This application relates to telecommunications systems in general, having for example application in UMTS (Universal Mobile Telecommunications System). In particular, this application relates to a method and apparatus for configuring compressed mode.
In a typical cellular radio system, a mobile communications apparatus communicates via a radio access network (RAN) to one or more core networks. The mobile communications apparatus or User Equipment (UE) comprises various types of equipment such as mobile telephones (also known as cellular or cell phones), laptops with wireless communication capability, personal digital assistants (PDAs) etc. These may be portable, hand held, pocket sized, installed in a vehicle etc and communicate voice and/or data signals with the radio access network.
In the following, reference will be made to UMTS and to particular standards. However it should be understood that the invention is not intended to be limited to any particular mobile telecommunications system or standard.
UMTS is a third generation public and mobile telecommunication system. Various standardization bodies are known to publish and set standards for UMTS, each in theft respective areas of competence. For instance, the 3GPP (Third Generation Partnership Project) has been known to publish and set standards for UMTS based upon GSM (Global System for Mobile Communications), and the 3GPP2 (Third Generation Partnership Project 2) has been known to publish and set standards for UMTS based upon CDMA (Code Division Multiple Access). Within the scope of a particular standardization body, specific partners publish and set standards in their respective areas.
Consider as an example a wireless mobile device which complies with the 3GPP specifications for the UMTS protocol. Such a wireless mobile device is generally referred to as user equipment (UE). The 3GPP technical specification 25.331, V5.18.0, referred to herein as the 25.331 specification, and incorporated herein by reference, addresses the subject of Radio Resource Control protocol for the UE-UTRAN (UTRA Network) interface.
A UMTS Terrestrial Radio Access Network (UTRAN) is designed to operate in bands using Frequency Division Duplex (FDD).
A UE arranged to communicate on an FDD network may utilize compressed mode (CM) during communication with the network. Compressed mode is needed inter alia when making measurements on another frequency (inter-frequency) or on a different Radio Access Technology (inter-RAT). Inter-frequency measurements are performed between the channels of different frequencies within the same or a different UMTS band. Inter-RAT measurements are performed between the channels of different Radio Access Technologies (e.g. GSM and UMTS). In the compressed mode, transmission and reception by the UE transceiver on the band the UE is camped on is stopped for a short time. This time is called the Transmission Gap. This allows the transceiver to be used to perform measurements on the other frequency. Once the measurement has been made, transmission and reception resumes on the band the UE is camped on. Compressed Mode (CM) is the term used to define the method whereby the average data rate is maintained by compressing data in the frame either side of the transmission gap required for the measurement.
FIG. 1 illustrates the implementation of compressed mode. Time is on the horizontal axis and instantaneous transmit power is on the vertical axis. In FIG. 1, one frame (e.g. 307) is shown as having duration of 10 milliseconds. Each frame comprises a plurality of slots. A series of frames 301 have transmission gaps 303 and 304. A more detailed view 302 of four frames around the transmission gap 303 is also shown. In compressed mode, a series of slots are not used for transmission of data. The number of consecutive slots in the series not used for transmission defines the transmission gap length where the gap is within the compressed mode frame. Either side of the transmission gap 303 the instantaneous transmit power of the slots of the frame remaining (305, 306) for data transmission is increased in order to keep the quality of the communication link unaffected by the reduced time available for transmission. Alternatively the transmission gap may occur at the respective end and start of consecutive CM frames. Examples of the measure of quality are Bit Error Rate (BER) and Frame Error Rate (FER), although any other appropriate measure of quality may be used. The size of the increase in instantaneous transmit power is dependent upon the transmission time reduction method and may be zero.
A transmission gap is necessary because UEs typically only have one transceiver. UE capabilities vary and the capabilities of a particular UE define whether it requires compressed mode in order to monitor cells on other frequencies. Accordingly, it is necessary for a UE to communicate its compressed mode requirements to the UTRAN. The compressed mode requirement may be expressed for any number of bands and radio access technologies.
The mechanism allowing information transmitted during at least one portion of a frame to be compressed in time, and a transmission gap to be created, include: reducing the spreading factor; and higher layer scheduling.
Reducing the Spreading Factor: the Spreading Factor is reduced by a factor of 2 so the data rate is doubled in the frame in which compression is to be carried out. The Spreading Factor is the ratio of the chips to base band information rate, the chips being the smallest element of a slot. Because the data rate is doubled the same amount of data can be transmitted in half the time. Measurements can be performed in the transmission gap that remains.
Higher Layer scheduling: The higher layers are aware of the compressed mode schedule, so they may lower the data rate in the frame in which measurements need to be performed. This avoids the need for a new spreading factor and new channelization codes. For example, higher layers may set restrictions so that only a subset of the allowed Transport Format Combinations (TFCs) are used in a compressed frame.
The bit rate available for communication between the UE and the UTRAN is determined by a Transport Format Combination. Accordingly, by defining a subset of Transport Format Combinations available for use, the maximum number of bits that will be delivered to the physical layer during the compressed radio frame is then known and a transmission gap can be generated. In the downlink, the Transport Format Combination Indicator field is expanded at the expense of the data fields and this shall also be taken into account by higher layers when setting restrictions on which TFCs may be used.
In both downlink and uplink, both the above methods are supported. The network decides which frames are to be compressed. In compressed mode, compressed frames can occur periodically, as indicated in FIG. 1. Alternatively, compressed frames can occur on request or upon demand. The rate and type of compressed frames used is variable and depends on the environment and measured variables.
The UTRAN can schedule compressed mode patterns for implementation by the UE. A pattern can be finite in which case it will terminate at a given connection frame number (CFN) or can be infinite and terminated by the UTRAN which subsequently specifies the CFN where the pattern should end. Frames are numbered 0 to 255 cyclically independent of CM gaps.
This can be further understood from FIG. 2. At step 200 the UTRAN provides a transmission gap pattern sequence. At step 202 the user equipment activates the transmission gap pattern sequence. At step 204 the UE may take appropriate additional steps for example performing a measurement as described above.
According to section 8.1.2 of the 25.133 standard the UTRAN must ensure that with the activation of one or more transmission gap pattern sequences, no more than two frames can contain a transmission gap within any window of three consecutive frames, if the UTRAN schedules three or more CM gaps in a row, then the signal to interference ratio (SIR) calculation can not be performed which can lead to incorrect UE operation. However, in practice, it is found that in some instances the UTRAN appears to schedule three or more CM frames in a row for example due to transmission delay. In particular this can occur in the scenario shown, for example, in FIG. 3 in which a first 310 is deactivated and a second pattern 312 is subsequently activated. The patterns include normal frames 314, and compressed mode frames 316 and it will be seen that the first sequence 310 terminates in two compressed mode frames containing transmission gaps whereas the second pattern 312 commences with two compressed modes frames containing transmission gaps. In such circumstances the device can reset as a failure mode.
The invention is set out in the claims: