Technology Evolution of Wireless Communications: A Survey and Look Forward
Communication networks and systems have been evolving at a rapid pace over the last decade. Ever since their inception, their scope and utility have also evolved tremendously - from basic speech communication to short texts to high-speed Internet connectivity to multimedia applications. At the heart of this evolution has been a transition from a carrier-centric approach toward a user-centric approach. The next generation of wireless systems will be a heterogeneous mix of multiple technologies, which will not just be competing with but also complementing one another, providing end users immersive experiences in real time along with innumerable applications. To provide such experiences and utility, the next generation of communication systems must satisfy stringent requirements such as high speed (10's of gigabits per second), submillisecond latency, and high density of connected devices while ensuring high quality of experience (QoE) for end users.
The next generation of wireless systems will be a heterogeneous mix of several communication technologies, such as millimeter wave (mmWave) communication, terahertz (THz) communication, and visible light communication (VLC), in addition to evolved versions of existing systems, which are expected to complement and cooperate with one another. Several wireless systems are emerging as candidate technologies for satisfying the requirements of next generation communication systems, which are required to provide connectivity not just in the traditional sense, but also in the rapidly emerging area of Internet of Things (IoT). To fulfill the requirements of next generation communication systems many different techniques need to be implemented. These techniques which fall into the scope of signal processing, coding theory, spectrum management, multiantenna systems (MIMO and massive MIMO), cloud computing, artificial intelligence, machine learning/deep learning, and security will be the enablers for future wireless systems.
Historical Background and Evolution of Wireless Systems
Subscriber numbers of mobile and wireless communication systems have witnessed enormous growth since their introduction. After the launch of Nordic Mobile Telephone (NMT), the first-generation analogue mobile system, the first digital cellular network was Advanced Mobile Phone Service (AMPS). Global System for Mobile (GSM) Communications was launched in 1991 and cellular subscriber numbers grew to 100 million in 1998 with figures reaching one billion in 2002 and two billion in 2005. By the end of 2007, cellular subscriptions exceeded three billion and network coverage extended over 80% of the world population. Mobile communication is therefore considered to be the enabler to bridge the so-called Digital Divide between the nations of the world. (Usman 2015). Such growth was possible due to mobile telephony and a vast array of affordable data services, enabled with the launch of packet-switched systems such as General Packet Radio Service (GPRS) in 2000 followed by 3G systems such as Wideband Code Division Multiple Access (WCDMA) in 2001. The 3GPP (Third Generation Partnership Project) family of standards gave a clear, cost-efficient road map from basic GSM voice and GPRS data services to true mobile broadband services based on Enhanced Data rates for GSM Evolution (EDGE) and WCDMA/HSPA technologies.
Second-generation (2G), systems which were based on time division multiple access (TDMA) offered low-rate data services in addition to conventional speech communication. GSM was the most popular 2G system holding 81.2% of global market share of active digital mobile subscriptions. GSM was used in most parts of the world except in Japan, where Personal Digital Cellular (PDC) was the 2G system used. GPRS, the packet-switched solution to GSM, provided data rates of up to 20
kbps per time slot. By using multiple time-slots per user in the downlink, attractive services were offered. GPRS was an important evolutionary step in realizing the idea of always connected, ubiquitous Internet with multimedia and real-time traffic. GPRS evolved to EDGE, which provided data rates three times that of GPRS. EDGE utilized higher-order modulation together with Link Adaptation (LA) and Incremental Redundancy (IR) to improve the data rate. (Usman, 2015). Such has been the rapid pace of development of wireless communication systems, that pre-3G systems which were state of the art only a couple of decades back, seem ancient in the present times.
While mobile communication systems continued to evolve and grow, the Internet, which brought multimedia applications, also developed at a staggering rate. The convergence of wireless systems and the Internet became necessary to realize mobile data communications that would enable new services such as location-based services. Such services are not meaningful in a fixed network, and the first attempt to realize such a convergence was 3G, referred to as Universal Mobile Telecommunication System (UMTS). Broadband connectivity helped make the Internet a richer experience with a higher utility factor. Much of the developed w'orld relied on fixed-line telecom networks to deliver broadband services, but emerging markets leapfrogged to using new mobile-based broadband technologies. High Speed Packet Access (HSPA), an upgrade to UMTS has by far been the most successful technology to deliver mobile broadband services. With around 900 million mobile broadband subscribers in 2012, more than 70% were being served by HSPA networks (Ericsson 2007). The mobile broadband system evolving from CDMA-2000 is called CDMA EV-DO (Evolution - Data Only). HSPA had the advantage that it could be built using the existing GSM networks and was a software upgrade of installed WCDMA networks. Another technology that competed to provide mobile broadband is IEEE 802.16e mobile Worldwide Interoperability for Microwave Access (WiMAX). HSPA and mobile WiMAX both used similar techniques to satisfy certain criteria required of mobile broadband systems - high data rates, low latency, good quality of service (QoS), good coverage, and high capacity. While HSPA and mobile WiMAX have comparable performance in certain areas such as peak data rates and spectral efficiency, the coverage range of HSPA was superior. Mobile WiMAX therefore, didn't have much technology advantage over HSPA and HSPA became the clear choice for mobile broadband services (Ericsson 2009). A major challenge is to keep the price of mobile broadband sendees low by making it imperative to utilize the available resources efficiently. Spectrum is a scarce and expensive resource in wireless communications and wireless channels are also the most challenging part of the radio network due to their inherent characteristics.
A rich variety of services has become possible with the advent of mobile broadband. Broadcast/multicast of high-resolution multimedia content, real-time streaming, virtual reality, online multiplayer gaming, real-time services in the cloud, and Internet of Things are examples of such services. Some of these service impose stringent requirements on the wireless communication systems in terms of data rate, latency, user density, and mobility to satisfy the end user's QoS) and QoE. To cater to these requirements the transition from 3G to 4G was made via an intermediate step called HSPA+. which could be referred to as 3.5G/3.75G. HSPA+ is an incremental upgrade to existing 3G networks as a migratory path toward achieving 4G speeds.
Average real-world download speed of 3G is around 2-3 Mbps while that of HSPA+ is around 6 Mbps with upload speeds of 0.4 Mbps and 3 Mbps, respectively (“How Fast Is 4G?” 2020). Of course, these vary across operators. The underlying technology in 3G UMTS is Wideband Code Division Multiple Access (WCDMA) and frequency division duplexing (FDD) for downlink and uplink transmissions. HSPA+ is also based on WCDMA but uses additional technologies - higher-order modulation schemes such as 16-quadrature amplitude modulation (QAM) and 64-QAM, multiple-input multiple-output (MIMO) schemes with beam-forming techniques with a further improvement to support dual-carrier (dual-cell) implementation (Rysavy 2006).
Although HSPA+ delivered significantly higher bit rates than 3G technologies, the opportunity remained for wireless operators to capitalize on the ever-increasing demand for wireless broadband, even lower latency, and multimegabit throughput. The solution is LTE (3GPP Long Term Evolution), the next generation network beyond 3G, which needs to satisfy the International Mobile Telecommunications - Advanced (IMT - Advanced) specifications to qualify as 4G technology. LTE encompasses the pillars of next generation networks:
>• Broadband wireless as the new access reality with download speed of at least 100 Mbps, upload speed of at least 50 Mbps, and latency of less than 10 milliseconds.
>• Achieving the aforementioned data rate even at high mobility (even traveling in cars and trains up to 350 km/h)
>• Convergence of technology and networks.
> Technology shift to all-IP (Internet protocol).
>• Download date rate of 1 Gbps for stationary and pedestrian users.
>• Support device-to-device (D2D) or machine-to-machine (M2M) communication in which the transmitter and receiver are both moving (even relative to each other).
Two key enabling technologies that enabled “beyond-3G” networks to achieve their objectives are orthogonal frequency division multiple access (OFDMA) and MIMO, the combined use of which improved spectral efficiency and capacity of wireless networks (Nortel Networks, 2008).
While several operators marketed HSPA+ as 4G, it could be designated as 3.75G at most. The first commercial launch of 4G LTE was in 2009 with OFDMA as the underlying technology, and it was completely IP based. Typical real-world download and upload speeds of 4G LTE are 20 Mbps and 10 Mbps, respectively. A trend that could be noticed in the download and upload speeds across different generation of wireless systems is the asymmetry between download and upload speeds, which became smaller and smaller moving toward newer generations of wireless systems. While 4G LTE does not satisfy the requirements of IMT - Advanced, a consensus was reached in 2010 to nevertheless consider LTE as 4G, based on other factors.
Some of the key technologies that help 4G achieve its performance metrics are MIMO-OFDM, frequency domain equalization techniques, transmit/receive and spatial diversity, smart antennas with beam-forming, IPv6 support, advanced and adaptive modulation and coding schemes, software-defined radio (SDR) to support diverse wireless standards, and multiuser MIMO (MU-MIMO). The initial version of LTE supported 2x2 MIMO for both downlink and uplink and later versions extended support for up to 4x4 MIMO (downlink). The number of antennas on the user equipment (UE) is constrained by the size of the devices. To exploit some of the benefits of MIMO, the antennas need to be spaced half a wavelength apart. The main LTE bands are centered around 1.8, 2.1. and 2.6 GHz. At these frequencies, a half-wave- length ranges from 5 to 8 cm, which should be the separation between antennas to get good spatial separation of the signals.
While LTE performance is significantly improved compared to 3G-HSPA, it still does not satisfy the requirements of IMT-A. An enhanced version of LTE called LTE- Advanced (LTE-A) was released in 2011, which achieves performance closer to IMT-A requirements. The download and upload speed on real-world LTE-A deployments are 42 Mbps and 25 Mbps, respectively, which still fall short of IMT-A requirements. LTE-A improves on LTE by adopting 8x8 MIMO for downlink, 4x4 MIMO for uplink, carrier aggregation, and higher-order modulation up to 256 QAM. LTE-A further supports heterogeneous networks with macrocells, picocells, and femtocells configuration. The limitations of LTE-A (for the kind of applications being envisioned) are latency, which averages 50 ms (can be around 5-10 ms in best-case scenario), and user density (2,000 devices per sq. km). A summary of performance metrics for different wireless systems is listed in Table 1.1 (“4G LTE Advanced,” 2020). It should be noted that these are metrics for best-case scenario and are poorer for typical real-world scenarios.
It becomes clear from the previous discussion that a host of different technologies act as enablers in approaching the performance requirements of wireless systems to support a variety of different applications for end users.