A Hierarchical Modulation for Upgrading Digital Broadcast Systems
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Published IEEE Transactions on Broadcasting, vol. 51, no. 2, 2005, pp. 223-229 A Hierarchical Modulation for Upgrading Digital Broadcast Systems Hong Jiang and Paul Wilford Bell Laboratories, Lucent Technologies Inc. 700 Mountain Ave, P.O. Box 636, Murray Hill, NJ 07974-0636 Abstract — A hierarchical modulation scheme is correction codes of different coding rates. It is desirable proposed to upgrade an existing digital broadcast system, for receivers in such a system to be capable of receiving such as satellite TV, or satellite radio, by adding more data at all of the specified bit rates. An advantage of data in its transmission. The hierarchical modulation such a system is that it is easy, at least in concept, to consists of a basic constellation, which is the same as in upgrade a deployed system to increase the bit rate, the original system, and a secondary constellation, which simply by switching to a modulation with a larger carries the additional data for the upgraded system. The constellation and/or an error correction code of higher upgraded system with the hierarchical modulation is backward compatible in the sense that receivers that have rate. A disadvantage of such an arrangement is that all been deployed in the original system can continue receivers have to be designed with capability of receiving data in the basic constellation. New receivers receiving all specified rates, and hence such receivers can be designed to receive data carried in the secondary may be expensive to manufacture. constellation, as well as those in the basic constellation. Many deployed systems, however, do not have such Analysis will be performed to show the tradeoff between flexibility; they are designed with a fixed modulation bit rate of the data in secondary constellation and the scheme, and error correction codes of fixed rates. penalty to the performance of receiving the basic Examples of such systems are the original ATSC constellation. terrestrial 8VSB system, and some digital satellite radio and TV systems. Oftentimes, after such a system is Index Terms — backward compatibility, bit rate, deployed and in service for a period of time, there are constellation mapping, digital broadcast, hierarchical rising needs to upgrade the system to provide more modulation, iterative decoding, local repeaters, penalty analysis, QAM, QPSK, receiver design, system upgrade services than the system was originally designed. The needs for upgrading the system may arise for many reasons. There is a market pressure for a service I. INTRODUCTION provider to add more services in order to differentiate itself from its competitors. New services may also Digital broadcast systems have increasingly been generate additional revenue. On the technical side, new deployed for services such as terrestrial digital TV, algorithms may have been developed to allow higher bit digital radio, satellite TV and radio, and digital cable rate to be transmitted through the same channel, and at TV. A digital broadcast system utilizes regulated the same time, advances in technology make the frequency bands with fixed bandwidths. The capacity of implementation of the new algorithms feasible and a digital broadcast system is limited by transmission cost-effective. This advance in algorithm development power of the system and channel impairments. Since in and technology makes it possible to build a new a broadcast system, the same data is transmitted to all generation of receivers that can have much better users, there is a tradeoff between transmitted bit rates performance under the same channel conditions. For and intended coverage areas. A digital broadcast system example, the utilization of iterative decoding algorithms is usually designed with a bit rate that can be reliably such as BCJR [1, 2, 3] and the MIMO technology [4] received by users in an intended coverage area for a can provide additional gains over receivers using given transmission power. traditional algorithms, or a single antenna. Some digital broadcast systems are designed with a Backward compatibility is a major difficulty in flexibility of transmitting with different bit rates by increasing the bit rate of a fixed rate digital broadcast allowing different modulating constellations (such as system that has been already deployed for a period of QPSK, 8PSK, 16QAM, 64QAM etc), and error time. Since the system has already been deployed, there Coded secondary information bits I I' Coded basic Secondary QPSK Mapper x(t) information Mapper Modulator (distance 2d ) Q Q' bits 1 (distance 2d2 ) Figure 1 QPSK/16QAM Modulation are a large number of receivers already used by minimum distances is given in Section III. Also, in subscribers that can only receive signals from the Section III, penalties of the hierarchical modulation to original system with the specific modulation and bit the originally designed receivers are derived. In Section rate. To replace these deployed receivers by new IV, a design of new generation receivers is presented generation of receivers may be prohibitively expensive. for the hierarchical modulation in which an iterative Therefore, any upgrade of the system must be backward decoding algorithm is used to provide additional compatible, in a way that the deployed receivers must channel gains for both basic and secondary information continue to operate in the upgraded system, even bits. The bit rate at which the secondary information though the deployed receivers may not be able to can be reliably received is also estimated. Finally in receive the supplementary data that is added to the Section V, we address the issue of the local information original system. transmission. In this paper, we propose a hierarchical modulation [5, 6, 10] method to increase the bit rate of a deployed digital broadcast system. The upgraded system is II. QPSK/16QAM HIERARCHICAL backward compatible, so that the upgrade is transparent MODULATION to the deployed receivers of the original system. The hierarchical modulation has already been included in Hierarchical modulations were initially proposed to the DVB-T standard [12]. provide different classes of data to users with different We will use the term basic information to mean reception conditions [5, 6, 10]. In that application, all data transmitted in the originally designed system, as is users within the coverage range can reliably receive customary in the literature of hierarchical modulations. basic information, while users under more favorable The data that is added in the upgraded system will be conditions can receive additional “refinement named secondary information. Thus, the originally information”. designed system with deployed receivers transmits In the context of the current application of basic information only, while the upgraded system upgrading a deployed QPSK system, the hierarchical transmits both basic and secondary information. QPSK/16QAM modulation can be described by the Although the concept of this paper may be applied block diagram in Figure 1. The basic and secondary to hierarchical modulation of any constellations, we information bits are channel encoded. The coded basic will limit our discussions to QPSK/16QAM hierarchical information bits are mapped to the QPSK constellation modulation in the remainder of this paper. The reason is in the same way as the original QPSK system. The twofold. First, most digital satellite systems use QPSK, minimum distance between points in the QPSK and due to its small constellation size, a QPSK system constellation is denoted by 2d1 . This represents the not only has the most urgent need for, but also can basic hierarchical constellation. The basic hierarchical benefit the most from the addition of a secondary constellation is next modified according to the coded information channel. Secondly, the use of a specific secondary information bits, and the combined hierarchical constellation, QPSK/16QAM in this case, hierarchical constellation is formed as shown in Figure simplifies the analysis. 2(b). The combined constellation is a 16QAM The remainder of this paper is organized as follows. constellation with the minimum distance between two In the next section, the hierarchical modulation for the points denoted by 2d . The mapping of the basic and basic and secondary information is described. An 2 secondary information bits indicated in Figure 2(b) is a analysis of the BER in terms of CNR and the ratio of Karnaugh map style Gray mapping. In Figure 2(b), the 2 two most significant bits (in bold face) represent the λ is an important parameter to characterize the system. basic information bits while the two remaining bits If λ = 0 , it is the QPSK system, transmitting only the represent the secondary information bits. 1 basic information. If λ = , it is the uniform 16QAM, Q 2 in which the basic and secondary information bits have same channel conditions. We will be only interested in 1 10 00 the case with 0 < λ < , where the constellation is truly 2 I hierarchical. When λ is small, the four points in each quadrant of the constellation form a “cloud”. To the originally designed receivers, a cloud represents a point in the 11 01 QPSK constellation. For example, any point in the first quadrant, (the ones labeled 0000, 0001, 0011, and 0010 (a) in Figure (b)) is treated as the point 00 in the QPSK constellation. The variation among the points in a cloud Q' has the same effect of white noise on the originally 1000 1010 0010 0000 designed receivers. Therefore, in the upgraded system after the secondary information is added, the originally designed receivers will continue to operate and receive 1001 1011 0011 0001 the basic information bits, with the only difference being that they may operate at a higher noise level. I' The additional noise due to the secondary 1101 1111 0111 0101 information in the upgraded system imposes a penalty on the performance of originally designed receivers. 2d We will analyze the penalty in terms of the hierarchy 1 parameter λ in the next section.