Fading and power control

The CDMA system needs a power control mechanism to overcome the effects of multiple users with different propagation characteristics transmitting simultaneously. This is often referred to as thenear–far problem, where a remote user can easily be drowned out by a user that is physically much closer to the base station. Power control endeavours to ensure that signals arriving at the receiver are almost equal in power, and at a level that meets the quality requirements in terms of SIR.

The three main features here are:

• attenuation due to increase in distance from the receiver;

• fading variations due to specific features of the environment;

• fading variations due to the movement of the mobile device.

Radio waves propagating in free space are modelled by an inverse square law whereby as the distance between the transmitter and receiver doubles, the signal loses half of its power. Thus in the equation belowais generally regarded to be of value 2 andxindicates the distance in metres:

Preceive= P transmit

This is not necessarily the case in a cellular system, where the terrain and buildings can have a major effect on the propagation model, and thus a is usually considered to be greater than 3. For example, in metropolitan areas,a=4 for planning purposes. As a user moves around, the power level at the receiver will fluctuate. These fluctuations can be broken down into two general categories: slow and fast fading.

Slow fading or shadow fading is as a result of obstructions, which will result in changes in received power level. Multiple versions of the same signal will form constructive and destructive interference at the receiver as the relative time shifts vary due to different path lengths and reflection/refraction characteristics of the surrounding environment. It is more pronounced in urban areas, with significant changes in received signal strength occurring over tens of metres.

Fast fading, or Rayleigh fading, is due to the Doppler shift, where the apparent wave- length of the transmitted signal will increase as the mobile device moves towards the receiver and decrease as the device moves in the opposite direction of the receiver. This appears at the receiver as a change of phase of the transmitted signal. Generally a num- ber of paths with different Doppler shifts will arrive at the receiver with changed phase shifts. As thesemultipathsare combined at the receiver, the signal will exhibit peaks and troughs of power corresponding to signals that are received in phase, and thus reinforce each other, and out of phase, where they cancel each other out. These variations are much faster than those occurring with environmental factors and can cause significant differences in power levels over relatively short distances. Consider the WCDMA sys- tem, where the transmit/receive frequency is in the 2 GHz range. The wavelength of this is 150 mm, and thus relatively small movements of the mobile device of the order of 75 mm will result in a different interference pattern, and consequently a different power level. This is why power control must be performed, and performed rapidly, in the system to attempt to maintain an ideal, even received power level. In the WCDMA system, as will be seen in Chapter 6 power control is performed 1500 times a second. In the IS-95 CDMA system, it is 800/second.

2.9

PROTECTING THE DATA

Despite the shift to data being transferred in digital format, there are still major problems in sending data across the air. In a fixed-line communications system, most of the problems of data transfer and ‘data loss’ are down to such issues as congestion, where data is stuck in a traffic jam, or buffer overflow, where a network device is being asked to process too much data. What is no longer considered to be a problem is the reliability of the medium over which the data is travelling. Consider a fibre optic cable, which can now be regarded as the standard for data transfer once out of the local loop. Fibre cables cite bit error rate figures of the order of 10−20, and generally bit errors that do occur are bit inversions, that is, a 1 that should be a 0 and vice versa. When this order of error rate is achieved, one can assume that the medium is completely reliable. In fact, many high-speed communications systems use this to their advantage; for example, as will be seen later, ATM provides no error protection whatsoever on data, and does not require a destination to acknowledge receipt of data. In general, fixed-line schemes provide, at

best, an error checking mechanism on data, usually in the form of a cyclic redundancy check (CRC). Should data arrive with errors, a rare occurrence, the sender is asked to retransmit, if that level of reliability is required. For example, Ethernet transmits frames of 1500 bytes of payload over which there is a 4-byte CRC, which introduces a relatively low overhead on the data.

However, a wireless communications system is notorious for corrupting data as it travels across the air. So far, cellular systems are focused on voice transmission, which is extremely tolerant of errors. Typically, a voice system can sustain about 1% of error before the errors become audible. With the introduction of mobile data solutions, more often the information being carried across the air is data, such as an IP packet. Unfortunately, data systems are very intolerant of errors, and generally require error-free delivery to an application. For that reason, cellular systems must now implement more rigorous error control mechanisms.

If a simple error checking scheme was introduced, there would be too much retrans- mission, and the system would spend the majority of the time retransmitting data, thus lowering the overall throughput. A better and more reliable scheme is required. The solution is to implement forward error correction (FEC). With this, a correction code is transmitted along with the data in the form of redundant bits distributed throughout the data, which allows the receiver to reconstruct the original data, removing as many errors as possible. For an efficient and robust wireless communications system, it is essential that a good FEC scheme is used to improve the quality of transmissions.

A problem common to all FEC schemes is the amount of overhead required to correct errors. If a very simple FEC scheme is considered, in which each bit is merely repeated to make the channel robust, then, as shown below, the amount of information to be transmitted is doubled. However, what is lost in bandwidth, by increasing the amount of information to be sent, is gained in the quality of the signal that is received.

Data: 10101011010100100100 1

Transmission: 110011001100111100110011000011000011000011

The standard terminology is that the data coming from a user application is quantified in bits per second. However, the actual transmission is quantified assymbols per second, since this transmission consists of data plus FEC bits. In the case above, one bit is represented by two symbols.