What is 3G?
The 3G standard was introduced in Australia in 2005 and uses a new technology called UMTS as its core network architecture - Universal Mobile Telecommunications System. The network build upon the circuit-switched technology used in the 2G GSM network, with some new technology and protocols to deliver significantly faster data rates.
3G UMTS implemented a brand new protocol called HSPA - High Speed Packet Access. The protocol uses more complex modulation schemes to transmit data at a faster rate. Where a 2G channel size was limited to 200 kHz, 3G UMTS channel bandwidths were 5 MHz, allowing much greater capacity and faster data speeds.
UMTS is an umbrella term for the third generation radio technologies developed by the 3GPP. UMTS specifies a complete network system, for which the radio access component, known as UTRAN, uses WCDMA technology to offer greater spectral efficiency and bandwidth to mobile network operators. The system also includes the Core Network; the entity that interfaces to external networks including the public phone network and other cellular telecommunications networks.
While the UMTS standard supports both FDD and TDD duplexing, the overwhelming majority of 3G UMTS deployments use FDD frequency bands.
Packet Data Protocols
Since UMTS was introduced in 3GPP R99 the need for improved support for downlink data services has increased. High Speed Packet data Access (HSPA) has been an upgrade to WCDMA networks (FDD and TDD) used to increase packet data performance. The introduction was done in steps; High Speed Down Link (DL) Packet data Access (HSDPA), was introduced in 3GPP Release 5, and Enhanced Up Link (UL), also referred to as High Speed UL Packet data Access (HSUPA), came in Release 6.
The combination of HSDPA and HSUPA is referred to as HSPA. HSPA evolution known as HSPA+ came in Release 7 with further improvements in later releases.
Dual-Carrier HSPA+ capability was introduced by 3GPP in the Release 8 specifications, and enables a theoretical peak downlink data throughput capability of 42 Mbps. DC-HSPA+ systems combine 64QAM modulation and double the bandwidth by using dual carriers (2 x 5 MHz = 10 MHz). To improve uplink data rates, the implementation of 16QAM over QPSK modulation made it possible to double peak data rates to 11.5 Mbps. Dual-Carrier technology further extended uplink performance to 23 Mbps.
The 3G Switch-Off
3G UMTS is beginning to reach an end-of-life status in many countries. Valuable spectrum is being refarmed into more spectrally efficient 4G and 5G networks.
In Australia mobile operators began switching off their 3G networks in late 2019, beginning with the mostly unused B1 (2100 MHz) spectrum. Before 4G, 2100 MHz provided the capacity layer of the 3G network. It's short wavelength allowed greater densification of cell towers compared to the long range 850 and 900 MHz transmissions. Most operators also held more 2100 MHz spectrum and refarming provided an easy way to add another 200 Mb/s of capacity with even a 10 MHz channel.
Will coverage decrease when 3G is switched-off?
The coverage of a cell tower depends on a few things. The first consideration is the frequency band. Most countries have their long range 3G network running on either 850 MHz or 900 MHz. Australia like many have a low-band 700 MHz 4G network which actually has a longer wavelength, allowing the network to reach a greater distance. However, channel size is very important - the wider the channel size the more difficult it is to detect and communicate over. 3G uses a 3.84 MHz channel size, compared to 4G often with a 10 or 20 MHz channel width. Telstra 4GX 700 MHz for example uses a 20 MHz channel size, which explains why as you move further from the base station often the network switches back to 3G briefly before dropping out completely.
The large channel sizing would ordinarily produce a much worse result, except LTE's use of OFDMA provides a far superior receive sensitivity than WCDMA. In simple terms, this means 4G can be received at much lower signal levels - down to about -122 dBm compared to about -114 dBm for 3G. This produces a balancing effect, where the coverage radius of 4G B28 (700 MHz) is almost as good as 3G B5 (850 MHz).
In short, yes, there will be a decrease in coverage but only minor and only for a short period of time. Mobile operators will quickly refarm the valuable spectrum into 4G LTE and the narrower channel sizing of the spectrum will once again restore (and under some conditions enhance) coverage.