WO2008136714A1

WO2008136714A1 - A base station, a mobile terminal and a method for wimax system

Info

Publication number: WO2008136714A1
Application number: PCT/SE2007/050300
Authority: WO
Inventors: Anders Lamm; Stefan Lindgren; Shiau-He Shawn Tsai
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-05-03
Filing date: 2007-05-03
Publication date: 2008-11-13
Also published as: JP2010526474A; EP2140690A1; US20100214969A1; EP2140690A4; EP2140690B1; JP5455893B2

Abstract

A base station for use in a WiMAX system comprise means for sending a wake-up signal to an idle terminal to indicate to the terminal that a paging signal is about to be sent to the terminal. The terminal receives and interprets the wake-up signal and prepares itself for receiving the paging signal. This saves battery power in the terminal since the wake-up signal can be detected more easily than the paging signal. The base station, in the mean time, can reduce its paging message traffic and saves overhead in downlink transmission.

Description

A base station, a mobile terminal and a method for WiMAX system.

Technical Field

The present invention relates to wireless communication between a base station and a mobile terminal in a communications network such as WiMAX. In particular, the invention relates to reducing the power consumption of the terminals in such a network.

Background and Related Art

WiMAX, which is an abbreviation of World- wide Interoperability for Microwave Access, is an implementation of the IEEE standard 802.16, for providing wireless network access. In particular, the development of IEEE 802.16e provides the specifications of WiMAX services to mobile terminals in a cellular environment. WiMAX is intended for use in wireless Metropolitan Access Networks, MAN, and is described as a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL.

When directing an incoming call to a mobile station in a cellular environment, paging must be first performed to locate the receiving mobile terminal and to establish an active radio connection with it for the following traffic. Due to the limited capacity of batteries, it is not practical for the mobile terminal to maintain a persistent radio connection to the base station. During the idle state, the mobile terminal only wakes up periodically to check base station paging signals. To maximize the stand-by time, the wake-up duty cycle should be kept as low as possible with an acceptable paging delay. Meanwhile, the power consumption during the wake-up period should also be kept as low as possible.

One technique generally used to save mobile power during the wake-up period is to insert a simple energy signal in advance to indicate that a paging message will soon be sent to the mobile terminal. Instead of demodulating and decoding the entire downlink paging message during every wake-up period, the mobile terminal only detects if there is a simple energy signal directing to it before activating its entire demodulation and decoding device. Such a signaling mechanism based on simple energy detection before triggering entire paging process is called paging indicator, as in WCDMA, or quick paging, as in CDMA. By turning off demodulators for regular paging message during most wake-up periods and by only monitoring a simple energy signal, significant power saving and stand-by time extension is achieved.

In the current WiMAX system, there is no paging indicator or quick paging signal. The mobile terminal, during stand-by mode, needs to wake up periodically and turn on the demodulator and decoder to read the Base Station Broadcast Paging (MOB_P AG- ADV) message. When waking up in paging intervals, the mobile terminal first reads the FCH and the DL-MAP to look for the location and the format of the broadcast CID. Then, the mobile terminal will demodulate the broadcast burst that contains the MAC PDU of MOB_PAG-ADV message. Most of the time, there is no paging information for the mobile terminal or only a dedicated Action Code ObOO (meaning no action required). The processing of reading FCH, DL-MAP, and the MAC PDU burst will invoke the entire baseband modem function, hence drains mobile's battery over time. During each paging interval, the mobile terminal has to be awake and demodulate the broadcast messages for 2 to 5 frames. In addition, a change in the downlink or uplink channel descriptor (DCD or UCD) also triggers a paging broadcast to wake up all mobile terminals to update the system parameters, although there is no dedicated page message to every one of them.

Object of the Invention

It is an object of the invention to reduce the power consumption during idle time in mobile terminals according to the 802.16 standard. The person skilled in the art can easily generalize this invention to OFDM-based systems with similar frame structure. Summary of the Invention

This object is achieved according to the present invention by a base station for use in a WiMAX system for communicating with a number of mobile terminals, said base station comprising communication means for sending a paging signal to a mobile terminal, the base station being characterized in that the communication means is arranged to send a wake-up signal to the mobile station, to indicate to the mobile terminal concerned that it should prepare itself for a paging signal, before sending the paging signal to the mobile terminal.

The object is also achieved by a mobile terminal arranged to communicate with a base station in a WiMAX system, said mobile terminal being arranged to go into idle mode when it is not communicating with the base station and comprising a receiving unit for receiving a paging signal when it should go into active mode, said mobile terminal being characterized in that the receiving unit is also arranged, when in idle mode, to receive a wake-up signal from the base station and upon receiving the wake-up signal, prepare itself to receive a paging signal.

The object is also achieved by a method for use in a base station in a WiMAX system, comprising the following steps:

- determining that a paging signal should be sent to an identified mobile terminal in the system,

- sending a wake-up signal, to the identified mobile terminal, to indicate that the identified mobile terminal is about to receive a paging signal,

- sending a paging signal according to the prior art to the mobile terminal shortly after sending the wake-up signal.

The invention also relates to a method for use in a mobile terminal in a WiMAX system, comprising the following steps:

- receiving a wake-up signal in the mobile terminal - upon receiving the wake-up signal, preparing the mobile terminal for receiving the paging signal by turning on the demodulator and decoder of the mobile terminal,

- receiving the paging signal, and

- upon receiving the paging signal, going into active mode to start communicating with network through the base station.

Hence, according to the invention a wake-up signal is achieved, which may be used to alarm a particular mobile terminal when a paging message is about to be sent to it. The wake-up signal is a physical signal that is easy to demodulate and detect, which means that the mobile station in stand-by mode does not need to turn on the whole baseband circuit and look for paging message, which requires demodulating and decoding the FCH, the DL-MAP, and the data burst. The wake-up signal is sent to the to-be-paged user at one or a few frames before the actual MOB_PAG-ADV message. A DCD/UCD change in the system may also trigger a wake-up signal to alarm idle mobile stations to look for the new DCD/UCD.

Preferably, the wake-up signal is sent as a simple signal that can be received by the mobile terminal by simple energy detection.

The wake-up signal may be placed in one of the transition gaps, TTG or RTG in a frame or, alternatively, at the end of the downlink subframe. A third option would be to place the wake-up signal immediately following the FCH in the DL subframe. In yet another embodiment the wake-up signal is placed by fixed block allocation.

The wake-up signal can be embedded at a specific location in the DL data region without ambiguity. In TDD mode, the wake-up signal can be located in the TTG, where the active users are transitioning from the receive mode to the transmit mode, while the standby users are not required to transmit. Alternatively, the wake-up signal can be located in the RTG. In one embodiment the wake-up signal may be sent as a set of evenly spaced tones with fractional symbol duration. The wake-up signal may also be sent as a set of single or multiple tones in the downlink subframe data region as the wake-up signal, or as single-carrier pulses.

The terminal that is to wake up may be identified in the wake-up signal by means of an address based on the terminal's MAC address. Alternatively, a sector specific address assigned to the terminal may be included in the wake-up signal to identify the terminal.

The wake-up signal may be comprised of a number of consecutive wake-up signals, for example, in consecutive frames, to the same mobile terminal.

With the method and apparatus of the invention the idle mode power consumption is reduced and the stand-by time for the mobile terminals is improved. The invention enables flexible allocation of frequency-time resources in WiMAX systems while reducing the DL overhead

Acronyms and Abbreviations

BS Base station

BW Bandwidth

CDMA Code Division Multiple Access

CID Connection ID

DCD Downlink channel descriptor

DL Downlink

FCH Frame control header

MAC Medium Access Control

OFDMA Orthogonal frequency division multiple access

PDU Protocol Data Unit

PS Physical slot = 4 OFDM time samples

RTG Receive-to-transmit Transition Gap TDD Time Division Duplex

TTG Transmit-to-receive Transition Gap

UCD Uplink channel descriptor

UL Uplink

WCDMA Wideband CDMA

WiMAX Worldwide interoperability for microwave access

Brief Description of the Drawings

The invention will be described in more detail in the following, by way of example and with reference to the appended drawings in which: Figure 1 illustrates a general WiMAX system. Figure 2 is an overall flow chart of the inventive method.

Figure 3 a illustrates a WiMAX TDD frame structure and Figure 3b illustrates the paging interval with such a frame structure. Figure 4 is a diagram illustrating timing advance in WiMAX. Figure 5 illustrates the location of a wake-up message according to one embodiment of the invention in a WiMAX TDD frame structure.

Figure 6 illustrates the location of a wake-up message according to another embodiment of the invention in a WiMAX TDD frame structure. Figures 7a, 7b and 7c illustrate possible formats of the quick paging signal.

Detailed Description of Embodiments

Figure 1 illustrates a general WiMAX system for providing wireless communication with mobile terminals. The system comprises a base station 1 communicating with a number of mobile terminals 3, 5, 7. As is common in the art, the mobile terminals 3, 5, 7 are arranged to go into standby mode when they are not engaged in communication with the base station 1. To wake up a mobile terminal when it should engage in communication, the base station sends a paging signal through a communication means 9 in the base station 1. This paging signal is received and interpreted in a receiving unit 11 in the mobile terminal. To interpret the paging signal the mobile terminal must turn on its demodulator and decoder (not shown), which must be done at regular intervals, in case a paging signal intended for the terminal is received.

According to the invention, before sending the paging signal, the communication means 9 of the base station sends a wake-up signal to indicate to the mobile terminal concerned that it should prepare itself for a paging signal. The wake-up signal is a simple signal that can be received by the mobile terminal by simple energy detection. Since the wake-up signal can be detected by the mobile terminal using less battery resources, this saves battery power in the mobile terminals. As long as no such wake-up signal has been received, the mobile terminal does not have to prepare itself for receiving a paging signal. The format and location of the wake-up signal will be discussed in more detail in the following.

Figure 2 is a flow chart of the overall method according to the invention. In step S 1 the base station 1 determines that a paging signal should be sent to an identified one of the mobile terminals 3, 5, 7.

In step S 2 the base station 1 first sends a wake-up signal, or quick paging signal, to the identified mobile terminal.

In step S3 the mobile terminal receives the wake-up signal. In step S4 the mobile terminal prepares itself to receive the proper paging signal. This includes turning on the demodulator and decoder to read the paging message. In step S 5 the base station 1 sends a paging signal according to the prior art to the mobile terminal. The time between the transmission of the wake-up signal and the paging message, that is, the time between steps S3 and S5, should be equal to or slightly longer than the propagation plus mobile processing delay. In step S 6 the mobile terminal receives the paging signal and goes into active mode to start communicating with network through the base station 1. Steps Sl, S 2 and S 5 are performed by the communication means 9 in the base station, whereas steps S3, S4 and S6 are performed by the receiving unit 11 in the mobile terminal 3, 5, 7 concerned. The WiMAX TDD frame structure is shown in Figure 3. The frame is divided into the Downlink (DL) and the Uplink (UL) subframes, separated by a transmit-to- receive transition gap (TTG), in which the base station switches off its transmitter and switch on their receivers to prepare for receiving from the mobile terminals. At the same time, the mobile terminals switch off their receivers and switch on their transmitters. Each DL subframe starts with a preamble followed by a frame control header (FCH); both have fixed length and location. The length and the format of the DL MAP are specified by the FCH. The length and the format of the UL MAP are given in the DL MAP. At the end of the UL subframe, there is a receive-to-transmit transition gap (RTG), in which the base station switches off the receiver and switches on its transmitter. During the RTG, the active mobile terminal switches off its UL transmitter and switches on its receiver for receiving the next DL subframe.

The paging process currently done in WiMAX is illustrated in Figure 3b. The time axis is divided into paging cycles. Each mobile terminal is addressed by the base station once every cycle. The location of each mobile terminal's paging signal in each cycle is determined by the paging offset. The paging message may span over several frames and a mobile may need to demodulate several frames to read the entire message. The length of the message is specified by the paging interval length in the unit of frames. According to the invention, a simple wake-up signal is inserted within each paging interval to reduce the mobile terminal's processing power consumption without reading the entire paging message. To extend coverage, the simple wake-up signal may also span over the entire paging interval of several frames for improved energy detection. The mobile terminal can accumulate more energy for reliable detection without turning on the entire demodulator/decoder chain.

The wake-up signal according to the invention, that is sent to a mobile terminal to indicate that it should prepare for receiving a paging signal, should be located in a fixed position of the frame, so that the terminal would know what part of the frame to consider. To this end, the wake-up signal may be located in different parts of the frame shown in Figure 3a, as will be discussed below. In a preferred embodiment, which is currently considered to be the best mode of the invention, the wake-up signal is inserted in one of the transition gaps, that is, the TTG or the RTG, that is, when the base station is turning off its transmitter and turning on its receiver or vice versa. Therefore, some of this time could be used by the base station to transmit a wake-up signal while there would still be time to perform the transmit to receive transition.

In the current WiMAX frame, TTG serves two purposes: the first is to reduce the interference in the beginning of a UL subframe caused by long propagation of the DL subframe from other cells, and the second is to allow far users to advance their timing for synchronized UL transmission. The typical TTG specified in the system profile is on the order of 100 μsec. Most DL signals from other cells become negligible after such a long propagation.

The far users, in addition to time needed to switch between receiving and transmitting, also need to compensate their UL propagation delays by advancing their transmit timing. This illustrated in Figure 4, which shows, as an example, the frame timing of four frames in TDD: a) for the base station b) for a first mobile terminal located near the base station c) for a second mobile terminal located far from the base station, with no timing advance d) for the second mobile terminal, with timing advance

The actual available switching gap between transmitting and receiving in each case is represented by a solid arrow. As can be seen, for the first mobile terminal, illustrated in b) which is located near the base station, the available switching gap is shortened by the propagation delay of one time unit in the downlink, and the timing advance of one time unit in the uplink to align with the base station's uplink sub- frame timing. For the second mobile terminal, as illustrated in c) with no timing advance, the available switching gap is shortened by the propagation delay in the downlink, but not in the uplink. The second mobile terminal will then have the same length of available switching gap as the first terminal, but will lose its synchronization with base station's uplink subframe timing by two time units. With timing advance of two time units, as shown in d), the transition gap will be further shortened by the propagation delay in the downlink and the timing advance in the uplink will be in sync with the uplink subframe timing.

From each mobile terminal, the TTG should be larger than the sum of its receive- transmit switching time and DL/UL propagation delays seen at its location. But for idle mode terminals waiting for paging, there is no need to prepare for uplink transmission right after the TTG. Therefore, the DL subframe can be prolonged into the TTG to include wake-up signals, which only the idle users listen to and the active users treat as non-existent, as long as the interference to other cells is maintained below an acceptable level.

As mentioned above, the wake-up signal could also be located in the RTG. In the current WiMAX, RTG allows the UL signal to fade before the next DL subframe takes place such that strong interference from near-by mobile terminals to the DL reception can be avoided. In the WiMAX system profile, the RTG value is either 60 μs or 74.4 μs, which roughly corresponds to 18km or 22km in distance. In many deployment scenarios, the signal from a mobile terminal is not likely to interfere with far users after long propagation and there will be room at the end of RTG with negligible amount of UL interference. Therefore, the base station can insert the wake-up signal right before the next preamble without it being severely corrupted by the residual UL propagation. There could be some degradation in the preamble acquisition when compared with an empty RTG, but with properly designed wake- up signal waveform, the impact can be reduced. Figure 5 illustrates a third possible location of the wake-up signal: at the end of the DL subframe in all subchannels, as illustrated by a block bl This can be achieved by modifying the DCD or hard coding new mobile terminals. The base station can simply avoid allocating any data bursts into this location by restricting its own DL MAP from using the end of a DL subframe.

The three possible locations of the wake-up signal discussed above have the advantage of being backwards compatible, that is, they can be implemented without any impact to the legacy mobile terminals.

A fourth possible location of the quick paging signal would be a fixed block allocation. The allocation is similar to the block allocation of ranging, fast-feedback, and P APR-reduction/Safety /Sounding zones currently used on the uplink for other purposes. Hence, it does not introduce any new resource allocation scheme. Through adding new fields in the DCD, the new mobile terminals can be assigned to this new DL block allocation for quick paging. This embodiment may not be backward compatible. Meanwhile, the base station can organize the DL MAP for the existing mobile terminals being consistently excluded from the new wake-up signal region. In this way, existing, or legacy, mobile terminals will not benefit from the wake-up signal but can still be used.

A fifth possible location of the quick paging signal would be immediately following the FCH, as illustrated in Figure 6 by a block b2. Because the preamble and the FCH have fixed lengths, this location does not change from frame to frame. However, inserting wake-up signal into this location generates backward compatibility issues, since existing mobile terminals are supposed to search for DL MAP at this location.

A series of consecutive wake-up signals for the same terminal may be sent in consecutive frames. In this case the mobile terminal will be arranged to monitor for a specified number of consecutive wake-up signals before it will prepare itself for re- ceiving the proper paging signal. The number of wake-up signals needed should be predetermined and may be selected arbitrarily. For example, two, three or four consecutive wake-up signals may be required.

The format of wake-up signal shall satisfy the fundamental requirement of being received by simple energy detection. Formats of wake-up signal include the following options:

• Single or multiple tones in DL subframe data region with or without a signature sequence

• A set of evenly spaced tones with fractional symbol duration

• Single-carrier pulses

Single or multiple tones in DL subframe data region with or without a signature sequence will work for all the different locations of the signal discussed above. This format can be applied to any location listed in the previous subsection. The wake-up signal can be indicated by the existence of one or multiple tones assigned to the idle user. In addition, orthogonal coding can be applied to those tones. Also, if more capacity is needed, wake-up signals of different users might be modulated by signature sequences with low cross-correlation, provided that the false alarm rate is maintained under a desired level.

A set of evenly spaced tones with fractional symbol duration can be used if the wake-up signal is located in the TTG or the RTG. Wake-up signal of one OFDMA symbol may be too long for the TTG. As shown in Table 1 below, in the first release of WiMAX system profile, which is the most typical implementation of the 802.16 OFDMA mode, the OFDMA symbol duration is chosen to be 102μs and the TTG 105μs. If the wake-up signal occupies one OFDMA symbol duration, it leaves only 3μs before the base stations start receiving UL signals. As a result, wake-up signals from other base stations will interfere with the UL signal. One solution is to transmit only some of the tones. This is illustrated in Figure 7a, which shows the duration of the OFDMA symbol for different frequencies 1/T_base=f_base, 2f_base, , etc. up to όf_base, where T_base is the useful OFDM symbol duration and its reciprocal is called the base frequency. As can be seen, the period for the frequency 2f_base is half of that for f_base; the period for the frequency 3f_base is one third of that for f_base, etc. Hence, using, for example, every other tone starting with 2f_base will reduce the wake-up OFDMA symbol duration to half. This is illustrated in Figure 7b, which shows the wake-up signal with Vi OFDM symbol duration based on integer multiples of 2f_base. Following the same principle, using only every third tone 3f_base, 6fbase, 9f_base etc., will reduce the wake-up symbol duration to one third. This is illustrated in Figure 7c, which shows the wake-up signal with 1/3 OFDM symbol duration based on integer multiples of 3f_base. As a general rule, using the subset of tones whose frequencies are integer multiples of the Nth harmonic of the base frequency, 1/T_base, the wake-up signal transmitted symbol duration can be effectively shortened to 1/N of the useful OFDM symbol duration, T_base. At the receiver side, the same FFT module for demodulating other OFDM symbols of T_base duration can be used by simply padding zeros outside the shortened T_base/N wake-up symbol duration to perform energy detection. At the transmitter side, only the first 1/N of the N_FFT samples is converted from digital to analogue waveform for RF transmission. That is, no new hardware is required to send or detect this type of wake-up signal. As explained, all of these frequencies may be used for wake-up signals, or a subset, such as every second, or every third frequency may be selected with a corresponding, fractional OFDM symbol time. This fractional frequency-time space usage can accommodate the newly inserted wake-up signals and still leaves time for residual DL signal energies to fade without interfering with the UL. The advantage of this approach is that the same FFT circuit can be used at the base station transmitter, while the mobile terminal's receiver can apply the same preamble detector, over a fractional symbol time window. The wake-up signal, when being sent with fractional symbol duration, might have coverage issues because of shorter symbol time to accumulate energy. A combination of multiple fractional- symbol-time wake-up signals over multiple frames that are dedicated to the same recipient(s) can be used to improve the coverage. Single-carrier pulses: Single-carrier pulses can be introduced to carry the wake-up energy signal in the TTG or RTG, outside the normal data region of an OFDMA frame. These pulses can be modulated by a user-dependent signature sequence to improve detection performance. The signature sequence can be mutually orthogonal or lowly cross correlated among different users. In the general sense, signature sequence modulation also includes time multiplexed pulses, since each individual one can be viewed as modulated by a sequence with non-zero values only at time instances the pulse is sent.

Table 1 : RTG and TTG in WiMAX System Profile

The wake-up signal should ideally be directed at one particular mobile terminal, which should be woken up in order to receive a proper paging signal. This will be optimal for saving battery time in the mobile terminals. On the other hand it may be easier to implement embodiments in which a wake-up signal can indicate a group comprising more than one mobile terminal. The mobile terminals in the group will all interpret the wake-up signal as intended for them and will start listening for a paging signal. Hence, for the mobile terminals of the group that are not about to receive a paging signal, some battery power will be wasted compared to the embodiments where a wake-up signal only indicates one mobile terminal. On the other hand an improvement will still be achieved compared to the prior art.

In a WiMAX system, the mobile terminal is recognized by its 48-bit MAC address. In the BS broadcast paging message, the mobile terminal identifies its paging in- struction by the "MS MAC Address hash" field. This 24-bit field has 2 24 =16769024 possible values. Hence, the wake-up signal cannot be mapped to one particular MS MAC Address hash. There are several ways to reduce the required wake-up signal space for mapping the identities of mobile terminals.

One option is to group hashed MAC addresses by frame number: In each frame, only a subset of MAC address will be mapped to the wake-up signal. For example, if there are 2⁷=128 signals in the wake-up signal space for one frame, then 2¹⁷= 131072 frames will be needed to complete the mapping to all possible 24-bit hashed MAC addresses. As can be understood, the drawback of this method is that it causes a delay in the paging response from the mobile terminal.

Alternatively the hashed MAC addresses can be divided into subsets by wake-up groups: One wake-up signal can be mapped to multiple MAC addresses. The set of MAC addresses being mapped to the same wake-up signal, that is, the addresses of all mobile terminals that will be woken up by the same energy signal, is a wake-up group. The drawback of this method is that other users in the same group that are not being paged may also be alarmed by the wake-up signal.

A third option would be to assign a sector- specific wake-up number to each mobile terminal that is actually in the relevant area, This wake-up number should be cancelled when the mobile terminal leaves the area, so that it can be reassigned to another mobile terminal. Some sector will not accommodate more than some thousands of mobile terminals. In reality a sector specific number can be assigned to each mobile terminal for wake-up during network entry. The set of such wake-up numbers should be much smaller than the set of hashed MAC addresses.

A specific embodiment of the third option would be to use an identification number already used in the 802.16 standard to identify mobile terminals, such as the Connection ID (CID), as a wake-up number. The CID, which is 16-bit long, or its reduced format, which can be H-, 7- or 3-bit long, may be used for the wake-up number purpose. That is, before a mobile terminal completely de-register from the pag- ing zone, one of its CID, e.g., basic CID, is still maintained in a network paging logic entity. There will be some additional constraints for a sector to coordinate its CID assignment with other sectors in the paging zone. In another embodiment, a dedicated segment of the 2¹⁶ CID space can be created for each paging zone as the wake-up identifier. The grouping techniques mentioned above is also applicable to CID-based wake-up identifier to reduce the number of signals in the space of wake- up signalling.

Any of the methods listed above for identifying the mobile terminal that is to receive a paging message may be used alone or in combinations of two or more. For example, different sectors may use different approaches.

Claims

1. A base station 1 for use in a WiMAX system for communicating with a number of mobile terminals 3, 5, 7, said base station comprising communication means (9) for sending a paging signal to a mobile terminal, the base station being characterized in that the communication means (9) is arranged to send a wake-up signal to the mobile station, to indicate to the mobile terminal concerned that it should prepare itself for a paging signal, before sending the paging signal to the mobile terminal.

2. A base station according to claim 1, wherein the communication means is arranged to send the wake-up signal as a simple signal that can be received by the mobile terminal by simple energy detection.

3. A base station according to any one of the preceding claims, wherein the communication means (9) is arranged to place the wake-up signal in one of the transition gaps, TTG or RTG in a frame.

4. A base station according to any one of the preceding claims, wherein the communication means (9) is arranged to place the wake-up signal at the end of the downlink subframe.

5. A base station according to any one of the preceding claims, wherein the communication means (9) is arranged to place the wake-up signal immediately following the FCH in the DL subframe.

6. A base station according to any one of the preceding claims, wherein the communication means (9) is arranged to place the wake-up signal by fixed block allocation.

7. A base station according to any one of the claims 1- 3, wherein the communication means (9) is arranged to send a set of evenly spaced tones with fractional symbol duration as the wake-up signal.

8. A base station according to any one of the claims 1-6, wherein the communication means (9) is arranged to send single or multiple tones in the downlink subframe data region as the wake-up signal.

9. A base station according to claim 3, wherein the communication means (9) is arranged to send single-carrier pulses as the wake-up signal.

10. A base station according to any one of the preceding claims, wherein the communication means is arranged to include an address based on the terminal's MAC address in the wake-up signal to identify the terminal.

11. A base station according to any one of the preceding claims, wherein the communication means is arranged to include a sector specific address assigned to the terminal in the wake-up signal to identify the terminal.

12. A base station according to any one of the preceding claims, wherein the communication means is arranged to include an identifier based on an identification number previously assigned to the terminal, such as the Connection ID.

13. A base station according to any one of the preceding claims, wherein the communication means is arranged to send a number of consecutive wake-up signals, for example, in consecutive frames, to the same mobile terminal to indicate that it should prepare itself for a paging signal.

14. A mobile terminal (3, 5, 7) arranged to communicate with a base station in a WiMAX system, said mobile terminal being arranged to go into idle mode when it is not communicating with the base station and comprising a receiving unit (11) for receiving a paging signal when it should go into active mode, said mobile terminal being characterized in that the receiving unit (11) is also arranged, when in idle mode, to receive a wake-up signal from the base station (1) and upon receiving the wake-up signal, prepare itself to receive a paging signal.

15. A mobile terminal according to claim 14, wherein the receiving unit (11) is arranged to receive the wake-up signal by simple energy detection.

16. A mobile terminal according to claim 14 or 15, wherein the receiving unit (11) is arranged to detect the wake-up signal in one of the transition gaps, TTG or RTG in a frame.

17. A mobile terminal according to any one of the claims 14-16, wherein the receiving unit (11) is arranged to detect the wake-up signal at the end of the downlink subframe.

18. A mobile terminal according to any one of the claims 14-17, wherein the receiving unit (11) is arranged to detect the wake-up signal immediately following the FCH in the DL subframe.

19. A mobile terminal according to any one of the claims 14-18, wherein the receiving unit (11) is arranged to detect the wake-up signal as allocated by fixed block allocation.

20. A mobile terminal according to any one of the claims 14- 16, wherein the receiving unit (11) is arranged to receive and interpret a set of evenly spaced tones with fractional symbol duration as the wake-up signal.

21. A mobile terminal according to any one of the claims 14-19, wherein the receiving unit (11) is arranged to receive and interpret single or multiple tones in the downlink subframe data region as the wake-up signal.

22. A mobile terminal according to claim 16, wherein the receiving unit (11) is arranged to receive and interpret single-carrier pulses as the wake-up signal.

23. A mobile terminal according to any one of the claims 13-21, wherein the receiving unit (11) is arranged to identify the paging signal as intended for it by means of its MAC address.

24. A mobile terminal according to any one of the claims 13-22, wherein the receiving unit (11) is arranged to identify the paging signal as intended for it by means of a sector specific address assigned to the terminal.

25. A mobile terminal according to any one of the claims 13-22, wherein the receiving unit (11) is arranged to identify the paging signal as intended for it by means of an identifier based on an identification number previously assigned to the terminal, such as the Connection ID.

26. A mobile terminal according to any one of the claims 14-25, wherein the receiving unit is arranged to prepare itself to receive a paging signal when a predetermined number of consecutive wake-up signals have been received.

27. A method for use in a base station (1) in a WiMAX system, comprising the following steps:

- determining that a paging signal should be sent to an identified mobile terminal (3, 5, 7) in the system;

- sending a wake-up signal, to the identified mobile terminal, to indicate that the identified mobile terminal is about to receive a paging signal;

- sending a paging signal according to the prior art to the mobile terminal shortly after sending the quick paging signal.

28. A method according to claim 27, wherein the wake-up signal is sent as a simple signal that can be received by the mobile terminal by simple energy detection.

29. A method according to claim 27 or 28, wherein the wake-up signal is sent in one of the transition gaps, TTG or RTG in a frame.

30. A method according to any one of the claims 27-29, wherein the wake-up signal is sent at the end of the downlink subframe.

31. A method according to any one of the claims 27-30, wherein the wake-up signal is sent immediately following the FCH in the DL subframe.

32. A method according to any one of the claims 27-31, wherein the wake-up signal is located in the frame by fixed block allocation.

33. A method according to any one of the claims 27-29, wherein the wake-up signal is sent as a set of evenly spaced tones with fractional symbol duration.

34. A method according to any one of the claims 27-32, wherein the wake-up signal is sent as single or multiple tones in the downlink subframe data region.

35. A method according to claim 29, wherein the wake-up signal is sent as single- carrier pulses.

36. A method according to any one of the claims 27-35, wherein the mobile terminal is identified in the wake-up signal by means of its MAC address.

37. A method according to any one of the claims 27-35, wherein the mobile terminal is identified in the wake-up signal by means of a sector specific address assigned to the terminal.

38. A method according to any one of the claims 27-35, wherein the mobile terminal is identified in the wake-up signal by means of an identifier based on an identification number previously assigned to the terminal, such as the Connection ID.

39. A method according to any one of the claims 27-38, wherein the wake-up signal comprises a predefined number of consecutive wake-up signals.

40. A method for use in a mobile terminal (3, 5, 7) in a WiMAX system, comprising the following steps:

- receiving a wake-up signal in the mobile terminal

- upon receiving the wake-up signal, preparing the mobile terminal for receiving the paging signal by turning on the demodulator and decoder of the mobile terminal,

- receiving the paging signal, and

- upon receiving the paging signal, going into active mode to start communicating with network through the base station (1).

41. A method according to claim 40, wherein the wake-up signal is a simple signal that can be received by the mobile terminal by simple energy detection.

42. A method according to claim 40 or 41, wherein the wake-up signal is received in one of the transition gaps, TTG or RTG in a frame.

43. A method according to any one of the claims 40-42, wherein the wake-up signal is received at the end of the downlink subframe.

44. A method according to any one of the claims 40-43, wherein the wake-up signal is received immediately following the FCH in the DL subframe.

45. A method according to any one of the claims 40-44, wherein the wake-up signal is located in the frame by fixed block allocation.

46. A method according to any one of the claims 40-44, wherein the wake-up signal is received as a set of evenly spaced tones with fractional symbol duration.

47. A method according to any one of the claims 40-45, wherein the wake-up signal is received as single or multiple tones in the downlink subframe data region.

48. A method according to claim 42, wherein the wake-up signal is received as single-carrier pulses.

49. A method according to any one of the claims 40-48, wherein the mobile terminal is identified in the wake-up signal by means of its MAC address.

50. A method according to any one of the claims 40-48, wherein the mobile terminal is identified in the wake-up signal by means of a sector specific address assigned to the terminal.

51. A method according to any one of the claims 40-48, wherein the mobile terminal is identified in the wake-up signal by means of an identifier based on an identification number previously assigned to the terminal, such as the Connection ID.

52. A method according to any one of the claims 40-51, wherein the receiving unit is arranged to prepare itself to receive a paging signal when a predetermined number of consecutive wake-up signals have been received.