GSM (Global System for Mobile Communications) is the most spread mobile telephone systems in the world. GSM differs from its predecessor technologies in that both signaling and speech channels are digital, and thus GSM is considered a second generation (2G) mobile phone system. The GSM standard also adds packet data capabilities by means of General Packet Radio Service (GPRS). GPRS is standardized by the 3rd Generation Partnership Project (3GPP).
Enhanced Data Rates for GSM Evolution (EDGE, also known as Enhanced GPRS) is a superset to GPRS and can function on any network with GPRS deployed on it, provided the carrier implements the necessary upgrades. EDGE provides higher speed data transmission. In addition to Gaussian minimum shift keying, EDGE uses 8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE is considered a third generation (3G) radio technology. Evolved EDGE is a further step in the evolution of the 3GPP standard providing reduced latency and more than doubled performance. Evolved EDGE improves on EDGE in a number of ways. Bit rates are e.g. increased up to 1 MBit/s peak bandwidth and latencies down to 800 ms using dual carriers, higher symbol rate and higher-order modulation, and turbo codes to improve error correction.
The Universal Mobile Telecommunication System (UMTS) is also one of the 3G mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (e-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, a radio access network typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 110a-c, commonly referred to as NodeB (NB) in UTRAN and eNodeB (eNB) in e-UTRAN. The RBS serves one or more areas referred to as cells 120a-c. In FIG. 1 the UE 150 is served by the serving cell 120a. Cells 120b and 120c are neighboring cells.
When performing maintenance of a an RBS it is sometimes necessary to stop the operation of the whole cell of the RBS for a short time, i.e. to turn off the output power for the frequency carriers of the cell. When turning off the output power of a whole cell, all the UEs in the cell that are connected to the RBS will lose their connections and start searching for and connecting to alternative neighboring cells and RBSs. If for example maintenance in an RBS 110a requires that the output power of the frequency carrier of cell 120a is turned off, the UE 150 will suddenly loose connection to the RBS, and will then try to connect e.g. to RBS 110b and cell 120b. In e-UTRAN, the UEs do not drop their calls immediately when they loose connection. Instead they will be disconnected for some time and will not be able to communicate during that time. If the disconnection time is short, the user of the UE will probably not notice the disconnection. If the cell is turned off suddenly and if there are many active UEs in the cell, all these UEs will at the same time start to connect to alternative cells and eNBs, which may lead to congestion for the alternative eNBs random access control channels, and thus longer disconnection times for the UEs.
In UTRAN, a solution to the problem of dropped calls when the output power of a NB is suddenly turned off is to instead gradually reduce the output power of the NB pilot or reference signal before turning off the power completely. As a handover is based on a comparison of measurements of the pilot signal transmitted by the serving NB and neighboring NBs, a gradual reduction of the output power of the pilot signal for a serving NB, will trigger the UEs connected to this NB to make handovers to neighbor cells or NBs. The UEs close to cell borders and alternative NBs will be the first to perform handovers. The number of UEs still connected to the NB at the time when it is finally turned off may be reduced in this way, thus reducing the amount of dropped calls. If the output power of the pilot signal is very low before the power is turned off, then very few UEs will still be connected to the NB.
The radio units in a RBS of a radio access network such as the one illustrated in FIG. 1, are capable of fulfilling the standardized requirements on transmission signal quality such as the error vector magnitude (EVM), unwanted emission, and spurious emission within a certain specified output power range, which is specific for each product type. For example a 40 W radio unit might be certified to transmit signals within the range of 1-40 W, which means that the radio unit will have acceptable EVM and emission within this range. The EVM is a measure of the allowed error in the transmitted signal compared to the theoretically ideal signal.
The power spectrum density of a radio unit will together with the amount of scheduled resources indicate what the total output power is. It is important to make the distinction between total output power and output power spectrum density for a system which allows scheduling of resources over a limited part of the system bandwidth in a sub frame, such as in an e-UTRAN. In one sub frame all resources may be scheduled, in the next maybe only half of the resources are scheduled. The total output power may thus vary from sub frame to sub frame depending on the amount of scheduled resources, although the available output power spectrum density is the same for both sub frames. In GSM though, it is not possible to schedule resources only on a part of the available system bandwidth, so the total output power will not vary in the same way.
Even though the total output power range for a radio unit in e.g. an e-UTRAN might be large (e.g. 1-40 W as in the example mentioned above), a major part of the range is typically dedicated for scheduling and/or power control purposes, and a minor part is dedicated for the reference signal. The power range corresponding to the reference signal is thus not that large which thus limits the possibility to reduce the output power of the reference signal. Furthermore, the relation between the output power of the reference signal and the output power used for scheduled data should typically be the same when the total output power is lowered in e-UTRAN. In most cases it is thus not possible to reduce only the reference signal power, without reducing the output power for scheduled data.
As already mentioned above, different modulation schemes may be used in the different technologies. Some examples of modulation schemes are the phase shift keying (PSK) modulations such as quaternary or quadrature PSK (QPSK) and 8PSK. With four phases, QPSK can encode two bits per symbol. Other examples are the combinations of PSK and amplitude-shift keying (ASK) modulations—also called quadrature amplitude modulation (QAM)—of different orders such as 16QAM and 64QAM. The use of a higher order modulation scheme provides a higher bandwidth utilization (transmission of more bits per symbol), but at the same time it puts higher requirements on the radio unit. The EVM requirement e.g. is higher for a radio unit using a higher order modulation scheme than for a radio unit with a lower order modulation scheme. The total power range of a radio unit will thus be affected by the modulation scheme used, thereby also affecting the possibility to reduce the power of the reference signal.
Consequently, the reduction of the output power of the reference signal that may be done in order to force UEs to perform a handover is in practice limited, both by the fix relation between the output power of the reference signal and the output power used for scheduled data, and by the limited output power range in an RBS using a higher order modulation.