Third-generation cellular telephony is on its way--not, unfortunately, as a single worldwide system, but as three incompatible ones. The main difference between the three lies in their choice of radio interface technology. This fact is crucial for several reasons, since the radio interface determines not only the fundamental capacity of a mobile radio network, but also how it deals with such issues as interference, multipath distortion, and handing off calls from one base station to another as users move around. Consequently, as might be expected, the choice of radio interface has a dramatic effect on the complexity of the system and its cost. Also, global travellers will need more than one phone with which to communicate, at least until trimode phones reach the market.
To understand what is being developed, and why, let's begin with one of the stated goals of third-generation (3G) systems, namely to support variable user data rates as high as 2 Mb/s. In one way or another, all three approaches provide for adaptive bandwidth-on-demand. Two of the systems use wideband code-division multiple access (WCDMA) for the radio interface. The other (of which more later) uses two variations of time-division multiple access (TDMA).
With WCDMA, a user's information bits are spread over an artificially broadened bandwidth. The job is done by multiplying them with a pseudorandom bit stream running several times as fast. The bits in the pseudorandom bit stream are referred to as chips, so the stream is known as a chipping, or spreading, code. It increases the bit-rate of the signal (and the amount of bandwidth it occupies) by a ratio known as the spreading factor, namely, the ratio of the chip rate to the original information rate.
The key devices in any CDMA system are its correlation receivers, which store exact copies of all of the system's chipping codes. These codes the receivers use to multiply a received data stream, selecting the same chipping code as was used in the transmitter. The devices also perform whatever other mathematical operations are needed to restore the original user data. The result is that at the receiver output, the amplitude of the de-spread signal is increased by the spreading factor relative to interfering signals. In the process, those interfering signals are diminished and simply add to the background noise level. In other words, correlation detection uses the spreading factor to raise the desired user signal from the interference. The effect is called processing gain.
Note that spectrum spreading by itself does not confer any benefits. It is the combination of spreading and de-spreading that works the magic in CDMA, allowing all the base stations in a network to use the same carrier frequency because every conversation on it is assigned a separate spreading code. The scheme can also resolve different propagation paths, turning multipath distortion from a destructive nuisance into a helpful ally.
To help understand this complex idea, remember that frequency-division multiple-access systems keep conversations from interfering with each other by assigning them to different frequency bands, whereas time-division systems do so by assigning them to different time slots. With CDMA, however, the conversations occupy the same frequency bands at the same time. But each interaction is multiplied by a different chipping code, and when the signals are de-spread, the only one that comes through intelligibly is the one whose code was used by the de-spreader. The others, as stated above, simply add to the background noise level (which ultimately limits the number of users that can share a channel).
For the system to work, two factors are key. First, only soft handovers may be employed, since with them mobile terminals can maintain simultaneous connections to different base stations as they move among them. Second, transmitter powers must be strictly controlled so that signals from all mobile terminals arrive at the base station with about the same strength, despite their differing distances from the base station. Strict power control is maintained with multiple real-time power control channels, plus control loops with different resolutions (coarse and fine). The control channels operate at power command rates between 800 Hz and 1.5 kHz. That is, base station equipment measures the power received from each mobile unit as much as 1500 times a second and issues commands to the mobiles at that rate to raise or lower their output power.
Many users can be accommodated. The maximum WCDMA chip rate is 3.84 megachips per second (Mch/s) and yields a modulated carrier about 5 MHz wide. System operators can deploy multiple carriers, each of which occupies 5 MHz. Moreover, in a WCDMA system, multiple end-users can share each 5-MHz channel.
[Fig. 1] Unlike second-generation narrowband code-division multiple-access systems, wideband CDMA, or WCDMA, allows network operators to dynamically reassign channel bandwidth [vertical axis] in response to user needs. These bandwidth allocations are updated every 10 ms, as shown. The bandwidth allocations in each of a system's channels can be changed independently of one another.
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In Fig. 1, for example, five users share a single channel. Three operate at fixed data rates, while the data rates of the remaining two are variable. The WCDMA system deals with this situation by continually changing the way it distributes the channel's bandwidth among the five users, adjusting the spreading factors of each of the users every 10 ms.
In sum, experience with narrowband CDMA (the current IS-95 second-generation systems), not to mention advanced experiments, field trials, and general diligence in wireless practice, has borne fruit. WCDMA is the favored radio interface for 3G in those situations where its increased bandwidth can be tolerated. But wideband options may not suit every situation, which is one reason why 3G will take more than one form. Each 3G option accommodates a specific second-generation predecessor, and uses that legacy protocol as one of its operating modes in 3G [Fig. 2].
[Fig. 2] Three main second-generation cellular technologies dominate the industry today: narrowband CDMA, GSM (a narrowband time-division multiple access, or TDMA, system as far as its radio interface is concerned), and a different narrowband TDMA system known as IS-136. Each has its own path mapped out for migrating to the third generation [right], meaning that there will be three incompatible versions.
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