3.1.1 High data rates in noise-limited scenarios
From the discussion above, some basic conclusions can be drawn regarding the provisioning of higher data rates in a mobile-communication system when noise is the main source of radio-link impairment (a noise-limited scenario):
- The data rates that can be provided in such scenarios are always limited by the available received signal power or, in the general case, the received signal-power-to-noise-power ratio. Furthermore, any increase of the achievable data rate within a given bandwidth will require at least the same relative increase of the received signal power. At the same time, if sufficient received signal power can be made available, basically any data rate can, at least in theory, be provided within a given limited bandwidth.
- In case of low-bandwidth utilization, i.e., as long as the radio-link data rate is substantially lower than the available bandwidth, any further increase of the data rate requires approximately the same relative increase in the received signal power. This can be referred to as power-limited operation (in contrast to bandwidth-limited operation, see below) as, in this case, an increase in the available bandwidth does not substantially impact what received signal power is required for a certain data rate.
- On the other hand, in case of high-bandwidth utilization, i.e. in case of data rates in the same order as or exceeding the available bandwidth, any further increase in the data rate requires a much larger relative increase in the received signal power unless the bandwidth is increased in proportion to the increase in data rate. This can be referred to bandwidth-limited operation as, in this case, an increase in the bandwidth will reduce the received signal power required for a certain data rate.
Thus, to make efficient use of the available received signal power or, in the general case, the available signal-to-noise ratio, the transmission bandwidth should at least be of the same order as the data rates to be provided.
Assuming a constant transmit power, the received signal power can always be increased by reducing the distance between the transmitter and the receiver, thereby reducing the attenuation of the signal as it propagates from the transmitter to the receiver. Thus, in a noise-limited scenario it is at least in theory always possible to increase the achievable data rates, assuming that one is prepared to accept a reduction in the transmitter/receiver distance, that is a reduced range. In a mobile-communication system this would correspond to a reduced cell size and thus the need for more cell sites to cover the same overall area. Especially, providing data rates in the same order as or larger than the available bandwidth, i.e. with a high-bandwidth utilization, would require a significant cell-size reduction. Alternatively, one has to accept that the high data rates are only available for mobile terminals in the center of the cell, i.e. not over the entire cell area.
Another means to increase the overall received signal power for a given transmit power is the use of additional antennas at the receiver side, also known as receive-antenna diversity. Multiple receive antennas can be applied at the base station (that is for the uplink) or at the mobile terminal (that is for the downlink). By proper combining of the signals received at the different antennas, the signal-to-noise ratio after the antenna combining can be increased in proportion to the number of receive antennas, thereby allowing for higher data rates for a given transmitter/receiver distance.
Multiple antennas can also be applied at the transmitter side, typically at the base station, and be used to focus a given total transmit power in the direction of the receiver, i.e. toward the target mobile terminal. This will increase the received signal power and thus, once again, allow for higher data rates for a given transmitter/receiver distance.
However, providing higher data rates by the use of multiple transmit or receive antennas is only efficient up to a certain level, i.e. as long as the data rates are power limited rather than bandwidth limited. Beyond this point, the achievable data rates start to saturate and any further increase in the number of transmit or receive antennas, although leading to a correspondingly improved signal-to-noise ratio at the receiver, will only provide a marginal increase in the achievable data rates. This saturation in achievable data rates can be avoided though, by the use of multiple antennas at both the transmitter and the receiver, enabling what can be referred to as spatial multiplexing, often also referred to as MIMO
(Multiple-Input Multiple-Output). Different types of multi-antenna techniques, including spatial multiplexing, will be discussed in more detail in Chapter 6. Multi-antenna techniques for the specific case of HSPA and LTE are discussed in Part III and IV of this book, respectively.
An alternative to increasing the received signal power is to reduce the noise power, or more exactly the noise power density, at the receiver. This can, at least to some extent, be achieved by more advanced receiver RF design, allowing for a reduced receiver noise figure.
3.1.2 Higher data rates in interference-limited scenarios
The discussion above assumed communication over a radio link only impaired by noise. However, in actual mobile-communication scenarios, interference from transmissions in neighbor cells, also referred to as inter-cell interference, is often the dominating source of radio-link impairment, more so than noise. This is especially the case in small-cell deployments with a high traffic load. Furthermore, in addition to inter-cell interference there may in some cases also be interference from other transmissions within the current cell, also referred to as intra-cell interference.
In many respects the impact of interference on a radio link is similar to that of noise. Especially, the basic principles discussed above apply also to a scenario where interference is the main radio-link impairment:
- The maximum data rate that can be achieved in a given bandwidth is limited by the available signal-power-to-interference-power ratio.
- Providing data rates larger than the available bandwidth (high-bandwidth utilization) is costly in the sense that it requires an un-proportionally high signal-to-interference ratio.
Also similar to a scenario where noise is the dominating radio-link impairment, reducing the cell size as well as the use of multi-antenna techniques are key means to increase the achievable data rates also in an interference-limited scenario:
- Reducing the cell size will obviously reduce the number of users, and thus also the overall traffic, per cell. This will reduce the relative interference level and thus allow for higher data rates.
- Similar to the increase in signal-to-noise ratio, proper combining of the signals received at multiple antennas will also increase the signal-to-interference ratio after the antenna combining.
- The use of beam-forming by means of multiple transmit antennas will focus the transmit power in the direction of the target receiver, leading to reduced interference to other radio links and thus improving the overall signal-to-interference ratio in the system.
One important difference between interference and noise is that interference, in contrast to noise, typically has a certain structure which makes it, at least to some extent, predictable and thus possible to further suppress or even remove completely. As an example, a dominant interfering signal may arrive from a certain direction in which case the corresponding interference can be further suppressed, or even completely removed, by means of spatial processing using multiple antennas at the receiver. This will be further discussed in Chapter 6. Also any differences in the spectrum properties between the target signal and an interfering signal can be used to suppress the interferer and thus reduce the overall interference level.
Part 2 discusses higher-order modulation, channel coding, and multi-carrier transmission.
Printed with permission from Academic Press, a division of Elsevier. Copyright 2009. "3G Evolution, HSPA and LTE for Mobile Broadband, 2e" by Erik Dahlman, Stefan Parkvall, Johan Skold, and Per Beming. For more information about this title and other similar books, please visit www.elsevierdirect.com.