Cellular radio systems are currently in widespread use throughout the world providing telecommunications to mobile users. In order to meet the capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell, there is a base station through which the mobile stations communicate, each base station typically being equipped with antenna arrays arranged sectors. Configurations of three or six sectors (sub-cells) are often employed, where the higher gain of correspondingly narrower beamwidth antennas improve the uplink from the lower power mobiles. The distance between the cells is determined such that co-channel interference is maintained at a tolerable level.
Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems. These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions. A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of the transmission. Since the various components arrive from different directions, there is also a Doppler spread in the received spectrum.
When a new cellular radio system is initially deployed, operators are often interested in maximising the uplink (mobile to base station) and downlink (base station to mobile station) range. The ranges in many systems are uplink limited due to the relatively low transmitted power levels of hand portable mobile stations. Any increase in range means that fewer cells are required to cover a given geographic area, hence reducing the number of base stations and associated infrastructure costs.
The range of the link, either the uplink or the downlink, can be controlled principally in two different ways: by adjusting either the power of the transmitter or the gain at the receiver. On the downlink the most obvious way of increasing the range is to increase the power of the base station transmitter. To balance the link the range of the uplink must also be increased by an equivalent amount. The output power of a transmitter on a mobile, however, is constrained to quite a low level to meet national regulations, which vary on a country to country basis. Accordingly the receive gain at the base station must be increased.
The principal method of improving the receive system gain and to reduce the effect of fading is to include some form of diversity gain in addition to the receive antenna gain. The object of a diverse system is to provide the receiver with more than one path, with the paths being differentiated from each other by some means, e.g. space, angle, frequency or polarisation. The use of these additional paths by the receiver provides the diversity gain. The amount of gain achieved depends upon the type of diversity, number of paths, and method of combination.
Cellular radio base stations frequently use two antennas for diversity reception on the uplink, spaced by many (e.g. 20) wavelengths. This large spacing is required because the angular spread of the incoming signals is narrow. This can be represented as a ring of scatterers around a mobile user who is transmitting to a base station otherwise known as the uplink path and such an arrangement is shown in FIG. 1. For example the radius of scatterers may be 50 to 100 meters, and the range to the base station may be up to 10 km, resulting in a narrow angular spread. A large antenna spacing is required at the base station to provide decorrelated fading, which can be calculated from the Fourier transform relationship between antenna array aperture and angular width (a large aperture in wavelengths provides a narrow beam).
In order to improve wanted signals and discriminate against interfering signals, antennas are being developed which utilise an array of antenna elements at the base station, allied with an "intelligent" beamformer. One such technique is to use a multichannel maximal ratio combiner on reception at the base station array. This operates by weighting the array signals s.sub.i (i=1 to N, where N=the number of elements in the array) with their complex conjugates s.sub.i * (assuming equal noise powers on each channel) and summing to give: ##EQU1##
For a N element array, this provides both array gain (approximately a factor N in power) and diversity gain, the latter only if at least some of the array elements are widely spaced. Thus a factor N improvement in mean signal level can be achieved, allowing extended range or lower mobile transmit power. The array provides narrower beams than a single antenna element, and hence also provides better protection against interference, improving carrier to interference ratios and hence allowing higher capacity systems by reducing re-use factors.
The limitation of the above is that the improvements are only for the uplink, and not for the downlink (base station transmit to the mobile). This invention is concerned with spatially diverse systems and in particular seeks to provide an arrangement wherein downlink performance is improved.
A standard feature of a number of cellular radio systems is that the sets of uplink and downlink frequencies are separated into two distinct bands spaced by a guard band, for example 1800-1850 MHz (uplink) and 1900.varies.1950 MHz (downlink). Up- and down- link frequencies are then paired off, e.g. 1800 with 1900, 1850 with 1950. There is therefore a significant change of frequency (e.g. 5%) between up and down links. There is consequently no correlation for the fast fading (as the mobile moves) between up and down links.