Wired Explains: Everything You Need to Know About 4G Wireless

*T-Mobile: *T-Mobile is in no hurry to get to 4G. Maybe because it first needs to get its 3G network in order. The wireless carrier is still talking about upgrading its 3G network to 3.5 G or HSPA+. HSPA+ will come to Los Angeles and other major cities in the United States this year, says T-Mobile. 4G is clearly a long way away.

*MetroPCS: *The first LTE 4G phone could come from MetroPCS, which is working with Samsung on the device. MetroPCS plans to start its 4G service later this year in cities such as Las Vegas.

__Give me some background: What’s the history here and how did we get to 4G? __

Cellular systems have evolved in four major phases or generations. The first generation, or 1G, technology was analog and transmitted voice calls only.

The second generation, or 2G, introduced digital transmissions and offered the first support for data, although the focus was still on voice calling. Second-generation phones are based on one of two standards: GSM (used by T-Mobile, AT&T, and most overseas carriers) and CDMA (used by Verizon and Sprint). In either case, average data speed on a 2G network is around 9.6 Kbps, about the same as a ’90s-era modems.

The intermediate “2.5G” level is where things get a little fuzzy. In the GSM camp, a standard called GPRS, which offered better data transmission capabilities, came to be known as 2.5G. GPRS has evolved into the EDGE networks (up to 400-kbps data speeds) still used by many phones today. As for the CDMA universe, 2.5G is referred to as 1XRTT, or just 1X. In the real world, it is known as EVDO and promises download speeds ranging from 600 kbps to 1.4 Mbps.

There’s been some debate on what constitutes 3G. Broadly speaking, 3G is defined not by the underlying technology, but by its speed: up to 2 Mbps. Verizon and Sprint’s 3G systems are referred to as EVDO, while AT&T and T-Mobile use HSDPA. A further level, known as HSDPA+ or Turbo 3G, can support download speeds of up to 14 Mbps.

The next step is 4G.

I want all the details, the nitty-gritty behind the two 4G standards. How do they differ?

LTE or Long Term Evolution, is a standard evolved by a group called the 3rd Generation Partnership Project. It is an all-IP network based upon the same core protocol of the internet, TCP/IP.

Both LTE and WiMax use the principle of Orthogonal Frequency-Division Multiple Access, which conceptually has been around since the 1960s. OFDMA is based on the idea of frequency-division multiplexing, which is a method to transmit multiple data streams over a channel. In case of OFDMA, a digital data stream that needs to be transmitted is split into multiple pieces, each of which is modulated onto a separate carrier. These sub-carriers are combined together at the end.

The difference between LTE and WiMax lies in how they handle the channel for processing data. WiMax processes all the information in a wide channel. In case of Clearwire’s implementation of WiMax, that means the about two-thirds of the channel is used for downloads, while a third is used to upload data.

LTE splits the channel into two parts using frequency-division multiplexing, so the download and upload speeds are better balanced.

Just say it. Is WiMax better or LTE?

Peanut butter or chocolate? Lost or The Wire? Havaianas or Birkenstocks? There’s no easy answer. It depends on where you are and what you can get access to.

If you want 4G now, WiMax is pretty much your only choice. It’s also an open standard supported by IEEE. And it is less expensive for service providers to upgrade their equipment to than LTE.

But LTE is coming and almost all the major U.S. carriers, except Sprint, will be supporting it. So ultimately, LTE might win in the long run.

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Photo: Relay tower(Jeff Kubina/Flickr)