Most of the attention with femtocells is focussed on the 3G UMTS standard, which evolved from 2G GSM. However, other mobile radio technologies are also adopting femtocells. Let's look at these and run through the alphabet soup of acronyms for them.

GSM: The dominant mobile phone technology worldwide is GSM, which has over 85% market share of the 3 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. These have been termed picocells, rather than femtocells, because they were not completely self-installing and required the operator to fit and configure the equipment. 

UMTS: This 3G (3rd Generation) mobile phone system evolved from GSM by replacing the radio subsystem with one based on CDMA (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.

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 14Mbit/s are achievable in lab 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 femtocells has been focussed on HSPA because of the large potential market size and compatible handsets.

TD-SCDMA: This is a variant of the UMTS specification developed primarily by the Chinese. 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 countries are expected to adopt this technology outside China, but the size of that market alone has justified some chip vendors investmenting and demonstrating their capability.

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 high speed data rates.

LTE:  Both GSM and CDMA communities have 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 (Offset Frequency Duplex Modulation) scheme is particularly effective at combatting multi-path and other aspects where radio propagation is difficult. There will also be a major change to the core network standard, which is called SAE (System Architecture Evolution), and uses the SIP protocol to setup sessions and voice calls. The standards bodies are incorporating femtocell aspects from the beginning, which will make the compatibility of LTE handsets, terminals and femtocells work together more effectively and avoid some of the workarounds required for HSPA. Again, there have been demonstrations of LTE femtocells by chipset vendors such as picoChip.

WiMAX: This is also an OFDM technology and competes with LTE, although there are talks to combine the two. It is marketed as a low cost means of delivering broadband data services and is likely to be popular in areas where there are no landline alternatives, but will be competing with HSPA and LTE. Chipsets and designs exist for WiMAX femtocells, but few of the mainstream femtocell vendors are yet offering these products, waiting first to determine the likely market size.

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