One of the aims of future cellular wireless communication systems is to enhance the achievable data throughput to mobile terminals (MT) situated close to a cell edge. This is important as, assuming a reasonably uniform distribution of MTs over the cell area, then a significant fraction of the MTs in a cell is close to its periphery. When the same carrier frequency is re-used in neighboring cells, the signal from the ‘wanted’ base station (BS) with which the MT is communicating is received at the cell edge at power levels similar to signals originating from BSs in neighboring cells. Cell edge MTs therefore experience strong interference in addition to low signal to noise ratio (SNR), which makes it difficult to achieve high data rates to these MTs.
In current third generation (3G) systems like the Universal Mobile Telecommunications System (UMTS), macro-diversity and soft-handover techniques are known. These allow simultaneous communication between more than one BS and a MT in order to improve the link quality to MTs at the boundaries between cells. In macro-diversity and soft-handover the same data is transmitted to a MT from multiple BSs. This is achieved by having all transmissions effected on the same carrier frequency, and discriminating transmissions from different BSs based on their different scrambling codes. The MT comprises a receiver arrangement for receiving multiple (CDMA) signals simultaneously, i.e. it has multiple receive signal paths (descrambling and decorrelation) and a combiner to combine the despread symbol streams.
Macro-diversity techniques in 3G systems rely upon code division multiple access (CDMA) techniques in order for a MT to receive a given data stream from more than one BS simultaneously. However, each wanted data stream causes interference to the reception of the other stream.
For future cellular systems, including 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and Wireless World Initiative New Radio (WINNER), multi-carrier (OFDM-based) transmission schemes are being proposed (at least for the downlink). Further, multiple access (sharing the time-frequency resources between MTs) is typically envisaged to be based on allocating different groups of subcarriers to different MTs (orthogonal frequency division multiple access (OFDMA)) rather than on CDMA. The macro-diversity techniques from 3G are therefore not directly applicable to these future systems.
One favored approach for improving cell-edge performance in these future OFDM-based cellular systems is to partition subcarriers between cells, wherein all cells may use all subcarriers at lower transmit powers. This gives coverage to the inner parts of the cell but does not reach the cell edges (and therefore does not cause interference to neighboring cells). For communication to MTs at the cell edges a BS then uses a subset of the total number of available subcarriers where the subsets are defined to be mutually exclusive with the subsets employed in neighboring cells. This prevents higher power transmissions to cell-edge MTs from causing high interference to cell-edge MTs in the neighboring cells.
This approach improves the inter-cell interference situation for cell-edge MTs at the expense of increasing the frequency re-use factor, which results in lower spectral efficiency than re-using all subcarriers in all parts of every cell (i.e., frequency re-use factor of one). It may also reduce the peak throughput to cell-edge users since only a subset of the total number of subcarriers are available for use.
A straight forward extension of the 3G macro-diversity ideas to these OFDM systems would be to use the cell-edge subcarrier subsets of two or more neighboring cells to transmit to a MT. The MT would then receive the same data from multiple BSs (via different subsets of subcarriers) and can combine these to enhance the data reception quality. The downside of this approach is of course that this consumes resources (subcarriers) in two or more cells for the benefit of one MT. This is analogous to 3G macro-diversity, which requires resources (spreading codes) to be allocated in two or more cells for the benefit of one MT, and requires the MT to receive and combine two or more signals.
A related piece of prior art is the operation of Single Frequency Networks (SFN), which are known in broadcast systems such as Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB). In these OFDM systems the same data signal is broadcast from all transmitters. In the regions approximately mid-way between two transmitters, the receiving terminal receives a super-position of the signals from both transmitters. This is equivalent to receiving the signal from a single source via the composite channel given by the summation of the two channels from each transmitter. With a suitably long guard interval the receiver in these OFDM systems can successfully receive the combined signal from the two sources with enhanced signal strength over reception from a single transmitter, without Inter-Symbol Interference (ISI), and without needing to be ‘aware’ that the signal originated from two separate sources.
FIG. 1 shows a schematic block diagram of a transmitter with a space-time coder 20 adapted to receive an input signal 10 and to generate two transmission signals 30 which can be jointly received at a receiver.
However, although the SFN concept means that a simple receiver can be used, the combined signal can still undergo fading.
For the case of two transmission paths with respective transfer functions h1 and h2 to the receiver antenna, the combined transfer function becomes (h1+h2), so that the SNR of the received signal is (h1+h2)2/n2 where n is the amplitude of noise and interference. But sometimes the particular values of h1 and h2 will cancel, significantly reducing the received power. Therefore the received signal quality could be highly variable.