (OWNC) combines the strategies of opportunistic scheduling and network coding. This paper applies two variants of OWNC on a two-user OFDMA system. Symbol-based OWNC decides a strategy from sum rate of the whole OFDM symbol. Subcarrier-based OWNC decides a strategy from the capacity on a subcarrier level. Novel algorithms are derived for power allocation on an OFDM symbol. They appear in the form of waterfilling algorithms but are specifically tailored to the OWNC. Simulation results show average capacity and outage capacity obtained by different approaches of the OWNC. It is shown that, for OWNC with two-user OFDMA, the proposed waterfilling power allocations have modest gains over uniform power allocation at low SNR.

Cooperative system has been receiving considerable attention recently as an effective approach to improve transmission reliability in slow fading wireless channels. In this paper, we exploit cooperative diversity in multiple relay networks to improve the performance of Gaussian source transmission with layered broadcast coding. Specifically, we study the distortion performance of two amplify-and-forward (AF) schemes, namely, distributed beamforming and selection AF. First, we analyze their asymptotic performance in terms of the distortion exponent and the expected distortion. We then show that, under stringent delay constraint, there exist SNR thresholds below which the AF systems achieve lower distortion than the direct transmission system.

Average and outage capacities for nonidentical two-user opportunistic scheduling for wireless network coding (OWNC) are derived in this paper. Nonidentical user case represents a more realistic scenario in which two users have different average signal-to-noise ratio (SNR). Our derived analytical results corroborate with the simulation results in the performance evaluation of OWNC. Insight on the performance of OWNC for the nonidentical user case is provided.

This paper provides an analytical formulation of average capacity and outage capacity for two-user opportunistic scheduling for wireless network coding (OWNC). It is shown that our derived capacity formulas can be easily evaluated without numerical integration and match exactly with the simulation results. At high SNR, it is shown that the average capacity of OWNC is double of logarithm (base 2) of average SNR subtracted by a loss term composed of the ‘scheduling loss’ and ‘concavity loss’ factors.

A novel algorithm called partial transmit sequence-active interference cancellation (PTS-AIC) is proposed to implement interference avoidance functionality in an ultra-wideband (UWB) OFDM device. It can be considered as another application of the PTS algorithm that has been applied to an OFDM transmission to reduce its peak-to-average power ratio. Specifically, PTS-AIC with adjacent subblock partitioning and interleaved subblock partitioning are proposed and compared. It is shown that the PTS-AIC can provide deep notch on the interference band. The x%-excess interference power is defined as a performance measure for PTS-AIC. Compared to existing approaches, PTS-AIC provides more flexibility in parametrizing the algorithm for trade-off between performance and complexity.

The performance of fast frequency hopping OFDM (FFH-OFDM) is evaluated under UWB channels with several types of receivers. It is shown that FFH-OFDM has a significant improvement over a conventional OFDM system. The receiver algorithms for the FFH-OFDM considered in this paper are zero-forcing (ZF), minimum mean square error (MMSE), QR- MMSE and sorted QR-MMSE (SQR-MMSE). Soft-output metric computation is proposed for the coded case that yields further gain over hard-output decision. The simulation results show the advantage of QR-MMSE and sorted QR-MMSE receivers in the uncoded case while MMSE has better performance for the coded case with a convolutional coding scheme. The insight on the performance advantage of MMSE over QR-based receivers for the coded case is also provided.

When an ultrawideband (UWB) transmitter is operating, it must not cause interference to existing primary users. Three new techniques are proposed in this paper in order to enhance the active interference cancellation (AIC), which is a technique to avoid causing the interference, for multiband-OFDM UWB transmitters. The first technique applies cyclic shifting while the second technique applies phase shifting to the frequency-domain data symbols. The third technique is a combination of these two techniques and is shown to improve the performance by about three orders of magnitudes. It is shown by simulation that the proposed techniques are able to enhance the performance of AIC significantly albeit higher complexity and a need for sending feed-forward bits.

Fast frequency hopping OFDM (FFH-OFDM) is a recently proposed concept to enhance conventional OFDM performance without using channel coding by fully exploiting frequency diversity available within one OFDM symbol. Previously, a linear receiver of FFH-OFDM requires zero-forcing (ZF) or minimum mean squared error (MMSE) detection algorithm. This paper introduces QR-based receivers for FFH-OFDM systems to further enhance the detection performance. By exploiting a special form of an effective channel matrix with the proposed scheme, a reduced complexity algorithm of QR decomposition is proposed. Simulation results indicate that the QR-based receivers outperform the MMSE and ZF receivers by about 4 dB and more than 5 dB at FER= 10-2, respectively, in an uncoded case. In a coded case, however, the MMSE receiver has similar performance to the proposed QR-based receivers.

Previously proposed technique called joint cooperative diversity and scheduling (JCDS) has been shown to increase aggregate throughput of multiuser OFDMA relay networks significantly while being able to maintain per-link throughput when the number of relays is high enough. This paper proposes a new technique so-called joint cooperative diversity and proportional fair scheduling (JCPFS) for the multiuser OFDMA relay networks which sacrifices a small amount of the aggregate throughput in return for significant improvement in the per-link throughput. Unlike JCDS, JCPFS provides increases in both aggregate and per-link throughputs as the number of users increases. The throughput performance of JCPFS is characterized by delay spread of the channels and its effect is shown by simulation.

Motivated by joint cooperative diversity and scheduling (JCDS) in frequency-flat fading relay systems, this paper proposes JCDS for frequency-selective fading relay systems with an OFDMA technique. While JCDS for frequency-flat fading relay systems uses time-varying phase rotation at the relays, JCDS in this paper uses fixed cyclic delay times instead. This technique can be considered as an extension of cyclic delay diversity in a conventional MIMO-OFDM system to a multiuser relay system in order to gain both multipath diversity and multiuser diversity. It is shown that JCDS using fixed cyclic delay has significant gains in aggregate throughput while per-link throughput can be maintained when there are enough relays. The effect of delay spread to the required number of relays and to the throughput performance are shown by simulation.

For emerging high data-rate communication systems in highly dispersive channels such as ultra-wideband systems, possible frequency offsets could be larger than the estimation range of the existing methods using training signals with several identical parts. This paper presents a novel improved maximum likelihood frequency offset estimator which can handle at least twice the estimation range of the existing methods using training signals with identical parts and achieves a better estimation performance. Based on the likelihood metric, a new design metric is introduced which is a pair-wise error probability (PEP) between the correct frequency offset point and a trial frequency offset point. The proposed PEP metric gives more theoretical insights on the performance of practical maximum likelihood estimators. How to design the PEP to achieve both a larger estimation range and a better estimation performance in fading channel environments is also presented and the corresponding estimator implementation is described.

This paper introduces a novel differential modulation scheme for a two-user cooperative diversity system which does not require channel state information at either the users or the destination. The performance of fixed decode-and-forward and selection relaying protocols is evaluated in both symmetric and asymmetric interuser-channel cases. The lower bound on the performance of the decode-and-forward protocol is given, while the exact bit-error probability of the selection relaying protocol is thoroughly derived. The decode-and-forward relaying protocol achieves a performance gain when the signal-to-noise ratios in the interuser channels are symmetric and sufficiently high. The selection relaying protocol shows a larger performance gain and does not exhibit an error floor like in the case of the decode-and-forward protocol. In addition, it is robust to the asymmetric interuser channels.

Cooperative transmission is an alternative approach to achieve diversity in wireless systems. So far the research has focused on the situation where perfect channel state information is available and coherent detection is employed at at the destination receiver. This paper proposes a differential modulation scheme for a two-user cooperative diversity system which does not require channel state information at either users or destination. The relaying protocols such as fixed decode-and-forward, selection relaying, and incremental relaying protocols are compared. The proposed scheme achieves spatial diversity gains when the signal-to-noise ratios between cooperative users are sufficiently high.

In this letter, concatenation of trellis-coded modulation (TCM) with differential space-time block codes (DSTBC) is considered under frequency-flat Rayleigh-fading channels with and without perfect interleaver. It is shown that the design criteria of TCM concatenated with DSTBC are exactly the same as that of TCM concatenated with space-time block codes (STBC) in perfectly known channels. They are effective code length over span two symbol intervals and minimum product-sum distance over span two symbol intervals. Based on the design criteria, several new rate-2/3 systematic Ungerboeck's TCM schemes have been found by computer search. They outperform existing optimal TCM schemes designed for additive white Gaussian noise or flat fading channels.

The usage of multichannel receivers that are followed by diversity combining can significantly improve the performance of wireless communications links. In this paper we investigate in some detail two popular techniques for diversity combining, namely maximal ratio combining (MRC) and equal gain combining (EGC). The main goal is to investigate under which circumstances it can be guaranteed that a certain technique does indeed decrease the probability of error of a multichannel receiver. The analyzed fading model is quite general with the only assumption that fading gains are scaled exchangeable random variables. The crucial mathematical tool used to produce the results of this paper is the powerful theory of stochastic majorization. Among other results, it was shown that the average bit error probability of the MRC receiver with equally distributed average powers always decreases with the addition of a new diversity branch if the total power is fixed.

Tarokh's differential space-time block codes (DSTBC) can provide a full spatial diversity and be decoded without channel state information. The bit error rate performance of a conventional differential detection has 3-dB degradation compared to a coherent detection in a quasi-static fading channel. Differential detection experiences more degradation in a fast fading channel and error floor appears at high SNR. In [E. Chiavaccini and G.M. Vitetta, May 2002], an approximate maximum likelihood receiver was derived and it reduced the error floor significantly. In this paper, we modify the receiver in [E. Chiavaccini and G.M. Vitetta, May 2002] and apply the concept of multistage decision-directed estimator with intrablock-interference cancellation to obtain further improvement. The proposed receiver works efficiently at moderate to high SNR and at the fading rate between f/sub d/T = 0.02 and f/sub d/T = 0.07.

Multiple symbol differential detection (MSDD) of space-time block code is considered in this paper. It is known that MSDD of space-time block code has very high complexity due to the block nature of the code. Therefore, two reduced complexity MSDD are proposed. The first scheme employs multiple differential feedback detection (MDFD). The second scheme, denoted as reduced search detection, works on reducing the number of symbols in the joint decision metrics. Simulation results show that MDFD is suitable for BPSK and reduced search detection is suitable for QPSK and 8-PSK. The improvement over differential detection is 1 dB with 11-block observation length with BPSK and 6-block observation length with QPSK and 8-PSK applied with reduced complexity schemes.

Trellis-coded modulation (TCM) is well-known for achieving coding gain in additive white Gaussian noise (AWGN) channels without sacrificing additional bandwidth. Block-coded modulation (BCM) has been studied as a counterpart to TCM. In this paper, we consider the possibility of applying BCM to a frequency-selective Rayleigh fading channel with soft-output equalizers. A comparison of error performance between TCM and BCM has been made. Our results show that BCM schemes have comparable performance to TCM schemes with less decoding complexity and the relative performance among coding schemes depends on the type of equalizers.