Adaptive Hierarchical Modulation Research Articles

Robust adaptive multi-service transmission with hierarchical modulation for OFDM systems in high mobility scenarios

Wireless communications play a key role in intelligent transportation systems for bearing multiple application services with different quality of service (QoS) requirements. Hierarchical modulation (HM) is an effective technique to increase the spectral efficiency (SE) of differentiated services multiplexing transmissions. In high mobility scenario, due to the impact of the feedback delay and channel estimation error, only the imperfect channel state information is available at the transmitter. Thus, the performance of the existing HM schemes degrades severely. In this paper, we propose a robust adaptive HM scheme to increase the overall SE in the mobile orthogonal frequency division multiplexing system. The bit error rate of the HM symbol is derived for the reliable QoS-aware multi-service transmission, and the strategies of bit assignment and modulation parameters selection for different services are designed. Finally, comparing with the traditional uniform modulation scheme, the proposed robust adaptive HM scheme increases the overall SE of multiplexing transmissions by $$15\%$$ at $$300\,\hbox { km}/\hbox {h}$$ .

Adaptive modulation for completion time minimization in wireless broadcast networks

In this work we study the effect of hierarchical modulation on completion time minimization in wireless broadcast channels. Time needed to transmit all bits intended for a particular user is defined as its completion time. In this work we investigate minimizing the maximum completion time over all users in a broadcast channel using hierarchical modulation. We compare time division, opportunistic scheduling, and three different adaptive modulation schemes: rate adaptive hierarchical modulation assisted transmission (HA), modified HA (MHA) and optimal HA (OHA). While HA aims at maximizing total throughput, OHA minimizes the maximum completion time, and MHA provides an intermediate solution. It is observed that OHA always chooses the best superposition order and the constellation size to minimize the maximum completion time.

Adaptive hierarchical modulation and power allocation for superposition-coded relaying

Abstract We propose a relaying scheme based on superposition coding (SC) with discrete adaptive modulation and coding (AMC) for a three-node wireless relay system, based on half duplex transmission, where each node decodes messages by successive interference cancelation (SIC). Unlike the previous works where the transmission rate of each link is assumed to achieve Gaussian channel capacity, we design a practical superposition-coded relaying scheme with discrete AMC while taking into account the effect of decoding errors at each stage of the SIC process at each node. In our scheme, hierarchical modulation (HM) is used to create an SC message composed of one basic and one superposed message with optimized power allocation. We firstly introduce the proposed scheme without forward error correction (FEC) for high signal-to-noise ratio (SNR) region and provide the optimal power allocation between the superimposed messages. Next, we extend the uncoded scheme to incorporate FEC to overcome bad channel conditions. The power allocation in this case is based on an approximated expression of the bit error rate (BER). Numerical results show the performance gains of the proposed SC relaying scheme with HM compared to conventional schemes, over a large range of SNRs.

Rate Adaptive Hierarchical Modulation-Assisted Two-User Opportunistic Scheduling

We propose and study a two-best user opportunistic scheduling scheme using fixed power discrete rate adaptive hierarchical constellations (known also as embedded, multi- resolution, or asymmetrical constellations). According to the classical opportunistic scheduling, the scheduler transmits information to the selected first best user in each transmission slot in order to maximize the spectral efficiency. Our newly proposed scheme transmits information not only to the first best user but also to the second best user if the channel qualities of the selected two users support. As such the frequency of channel access of the users is increased without any degradation of overall spectral efficiency compared to the classical single best user scheduling. The key idea is to rely on hierarchical constellations to transmit information to two best users simultaneously. Specifically, according to the channel quality of the first best user, the alphabet size of the square quadrature amplitude modulation (QAM) is selected in order to maximize the spectral efficiency. However, if the first best user's channel quality is good enough to decode the information transmitted in the second hierarchy of the selected size hierarchical QAM constellation while the second best user's channel quality is capable of decoding the information transmitted in the first hierarchy, information is transmitted to both the selected first and second best users using the hierarchical QAM constellation. Otherwise, information is transmitted only to the first best user using the selected size uniform QAM constellation. We present expressions for the information transmission probability and the overall average spectral efficiency [bits/sec/Hz] defined as the overall average transmitted data rate per unit bandwidth for a specified number of users, a specified average carrier-to-noise ratio (CNR), and bit error rate (BER). Numerical results for a fading environment where the channel variation among the users is identically and independently distributed (i.i.d.) are presented. These results show that, in such i.i.d. fading environment, our newly proposed fixed power discrete rate adaptive hierarchical modulation- assisted two-best user scheduling provides more frequent access to the information (almost double) without any average spectral efficiency degradation compared to the classical single best user opportunistic scheduling.

Adaptive Hierarchical Modulation for Simultaneous Voice and Multiclass Data Transmission Over Fading Channels

In this paper, a new technique for simultaneous voice and multiclass data transmission over fading channels using adaptive hierarchical modulation is proposed. According to the link quality, the proposed scheme changes the constellation size as well as the priority parameters of the hierarchical signal constellations and assigns available subchannels (i.e., different bit positions) to different kinds of bits. Specifically, for very bad channel conditions, it only transmits voice with binary phase-shift keying (BPSK). As the channel condition improves, a variable-rate adaptive hierarchical M-ary quadrature amplitude modulation (M-QAM) is used to increase the data throughput. The voice bits are always transmitted in the lowest priority subchannel (i.e., the least significant bit (LSB) position) of the quadrature (Q) channel of the hierarchical M-QAM. The remaining (log/sub 2/M-1) subchannels, called data subchannels, are assigned to two different classes of data according to the selected priority parameters. Closed-form expressions as well as numerical results for outage probability, achievable spectral efficiency, and average bit error rate (BER) for voice and data transmission over Nakagami-m fading channels are presented. The adaptive techniques employing hybrid binary shift keying (BPSK)/M-ary AM (M-AM) and uniform M-QAM for simultaneous voice and two different classes of data transmission are also extended. Compared to the extended schemes, the new proposed scheme is spectrally more efficient for data transmission, while keeping the same outage probability for voice and data (both classes) as the scheme employing BPSK/M-AM. The new scheme also provides, as a by-product, a spectrally efficient way of transmitting voice and a single-class data.