You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Body-Centric Wireless Communications: A Survey of Hearing-Instrument Antennas 
Chapter Title:  IV. HEARING-INSTRUMENT ANTENNAS 
1. Abstract of Cited Paper (BIB001):  Two antenna configurations are considered for ear-to-ear on-body communications at 2.45 GHz. Both consist of monopole antennas operated on small ground planes that are placed next to the human head. Their performances are compared in terms of maximum path gain (|S 21 |) and obtainable bandwidth of the antenna structures. It is found that the bandwidth, as well as the ear-to-ear path gain can be improved by the addition of a parasitic monopole antenna element. The parasitic antenna element affects the electric current distribution on the ground plane, which has a favorable impact on the on-body antenna polarization. 
2. Abstract of Cited Paper (BIB002):  A balanced PIFA-inspired antenna design is presented for use with the 2.45 GHz ear-to-ear radio channel. The antenna is designed such that the radiated electric fields are primarily polarized normal to the surface of the head, in order to obtain a high on-body path gain (|S21 |). The antenna structure can be made conformal to the outer surface of a hearing instrument, such that the bandwidth of the antenna is optimized given the available volume. The radiation patterns, ear-to-ear path gain and available bandwidth is measured and compared to the simulated results. It is found that the antenna obtains a relatively high ear-to-ear on-body path gain, as well as a bandwidth that is large enough to cover the entire 2.45 GHz ISMband. 
3. Abstract of Cited Paper (BIB003):  A novel concept for an electrically small on-body antenna targeted for 2.4 GHz ISM band custom in-the-ear (ITE) hearing instrument (HI) applications is introduced. The antenna is based upon a cavity-backed design in order to take advantage of the maximum volume available ITE while providing isolation from the user’s body, and it occupies only $40\%$ of the volume of the sphere with radius $a=12 \,\text{mm}$ . The antenna is implemented on a realistic three-dimensional (3-D)-printed lossy substrate and exhibits high efficiency of 70% and 22%, and a 6-dB impedance bandwidth of 108 and 149 MHz, when the antenna is measured in free space (FS) and ITE, respectively. A measurement campaign conducted in FS and on a specific anthropomorphic mannequin (SAM) head with ears shows that the radiation pattern is optimal for HI applications. Furthermore, the antenna is primarily polarized normal to the surface of the head to ensure the best on-body path gain (PG). This is substantiated by the study of the ear-to-ear (E2E) PG, which is measured and compared to analytic and numerical results.

Literature Review:
To obtain a low path loss, i.e., high on-body path gain (|S 21 |) the on-body antenna is oriented such that the electric field is perpendicular to the surface of the head. This will ensure an efficient launch of a creeping wave BIB001 . Further, the antenna should radiate along the surface of the body. Many hearing-instrument antennas have been presented in the literature, e.g.,  , BIB002 , BIB003 - BIB004 . However, to be useful, e.g., in HIs, the antenna must be physically small in order to fit the devices-but not necessarily electrically small at 2.4 GHz. Typical measured fractional bandwidths are in the range 1-10%. Examples of hearing-instrument antennas for body-centric wireless communications are shown in Fig. 3 . V. CONCLUSION The main types (BTE, RIE, ITE, and CIC) of hearing instruments have been introduced. The different hearing-instrument antenna types used for body-centric wireless communications at the 2.4 GHz ISM band are presented. The ear-to-ear propagation communication relies on creeping waves on the surface of the head.