Most of the attention with small cells is focussed on the 3G UMTS standard, which evolved from 2G GSM. However, other mobile radio technologies are also adopting small cells. Let's look at these and run through the alphabet soup of acronyms for them.
2G GSM: The dominant mobile phone technology worldwide is GSM, which has over 95% market share of the 7 billion (and growing) subscriptions worldwide. There have been small GSM basestations for many years, with independent vendors such as ip.access providing complete solutions such as those now being installed onboard aircraft and ships. Historically, these have been termed picocells, rather than femtocells, perhaps because they were not completely self-installing and required the operator to fit and configure the equipment. They are more commonly found in business/enterprise and for maritime use on ships; residential GSM small cells have not been commercially significant.
3G UMTS: This 3G (3rd Generation) mobile phone system evolved from GSM by replacing the radio subsystem with one based on WCDMA (Wideband Code Division Multiple Access), which offers higher capacity and performance than 2G. By squeezing more phone calls into the same spectrum, fewer cellsites are required and/or higher data rates can be achieved. Almost all UMTS networks are owned or directly interwork with an existing 2G GSM network, so that in areas of poor coverage calls can be handed over and continue on the other network.
3G HSPA: Often termed 3.5G, this is an improved version of UMTS by increasing the coding used on the radio transmissions to dramatically improve the data throughput. Peak rates of 42Mbit/s are achievable in ideal conditions, with even higher capacity promised. These systems are completely backward compatible with the original UMTS systems, although newer handsets or data dongles would be required to take advantage of the higher data rates. Most of the attention of small cell has been focussed on HSPA because of the large potential market size and compatible handsets. (More details on 3G UMTS/HSPA residential femtocells).
3G TD-SCDMA: This is a variant of the UMTS specification developed primarily by the Chinese and in operation by China Mobile. It uses TDD (time division duplexing) and so can efficiently adapt to handle situations where the proportions of upload and download data traffic are not balanced - for example when web surfing. At this time, few if any countries are expected to adopt this technology outside China, but the size of that market alone has justified some chip vendors to invest and demonstrate their capability. (More details on TD-SCDMA femtocells.)
3G CDMA: Not to be confused with Wideband CDMA, this earlier technology was popular in the US, Japan and Korea but did not achieve the global takeup it had hoped. The 2G version of CDMA is known as 1xRTT and is efficient for voice and text services. The 3G version called EV-DO provides higher speed data rates of a few megabits.
4G LTE: Both GSM and CDMA communities jointly agreed to move toward a common standard for their next step. Long Term Evolution (LTE) is their 4G standard and the radio interface has already been demonstrated achieving over 100Mbit/s. The OFDM (Orthogonal Frequency Division Modulation) scheme is particularly effective at combatting multi-path and other aspects where radio propagation is difficult. There is also be a major change to the core network standard, which is called SAE (System Architecture Evolution), and radically changes the way voice calls are handled. The SIP protocol is used to setup sessions and voice calls through an IMS core. The standards bodies incorporated small cell and femtocell technology aspects from the beginning, ensuring compatibility between LTE handsets, macrocells and small cells work and avoiding some of the workarounds required for HSPA. Residential, enterprise and outdoor LTE small cell products are already available on the market.
LTE-A: (LTE Adanced). A range of additional features and advanced capabilities for LTE, increased speed and capacity. The most popular feature is probably Carrier Aggregation (CA) which bonds two or more different frequency bands together to achieve faster speeds of 300Mbps or more. Other features such as CoMP (Co-Ordinated Multipath) achieve high spectral efficiency for specific areas of peak demand but require very tight phase timing co-ordination between cellsites. These different optional features can be adopted piecemeal and don't all have to be deployed or used as a complete package.
Wi-Max: This radio interface was designed to be used in unlicensed spectrum, enabling mass market low-cost wireless connectivity witout some of the IPR (Intellectual Property Rights) costs of other standards. Although deployed in many smaller networks, the technology was not as widely adopted and failed to reach mass market penetration. Vendors and operators have almost entirely abandoned it for LTE.
Wi-Fi: Not shown on the chart because not a wide area radio technology. Wi-Fi is good for nomadic/static use - sessions are not actively handed off if you move between access points. There have been many updates and revisions to the various 802.11 series of specifications, increasing the speed, security and performance of the system. Popular for use at home and in the enterprise, the technology can be less useful in public or shared areas where many operators and others contend for the same spectrum. The latest standard 802.11a/c and next generation hotspot combined with commercial procedures for operators should enable more seamless and secure access to Wi-Fi when away from base. Network operators are actively deploying large number of Wi-Fi hotspots and many small cell products now also include Wi-Fi. Spectrum use at 60GHz, previously called Wi-Gig, has now also been incorporated into the same standard and promises to achieve 1Gbps rates or more at very short range within the same room.
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