GSM, GPRS, EDGE, UMTS, HSPA, LTE and WiMAX are successive generations of mobile radio standards. GSM (2G) carried digital voice and slow circuit data; GPRS (2.5G) and EDGE (2.75G) added packet data on the same GSM carriers; UMTS (3G) moved to wideband CDMA and HSPA pushed it to tens of megabits per second; LTE and WiMAX (4G) switched to OFDM and wider channels. 5G NR, the current generation, builds on the LTE toolkit with much wider channels, new spectrum and a new core network.
This is a crash course in the fundamentals of mobile standards. It does not matter whether you are a common man trying to understand the jargon or a professional trying to get a grip of things: it should get you started. Please leave a comment if any topic is unclear and I will be happy to clarify.
To understand GSM timeslots and frequencies in more detail, you can read here.
Key takeaways
- Each generation changed the way users share the radio channel: TDMA for GSM, CDMA for UMTS and CDMA2000, OFDMA for LTE, WiMAX and 5G NR.
- GPRS and EDGE are upgrades to GSM, not new networks. They reuse the same 200 kHz carriers and eight timeslots, with better coding (GPRS) and 8-PSK modulation (EDGE).
- HSPA and HSPA+ are upgrades to UMTS. The big jumps came from shared high-speed channels, higher-order modulation and MIMO.
- Peak rates in standards are theoretical. Real throughput depends on how many users share the cell, signal quality and what the handset category supports.
- LTE won the 4G race; mobile WiMAX has largely been switched off. 5G NR is now the mainstream generation, and many operators have already retired 3G.
The generations at a glance
| Standard | Generation | Access method | Channel width | Headline peak downlink | First commercial use |
|---|---|---|---|---|---|
| GSM | 2G | TDMA (8 slots per carrier) | 200 kHz | 9.6 to 14.4 kbit/s per circuit data channel | 1991 (Finland) |
| GPRS | 2.5G | TDMA, packet switched | 200 kHz | About 85 kbit/s with 4 slots (CS-4) | About 2000 |
| EDGE | 2.75G | TDMA, 8-PSK | 200 kHz | About 237 kbit/s with 4 slots (MCS-9) | 2003 |
| UMTS (WCDMA) | 3G | CDMA | 5 MHz | 384 kbit/s in practice (Release 99) | 2001 (Japan) |
| HSDPA / HSUPA | 3.5G / 3.75G | CDMA, shared channel | 5 MHz | 14.4 Mbit/s down, 5.76 Mbit/s up | 2005 / 2007 |
| HSPA+ | 3.9G | CDMA, 64QAM, MIMO | 5 MHz (or 2 x 5 MHz) | 42 Mbit/s (later releases more) | About 2009 |
| LTE | 4G | OFDMA down, SC-FDMA up | 1.4 to 20 MHz | About 150 Mbit/s (2x2 MIMO, 20 MHz) | 2009 (Sweden, Norway) |
| Mobile WiMAX | 4G | OFDMA | Up to 20 MHz (commonly 10 MHz) | Tens of Mbit/s | About 2006 to 2008 |
| 5G NR | 5G | OFDMA (both directions) | Up to 100 MHz (FR1), 400 MHz (FR2) | Several Gbit/s | 2019 (South Korea, USA) |
GSM: the 2G baseline
GSM stands for Global System for Mobile Communications (it began as Groupe Spécial Mobile, a European working group). It became the most widely used 2G standard in the world. In Europe a 1987 EU directive reserved the 900 MHz band for GSM, which is why it was effectively the only 2G system deployed there; that directive was relaxed in 2009 so the band could also be used for 3G and later 4G. As a digital standard, GSM carries both voice and data, although the circuit-switched data rate per channel is only 9.6 kbit/s, or 14.4 kbit/s with a lighter coding scheme.
North America, Korea and pockets worldwide used another 2G standard based on CDMA technology. This protocol, cdmaOne, was developed by Qualcomm and is also known as IS-95 or, more commonly, CDMA.
How GSM shares the channel
GSM is based on time-division multiple access (TDMA). Each 200 kHz carrier is divided into eight timeslots, and each caller uses a particular slot. The carrier runs at a gross rate of about 270.8 kbit/s using GMSK modulation, and one TDMA frame of eight slots lasts about 4.615 ms. Uplink and downlink use separate frequencies (frequency-division duplex): the downlink goes from the base station to the user and the uplink from the user to the base station.
Most GSM networks operate in two bands, 900 MHz and 1,800 MHz. In North America the allocated bands are 850 MHz and 1,900 MHz. So, to support global roaming, a handset must support all four bands, which is why "quad-band" phones were a selling point. The number of carriers depends on the band:
| Band | Main regions | Carriers per direction (200 kHz each) |
|---|---|---|
| GSM 850 | Americas | 124 |
| GSM 900 (primary band) | Europe, Asia, Africa | 124 (more with the extended band) |
| DCS 1800 | Europe, Asia, Africa | 374 |
| PCS 1900 | Americas | 299 |
GPRS: packet data on GSM (2.5G)
GPRS stands for General Packet Radio Service. It is a 2.5G standard that adds packet-switched data to GSM, so a user pays for data sent rather than for time connected. A GPRS handset can be given several of the eight timeslots on a carrier at once. How many depends on its multislot class: a common class 10 device can use up to four slots on the downlink and two on the uplink, with no more than five in total.
GPRS defines four coding schemes, CS-1 to CS-4. They trade error protection for speed, from about 9 kbit/s per slot (CS-1, robust) to about 21.4 kbit/s per slot (CS-4, little protection). So the original rule of thumb of "about 20 kbit/s per slot, about 80 kbit/s with four slots" is right for the fastest coding scheme. In practice most networks used CS-1 and CS-2, giving 30 to 50 kbit/s.
Worked example: GPRS and EDGE throughput
Peak user rate is simply slots multiplied by the rate per slot:
- GPRS, CS-2 (13.4 kbit/s per slot), 4 downlink slots: 4 x 13.4 = 53.6 kbit/s.
- GPRS, CS-4 (21.4 kbit/s per slot), 4 slots: 4 x 21.4 = 85.6 kbit/s.
- EDGE, MCS-9 (59.2 kbit/s per slot), 4 slots: 4 x 59.2 = 236.8 kbit/s.
- EDGE, MCS-9, all 8 slots (theoretical only, no handset did this): 8 x 59.2 = 473.6 kbit/s.
These are peak link rates before protocol overhead, and they assume good signal quality. Move to the cell edge and the network drops to more robust coding with lower rates.
EDGE: 8-PSK on the same carriers (2.75G)
EDGE stands for Enhanced Data rates for GSM Evolution (it was earlier called Enhanced Data GSM Environment). It is a 2.75G standard that raises the rate of each timeslot by switching from GMSK to 8-PSK modulation, which carries three bits per symbol instead of one. EDGE defines nine modulation and coding schemes, MCS-1 to MCS-9, reaching about 59.2 kbit/s per slot at the top. Earlier figures of around 48 kbit/s per slot reflect the more commonly achieved schemes.
Baseband processors commonly supported multislot class 10 or class 12, both of which allow up to four downlink slots, which works out at roughly 200 to 240 kbit/s peak. EDGE met the ITU's 384 kbit/s figure for 3G on paper, which is why it is sometimes counted as a 3G technology, but the rates operators and chips actually delivered were lower.
UMTS: wideband CDMA (3G)
UMTS stands for Universal Mobile Telecommunications System. It is the 3G follow-on to GSM, specified by 3GPP (the partnership of standards bodies formed in 1998) and accepted by the ITU as one of the IMT-2000 family of 3G standards. Its radio interface is WCDMA: every user transmits across the whole 5 MHz channel, and users are separated by codes rather than by time slots or frequencies.
With tremendous growth in mobile phone use during the 1990s, vendors sought new frequency bands and new technologies to increase capacity, and the term 3G marks that generational leap. UMTS uses new spectrum around 2 GHz and an air interface that can carry more data, and therefore more digital voice calls, than 2G. The first release delivered up to 384 kbit/s to handsets in practice (2 Mbit/s was the theoretical figure). Unlike GSM, a UMTS phone can hold a voice call and a data session at the same time without special support. On GSM this needs a feature called dual transfer mode (DTM), which few networks enabled.
NTT DoCoMo launched the first commercial WCDMA network, FOMA, in Tokyo in October 2001, using a version that was not fully aligned with the final standard at first. 3G was then deployed by operators around the world. Most operators did not cover their whole networks with 3G, so 3G devices needed 2G fallback for the widest coverage.
The CDMA2000 and TD-SCDMA branches
Qualcomm continued to develop its own CDMA2000 air interface, giving cdmaOne operators an upgrade path. The data-optimised 3G version, EV-DO, was first deployed in Korea in 2002 and then widely in North America and elsewhere. The initial version, Revision 0, supports up to 2.4 Mbit/s on the downlink.
TD-SCDMA was developed in China, accepted by the ITU as an IMT-2000 standard, and adopted as a Chinese national standard in January 2006. China Mobile began trial service in 2008 and full commercial service in 2009. As the name implies, it combines TDMA and CDMA. Uplink and downlink share the same frequency in different time slots (time-division duplex), which gives operators some flexibility in how they split capacity. Aggregate data rates are around 2 Mbit/s, broadly similar to UMTS.
HSPA and HSPA+: making 3G fast
HSPA stands for High-Speed Packet Access. It combines HSDPA (downlink, often called 3.5G) and HSUPA (uplink, often called 3.75G). As with the 2.5G technologies, these enhance UMTS data rates while keeping radio compatibility.
HSDPA shipped first, with commercial service from late 2005. It adds a high-speed downlink channel that is shared by all users actively downloading. The base station schedules users every 2 ms, picks the modulation (QPSK or 16QAM) to suit each user's signal, and retransmits lost blocks quickly (hybrid ARQ). The headline peak is 14.4 Mbit/s for the top handset category. As with any shared medium, actual speed depends on how many users share the channel, and each handset category caps how much it can accept.
HSUPA addresses the uplink, which matters for users sending high-resolution photos or video. It raises the uplink peak to 5.76 Mbit/s, and commercial deployments began in 2007. Together, HSDPA and HSUPA brought several megabits per second to phones and USB modems, similar to what older DSL and cable services delivered, and mobile broadband became a real product.
HSPA+ boosts rates further in two ways. One is higher-order modulation (64QAM on the downlink), which packs more bits into each symbol. The other is MIMO (multiple input, multiple output), which can double throughput by sending two data streams at once; this needs two antennas at the base station and two on the handset. Used together they give 42 Mbit/s. Later releases added dual-carrier operation (two 5 MHz carriers bonded), with peaks of 42 Mbit/s without MIMO, then 84 Mbit/s and beyond.
The TD-SCDMA standard was extended in the same way (TD-HSDPA and TD-HSUPA). On the CDMA2000 side, Qualcomm developed EV-DO Revision A, with up to 3.1 Mbit/s down and 1.8 Mbit/s up, and Revision B, which bonds up to three carriers for 14.7 Mbit/s down and 5.4 Mbit/s up, similar to HSPA. Qualcomm sampled the first EV-DO Rev B chips in late 2008.
LTE: Long Term Evolution (4G)
The UMTS camp developed LTE as its 4G technology. 3GPP approved the core LTE specifications at the end of 2007 and froze them as Release 8 in December 2008. In December 2009 TeliaSonera launched the first commercial LTE networks, in Stockholm and Oslo.
LTE uses OFDM (orthogonal frequency-division multiplexing) and wider channels of up to 20 MHz. The downlink uses OFDMA. The uplink uses a variant called SC-FDMA, which has a lower peak-to-average power ratio, so the handset's power amplifier can run more efficiently and the battery lasts longer. The 3GPP design target was 100 Mbit/s downlink in 20 MHz. The often-quoted peak figures of 326 Mbit/s downlink (four-antenna MIMO) and 86 Mbit/s uplink are the physical-layer maximums of the first release. Because of the space required for multiple antennas, phones commonly use two receive antennas while fixed broadband routers may use four, provided the network supports that mode.
Worked example: where LTE's peak rate comes from
A 20 MHz LTE carrier has 100 resource blocks of 12 subcarriers, so 1,200 subcarriers. Each carries 14 OFDM symbols per millisecond, or 14,000 symbols per second. With 64QAM (6 bits per symbol):
- Raw rate per layer: 1,200 x 14,000 x 6 = 100.8 Mbit/s.
- With 2x2 MIMO (two layers): about 201.6 Mbit/s raw.
- After control channels, reference signals and coding overhead (roughly a quarter, an approximate figure), about 150 Mbit/s, which matches the common Category 4 handset peak.
The same arithmetic explains why 5G gets faster: multiply the bandwidth by five (100 MHz), add more MIMO layers and 256QAM, and the peak moves into gigabits.
As a result of these changes, LTE gives roughly three times the spectral efficiency of early HSPA, meaning that an operator can carry about three times the data (or the number of voice calls) in the same spectrum. LTE has no circuit-switched voice: voice runs as packets using VoLTE, and early phones fell back to 3G or 2G for calls until VoLTE was widely deployed. LTE did bring down the cost of mobile data significantly, though it took several years of base station roll-out and cheaper modems.
WiMAX: the other 4G contender
WiMAX is based on the IEEE 802.16 standard, originally designed for fixed wireless broadband. Intel was its most prominent backer, and the mobile version (IEEE 802.16e-2005) was used by some mobile service providers. An industry group, the WiMAX Forum, promotes the standard and certifies products for interoperability. Originally conceived as a data standard, WiMAX can carry voice over IP as well.
Like LTE, WiMAX uses OFDM with channel widths of up to 20 MHz, and it supports MIMO. Typical published figures put the theoretical maximum at around 75 Mbit/s, with mobile users seeing much less in practice. A significant difference is that WiMAX used OFDMA on the uplink, which has a higher peak-to-average power ratio than LTE's SC-FDMA, making it less power-efficient for handheld devices.
Most mobile WiMAX networks were switched off in the mid-2010s as operators moved to LTE, often to TD-LTE, which shares spectrum in the same way. WiMAX survives mainly in some fixed and private networks.
Was LTE really 4G?
The ITU's IMT-Advanced requirements for 4G called for peak rates of about 1 Gbit/s for low-mobility users and 100 Mbit/s for high-mobility users. Neither the first LTE release nor the first mobile WiMAX met this. LTE-Advanced (3GPP Release 10) and WirelessMAN-Advanced (IEEE 802.16m) were the technologies the ITU accepted as IMT-Advanced in 2012. In practice the ITU accepted the market's use of "4G" for LTE and WiMAX in December 2010, and the name stuck.
Bringing it up to date: 5G NR
5G New Radio (NR) is 3GPP's fifth-generation radio standard, first specified in Release 15 (non-standalone in late 2017, standalone in mid 2018). It meets the ITU's IMT-2020 requirements, whose headline figures include a 20 Gbit/s peak downlink, 1 ms user-plane latency for low-latency services and up to a million connected devices per square kilometre. The first commercial 5G smartphone services launched in April 2019 in South Korea and the United States.
5G NR keeps OFDM from LTE but makes it far more flexible:
- Two frequency ranges. FR1 covers bands below about 7 GHz (the 3.5 GHz band is the workhorse in many countries) with channels up to 100 MHz. FR2 covers millimetre-wave bands from about 24 GHz upward, with channels up to 400 MHz.
- Scalable numerology. Subcarrier spacing can be 15, 30, 60 or 120 kHz instead of LTE's fixed 15 kHz, so short slots and low latency are possible where needed.
- Massive MIMO and beamforming. Base stations with 32 or 64 antenna elements steer beams to users, which is essential at millimetre-wave frequencies.
- New core network. The 5G core (5GC) is service-based and supports network slicing. Early 5G ran "non-standalone", with an LTE anchor and the LTE core; standalone 5G uses the new core directly.
The direction has been consistent since GSM: wider channels, more antennas, higher-order modulation and smarter scheduling. 5G-Advanced (Release 18 onwards) continues that line, and 3GPP has started work on 6G, with ITU targets under the IMT-2030 programme and commercial service generally expected around 2030.
What happened to 2G and 3G?
Many operators have switched off 3G to reuse its spectrum for 4G and 5G. In the United States all three national carriers closed their 3G networks during 2022. In Europe and Asia the picture varies: some operators keep 2G running longer than 3G because many alarms, meters and cars still use it for low-rate data. Low-power IoT now moves to LTE-M and NB-IoT, which are part of the LTE and 5G families.
Common mistakes and misconceptions
- Quoting peak rates as user speeds. Every figure in this post is a peak for one user in ideal conditions. Shared channels mean real speeds are lower, often by a large factor.
- Treating GPRS and EDGE as separate networks. They are features of the GSM network that reuse the same carriers and timeslots.
- Mixing up UMTS and WCDMA. UMTS is the whole system; WCDMA is its main radio interface.
- Assuming 5G always means millimetre wave. Most 5G traffic runs in FR1 bands below 7 GHz; millimetre wave is used for dense hot spots and fixed wireless.
- Assuming every phone supports every band. Roaming has always depended on band support, from quad-band GSM to the dozens of LTE and NR bands in a modern phone.
Frequently asked questions
What is the difference between GSM and CDMA?
GSM separates users by giving each one a time slot on a 200 kHz carrier (TDMA). CDMA systems such as cdmaOne and CDMA2000 let all users transmit across the same wider channel at once and separate them using codes. In 3G, the GSM family also moved to CDMA, in the form of WCDMA.
Is EDGE 2G or 3G?
EDGE runs on GSM carriers and is usually called 2.75G. It met the ITU's 3G data-rate figure on paper, so it was formally part of the IMT-2000 family, but in practice it is treated as an enhanced 2G technology.
Why did LTE win over WiMAX?
LTE offered a smooth path for the large installed base of GSM and UMTS operators, better uplink power efficiency for handsets, and an ecosystem that later CDMA2000 operators also joined. WiMAX had an earlier start but a much smaller operator base.
Does 5G replace LTE?
Not yet. Many 5G networks still rely on LTE for coverage and signalling, and phones use LTE where 5G is not available. LTE will remain in service for many years, much as 2G outlived 3G in some countries.
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