Mobile Wireless Systems and Evolution
Since the dawn of modern communications, consumer adoption and usage growth have been predicated on unique content, services, and applications. These services began with the first telephony networks, and have evolved to include video services such as broadcast television and video-on-demand, as well as data communications such as Internet browsing and instant messaging. A common thread in this evolution has been the broad-based adoption of the Internet Protocol (IP) as the network-layer protocol for the delivery of these services and applications.
So, too, have wireless communications evolved from a voice-centric architecture and service offering to one in which users have access to browsing services (based on Wireless Application Protocol [WAP] or Hypertext Transfer Protocol [HTTP]), video (both live streaming video and video communications), and transactional services, all complemented with features and functions that only a mobile network and device can provide. With this, mobile services are unique in both content and experience.
This article is part of a four-part series that will explore the wireless industry, starting with a basic understanding of the evolution of the mobile network itself, from first-generation (1G) cellular network to fourth-generation (4G) high-speed mobile broadband networks.
First-Generation Wireless Systems (1G)
First-generation wireless systems, known as 1G, were designed as a mechanism to carry low-quality voice traffic. These analog, circuit-switched systems began deployment in the late 1970s with the predominant technologies being Advanced Mobile Phone Service (AMPS) in North and South America, and Total Access Communications Systems (TACS) in Europe.
All wireless networks rely on Multiple Access technologies designed to support multiple users sharing a single physical communications resource. Different channelization protocols have been developed to optimize this support. First-generation wireless networks used Frequency Division Multiple Access (FDMA) to allow a number of users to share the available frequency by segmenting the frequency block into smaller subcarriers, and allocating those subcarriers on a per-user basis. The figure below illustrates how FDMA would allocate a subcarrier to a particular user in a 1G wireless system.
FDMA systems are the least efficient systems for Multiple Access due to a number of reasons:
Second-Generation Wireless Systems
Second-generation (2G) wireless systems, were designed to optimize and improve the performance and scalability of voice traffic, as well as preserve the most “precious” asset to a wireless carrier—spectrum. Relying on digital channels over a circuit-switched infrastructure for transmission of voice traffic, 2G networks used either Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) for channelization.
Digital channels provided the first real advantage over 1G wireless systems—specifically, because digital signals are less susceptible to noise. All voice traffic passes through a voice encoder, or vocoder, that outputs a digital representation of the analog voice. Although the digital signal will also pick up noise when converted to an electromagnetic signal, the digital signal is still clearly identifiable. This is because the digital signal only needs to be readable as a 1 or a 0 to be processed. The figure below illustrates a digital signal.
TDMA technology alleviates the channel capacity issues of FDMA by dividing the channel into individual timeslots and allocating a timeslot per user. The most pervasive 2G voice system—Global System for Mobile Communications (GSM)—assigns eight timeslots per channel. Since accurate timing is needed to ensure accurate communication, the cellular network notifies the handset of transmission times in order to offset the impact of propagation delay. In order to ensure no overlap between handset transmissions, a guard band, or interval during which no information is sent or received, is allocated between each timeslot. The number of voice calls supported in a TDMA network is a function of the number of timeslots available per channel, the size of each timeslot, and the number of bits that can be transmitted within that timeslot (spectral efficiency). The figure below illustrates how TDMA would allocate a timeslot to a particular subscriber in a 2G wireless system.
CDMA technology alleviates the channel capacity issue of FDMA in a completely different way—each voice call is allocated a unique “code” prior to being put on the radio channel. Each cellular device transmits using this unique “code.” Since there is no sub-division of a particular channel, all cellular devices receive all voice calls, but treat any information with a different code as noise and filter it out. The noise level, which is a function of background noise plus the noise created by every call within the channel, is the limiting factor as to how many voice calls can be supported in a CDMA system.
Second-generation wireless systems were at the forefront of the cellular voice revolution, providing enhanced capabilities similar to those previously only found on a landline voice network, including caller ID, call waiting, etc. These wireless systems also brought about the beginnings of cellular data with Short Message Service (SMS). SMS, or test messaging, provides a method of transmitting textual data via the existing voice network. Over 1 trillion SMS messages are sent each year. In addition, low-speed circuit-switched data solutions, such as General Packet Radio Service (GPRS) and CDMA-1x, evolved as 2.5G wireless systems overlaid on existing 2G.
Third-Generation Wireless Systems
Third-generation (3G) wireless systems are the most commonly deployed networks today, and are at the forefront of the cellular data revolution. With 3G wireless, subscribers are not only able to place and receive voice calls, but are also able to use available resources for data services, including picture messaging, web browsing, email, and mobile video. These all-digital wireless systems allow for significantly higher data transfer rates than 2G systems while still relying on the same channelization protocols—CDMA and TDMA. For this reason, 3G systems are evolutionary from previous generation technology.
GSM networks have evolved from TDMA-based solutions for voice to what is now known as the Universal Mobile Telecommunications System (UMTS) High Speed Packet Access (HSPA), based on CDMA technology for data communications. The CDMA networks have evolved to what is now known as CDMA Evolution – Data Only (CDMA EVDO), or just EVDO.
Fourth-Generation Wireless Systems (4G)
Fourth-generation (4G) wireless systems are at the peak of the “hype” curve. These networks, such as Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE), promise data rates in excess of 2Mbps per subscriber. Unlike 3G networks that provided an evolutionary step from previous generation technologies, 4G networks are revolutionary in their implementation. End-to-End, these systems are designed to be packet-switched networks, and rely on Orthogonal Frequency Division Multiplexing (OFDMA) to provide channelization.
OFDMA provides a sub-carrier model similar to that of TDMA by dividing a channel into multiple carriers, but also provides the ability to group a number of sub-carriers together to create a sub-channel. These sub-channels adapt dynamically to changing conditions in the network. Using this sub-channelization, a mobile system can allocate more radio resources to devices that have better transmission capabilities, and less radio resources to others through the use of different modulation schemes. The sub-channelization also allows for the OFDMA system to scale to greater capacity than TDMA or CDMA, since each channel may be divided into thousands of sub-channels. The figure below illustrates how OFDMA would allocate a timeslot to a particular subscriber in a 4G wireless system.
The “Mobile Internet”
As technologies converge, specifically around multi-access channelization schemes, the “Mobile Internet” model is beginning to take shape. This Mobile Internet relies on any number of access technologies (WiFi, 3G, 4G, etc.) to provide a ubiquitous and seamless wireless experience for end devices. This gives rise to numerous concerns regarding connection management, coverage, network capacity, and protocol-specific mobility requirements. Future articles in this series will cover these topics in more depth.
If you have any questions regarding the topics discussed in this article, you can email questions to Kevin from now through July 2, 2009.