# Review of Channel Estimation for Candidate Waveforms of Next Generation Networks
## 1 Introduction  
The speedy growth of wireless communication in recent years has placed a huge demand on the available frequency spectrum, making this resource a scarce commodity [1; 2]. The concept ofmulti-carrier modulation (MCM) is usually adopted in modern communication systems so as to ensure efficient utilisation of the scarce frequency spectrum and yield higher system throughput. In MCM technologies, wideband signals are split at high symbol rate into multiple lower rate signals, where the low rate signals are modulated onto narrowband orthogonal sub-channels (sub-carriers) for the simultaneous transmission of data over the wireless channel [3]. MCM schemes offer more benefits than their single carrier modulation (SCM) counterparts. One benefit of MCM schemes over SCM schemes is in their ability to offer higher spectral efficiency mainly due to the channel division as well as overlapping of the orthogonal sub-carriers. For efficient sharing of the scarce frequency spectrum and for higher data rate applications, several wireless channel access methods have been developed over the years. Multiple access methods provide an efficient way for several communication users to effectively share the same spectrum resource without causing interference with one another. In wireless networks, these access schemes are generally divided into two basic groups: contention-free channel access schemes and contention-based random channel access schemes [4; 5]. In contention-free access schemes, a central entity allocates several nodes using radio resources such as time slots, frequency and code, where data are transmitted through these nodes by the use of the allocated radio resource. Examples of contention-free channel access schemes include frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA) schemes. On the other hand, the nodes in contention-based random channel access schemes compete with one another for the transmission of data over the wireless channel. The data in contention-based random channel access schemes are usually received without collision. Carrier sense multiple access (CSMA) and Aloha method are examples of the contention-based random channel access schemes. In FDMA schemes, each user is allocated a frequency channel where they can transmit their respective signals at the same time over the available spectrum resource while multiple users are allowed to transmit their signals at the same frequency over the available frequency spectrum at different time slots in TDMA schemes. CDMA schemes are multiple access technologies that allow multiple users to simultaneously and asynchronously access a spectrum resource by modulating and spreading their information signals with pre-assigned signature codes or spreading codes which are orthogonal to one another [6]. CDMA offers several advantages over FDMA and TDMA such as improvement in spectral efficiency, larger system capacity, higher security and better anti-interference ability [7]. The first to the third generation of wireless networks is built based on the contention-free channel access schemes. The first generation (1G) of wireless networks became operational in 1979 [7]. It is known as analogue cellular systems because the design of these cellular systems is based on FDMA and analogue frequency modulation technology. The second generation (2G) of wireless systems is designed based on either TDMA or CDMA technology while the third generation (3G) of wireless networks is based on CDMA, where air interface standards such as wideband-CDMA (WCDMA), CMDA 2000 and time division synchronous CDMA (TD-SCDMA) were developed. In recent decade, an improved version of FDMA scheme has been proposed and developed to offer higher data rates, improved spectral efficiency and better anti-interference ability. This access scheme is known as orthogonal frequency division multiple access (OFDMA) with cyclic prefix (CP).