WO1999039460A1

WO1999039460A1 - Tdma

Info

Publication number: WO1999039460A1
Application number: PCT/SE1999/000101
Authority: WO
Inventors: Per-Erik ÖSTLING
Original assignee: Radio Design Innovation Tj Ab
Priority date: 1998-01-29
Filing date: 1999-01-26
Publication date: 1999-08-05
Also published as: SE9800250L; AU2555199A; SE9800250D0

Abstract

The invention relates to a method for transmitting information using multiframe TDMA. In the invention the frames of a conventional TDMA-system are grouped into multiframes, which consist of at least one frame that is used for measurement or transmission of other data than user data.

Description

APPLICANT: RADIO DESIGN INNOVATION AB TITLE OF INVENTION: TDMA

1. FIELD OF INVENTION

The present invention relates to a method for transmitting information between telecommunication devices using multiframe TDMA.

2. BACKGROUND OF THE INVENTION

2.1 TDMA General

Time division multiple access (TDMA) is a multiple access method used to distinguish signals from different users in digital wireless systems. A number of users share the same transmitter/receiver (TRX) in the base station by transmitting in alternating slots in a frame, Figure 1. In each frame the sequence of slots are repeated and each slot uses a given frequency; the frequency could be the same or different frequencies could be used. One key feature of TDMA is that it is possible to assign a particular transmit and receive slot that do not coincide in time. Figure 1 discloses a conventional TDMA structure with N time slots per frame where each slot is allocated to a different user. The frame structure in the uplink is delayed compared to the downlink (or vice versa) to prevent that the terminal is simultaneously transmitting and receiving in order to eliminate the need for duplex filters in the terminal. 2 2.1.1 Advantages with conventional TDMA

Two major advantages with TDMA are: no duplex filter is needed in the terminal, and idle slots (used by other users) can be utilized for measurements by the terminal. The first feature makes terminals cheaper and smaller, and the second is desirable in modern cellular systems where the terminals participate to a high degree in allocation of radio resources. For example, in idle slots the terminal can measure the signal strengths to other base stations for handoff purposes or in interference based dynamic channel allocation (DCA) systems the terminal can measure interference levels on other frequencies.

2.1.2 Limitations of TDMA

There must be at least two time slots per frame in order to remove the duplex filter in the terminal and three or more slots are needed if any idle time slot should be available for measurements. Unless a separate measurement receiver and a duplex filter is used it is not possible to make measurements of other frequencies during the time slots the terminal is receiving or transmitting. The more time slots (users) there are per frame the more time will be available for the terminal to make measurements.

A major disadvantage with TDMA, however, is that the bit rate on the channel increases making the communication link more sensitive to time dispersion. This will reduce the link quality and may require a complex equalizer in order to enable communication. The channel bit rate is basically proportional to the number of time slots (users) per frame.

2.2 Problem solved by means of invention

Apparently there is a contradiction in simultaneously achieving a lot of time for measurements and low bit rate on the channel. For instance, with only two time slots per frame it is not possible to make any measurements at all in the terminal since the terminal will always be busy transmitting or receiving. The smallest number of time slots per frame (i.e., the lowest channel bit rate) that can be allowed while providing possibilities for measurements is three, but then it is only possible 3 to make measurements of 33% of the channels since the terminal can only measure during one of three time slots (a channel is defined as one time slot in a frame on a given carrier frequency).

Thus, the object of the present invention is to overcome the above contra- diction and allow TDMA with only a few time slots per frame while still obtaining high capacity for measurements.

3. PREVIOUS WORK

In order to find out whether the prior art overcomes this contradiction, a novelty search was performed, the result of which is presented below: Dl : WO 96/21998

D2: EP 0 444 592

D3 : US 5 617 412

D4: IBM Technical Disclosure Bulletin, Vol. 39, No. 1, January 1996, page 353. Dl discloses a system using multiframes, wherein at least two measurement periods are used during a multiframe.

D2 discloses a TDMA-system using multiframes.

D3 discloses a FDMA-system using multiframes which comprise a predetermined number of frames including at least on control frame. D4 discloses slot rotation using pseudo random number generator in a TDMA- system for improving the carrier-to-interference ratio.

However, these presented documents do no overcome the above discussed contradiction.

4. SUMMARY OF THE INVENTION The above object is achieved by means of a method as claimed in claim 1.

An advantage of the method in the present invention is that it alleviates the problem of poor possibilities of performing measurements at the terminal when the number of users per carrier is low (2 or a few more users per carrier). It also allows a terminal to make measurements of other frequencies in the time slots allocated to that user for transmission and reception. Another advantage is that the invention utilizes the main benefits of TDMA (no duplex filter in terminal and time for measurements by terminal) while maintaining minimum increase of channel bit rate and is most useful when there are only a few time slots per frame (i.e., 2-4 time slots).

Other characteristics of the present invention will be described in the dependent claims.

5. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, with refer- ence to the accompanying drawings, in which:

Figure 1 is a conventional TDMA structure with N users on the same carrier;

Figure 2 is a multiframe TDMA structure according to the invention;

Figure 3 is an example of the multiframe TDMA structure according to Figure 2; Figure 4 is an example of a measurement frame, the time slots of which can be used on different carriers;

Figure 5 discloses the general frequency hopping case for user i and user k;

Figure 6 discloses a special case where users on the same carrier use the same measurement frame.

6. DETAILED DESCRIPTION OF THE INVENTION

6.1 Multiframe

The invention will now be described with reference to Figures 1-3. Figure 2 discloses a frame structure of the inventive multiframe TDMA scheme with M frames per multiframe. Of these frames are MA conventional frames and at least one measurement frame as described below.

The capacity for making measurements can readily be changed by altering the number of frames in the multiframe (at the cost of increased channel bit rate) or by changing the number of measurement frames in the multiframe. Furthermore, the number of frames per multiframe may, or may not, vary between users. It may also change with time for a given user. Of course, if there are more than two time slots per frame the terminal can make measurements during the idle slots in the frame. The terminal can also steal conventional frames and make measurements in these frames. With the concept "steal", we mean that terminals use conventional data frames for measurements.

6.1.1 Conventional frame

Each conventional frame is constituted by Ntime slots with a possibility to allocate each slot to a different user, Figure 1. The slots can be transmitted on the same carrier frequency or on different carrier frequencies.

6.1.2 Measurement frame

The measurement frame is used by the terminal for reception of dummy 1 data or measurements. The quantities being measured could be, for example, interference levels of other traffic channels or signal strengths of broadcast signals from other base stations. It should be realized that the duration of the measurement frame not necessarily has to be of the same duration as a conventional frame. The duration of the measurement frame may be a fraction of a conventional frame as well as longer (e.g., a multiple) than a conventional frame.

In contrast to other multiframe approaches where an extra frame is defined per carrier (i.e., all users belonging to the carrier are forced to use that extra frame), the multiframe structure is here defined per user basis. This also allows the multiframe approach to be used also in systems with frequency hopping where individual users have independent hopping sequences.

The position of the measurement frame in the multiframe may, or may not, vary between multiframes for a given user, independently of the positions of the measurement frames of the other users.

As in other TDMA systems better measurement performance is obtained if slot synchronization among base stations is used. If the measurement frame is of different length than the conventional frames it may also be necessary to use multiframe synchronization so that the measurement frame occurs simultaneously at all base stations. As an example of a multiframe structure Figure 3 discloses the particular frame structure for a TDMA scheme with two users (two time slots) per frame. The figure shows that the measurement frame is the only possibility to make measurements since a terminal will transmit in time slot 1 and receive in time slot 2, or vice versa.

6.2 Terminal measurements

During the measurement frame for a given user the corresponding terminal can make measurements on an arbitrary carrier frequency in each time slot (or even on several frequencies during a slot) of the measurement frame independently of which frequencies other users allocated to the same measurement frame measure. This is illustrated in Figure 4 for a measurement frame of the same duration as a conventional frame (Ntime slots). In each time slot the terminal can measure on anyone of the carrier frequencies F\-Fj_ (indicated by an X).

One event that can occur in a system with measurement frames is that a user makes interference measurements on a channel allocated to another user which has its measurement frame simultaneously. Since that user will also be busy making measurements there will not be any traffic data conveyed to the terminal, and unless some other kind of signal is transmitted by the base station the first user will perceive this channel as non-interfered. Such an event is called a measurement collision.

6.3 Methods for avoiding measurement collisions

Measurement collisions should be avoided since they may result in allocation of interfered channels. There are basically three methods two avoid or reduce the impact of measurement collisions: scheduling of measurement frame position,

¹ The term "dummy" in this case refers to non-traffic data. In reality this could be some kind of control alternating measurement frame position, and transmission of dummy information by the base station during measurement frames. The best choice depends on the system configuration and especially whether the base station transmits dummy information or not during measurement frames. We will first describe the principles of these methods and present examples of their functions later.

6.3.1 Base station silent during measurement frames

Whether or not it is possible to completely avoid the measurement collisions when the base station does not transmit during measurement frames depends on the number of carrier frequencies L in the system and the number of measurement frames M.

6.3.1.1 Fewer frequencies than frames

If there a.τ fewer carrier frequencies than there are frames in a multiframe (L≤M) it is possible to completely avoid measurement collisions. This can be achieved, for instance, if all users allocated to the same carrier frequency also use the same measurement frame, while different carrier frequencies use different measurement frames. Hence, a user on a particular frequency can make measurements on any other frequency without risking that users on that frequency also make measurements^. This approach requires that all base stations are synchronized in the sense that identical frequencies have their measurement frames simultaneously at the different base stations. Furthermore it is not possible to allocate measurement frames individually to users since this approach is based on the fact that all users on the same carrier frequency shall have the same measurement frame.

6.3.1.2 More frequencies than measurement frames

If there are more carrier frequencies than there are frames in a multiframe (L>M) it is not possible to use the previous approach to avoid measurement

information instead of random data. collisions since there will always be at least two carrier frequencies having their measurement frames at the same time. However, by changing the position of the measurement frame for each multiframe the collisions will not occur between the same users all the time. With this approach it is possible to define which frame to be the measurement frame for each user individually. It is even possible to use different lengths of multiframes for different users.

6.3.2 The base station transmits dummy- or other information during idle frame: Another alternative which avoids measurement collisions and provides true measurements of interference levels is to have the base stations transmit dummy information during the idle frames. If the transmitter power of the dummy information is the same as the previous slot with true information, other terminals that make measurements of that frequency will not notice that the particular user on that frequency is making measurements.

Just as in the previous case it is possible to define the measurement frames and multiframes individually for each user. It is also possible to use a centrally controlled approach where a pre-determined subset transmit dummy information in the measurement frame. Such an approach can be useful, for example, if the measurement frame is shorter than a conventional frame and it takes several measurement frames until all users have transmitted dummy information.

6.4 Examples of multiframe scheduling

To exemplify different methods of implementing the invention we describe two entirely different cases. One case with different number of frames per multiframe for each user. This could for instance represent the way frequency hopping terminals measure signal strengths of beacon signals for handoff purposes. The second case has more constraints in that all users allocated to the same frequency use the same measurement frame. In both examples the measurement

² Actually a measurement collision will occur if a user on a particular carrier frequency measures on the frame has the same length as a conventional frame.

6.4.1 Measurements in a frequency hopping case

Figure 5 discloses the general case (Frequency hopping case) where a user is not bound to a specific carrier (F\-Ff) and the position of the measurement frame in a multiframe and the length of the multiframe is individually defined for each user. During the measurement frames the terminals can measure on different carriers (F\ - Fj as shown in Figure 4.

Figure 5 discloses the situation for user and user k, respectively, wherein the user's carrier frequency in its assigned slot in a frame is indicated by a bold lined square marked with the user's identity. The measurement frames are marked by shadowed rectangles that span across the whole frequency band to indicate that the terminals can measure on any carrier frequency. The figure emphasizes independent assignment of positions of measurement frames and lengths of multiframes for each user. The first measurement frame (in both users' multiframe A) in Figure 5 for both users coincide, i.e., both terminals are simultaneously performing measurements on arbitrary carriers (F\ - Ff) and therefore there would be a measurement collision if user i would measure on the frequency allocated to user k during user k's time slot in the frame, and vice versa. In multiframe B for user t, frame 2, user k has a measurement frame, and user i is transmitting on carrier 4 in its assigned time slot. In multiframe B for user k (frame M , user k is transmitting on carrier FT,. User i in the same frame M^ is measuring on arbitrary carriers in the rectangle. It should be realized that the length of a multiframe can change in time and can be different for different users.

6.4.2 Measurements in a fix frequency case

Figure 6 discloses a special case, wherein users allocated to time slots on the same carrier are bound to use the same measurement frames. The dashed squares indicate which users currently have their measurement frame, i.e., all users on the

same frequency but during another time slot. same carrier. During a measurement frame a user can make measurements on different carriers in each time slot (X) in accordance with Figure 4. It is assumed that base stations do not transmit in measurement frames and therefore the positions of the measurement frame for a given carrier is changed for each multiframe to reduce the impact of measurement collisions.

Referring to the example in Figure 6 all users on frequency F__ have their measurement frame in the first frame in multiframe A. Similarly, in frame 2 all users allocated to frequency 4 perform measurements, and so on. In multiframe B the positions of each carriers' measurement frame is changed. In this special case a measurement collision may occurs in frame 3 of multiframe A when users assigned to carrier F3 measure interference levels on time slots on carrier F\, and vice versa. The users of -F3 will then not experience any interference since users on F\ are listening (measuring) and not transmitting information. Since the positions of the measurement frames change for each multiframe the measurement collisions in multiframe B hit the users assigned to frequency E2 ^an(^ F , respectively. By changing the positions of the measurement frames the effect of measurement collisions to individual users is reduced. If the base station would have transmitted dummy information during the measurement frames the impact of collisions could have been completely eliminated.

7. PERFORMANCE

To quantify the performance of the invention some calculations of "equivalent" number of slots per frame, and the number of channels that can continuously be monitored are provided.

7.1 Equivalent number of slots per frame

The use of a measurement frame leads to an increase of the channel bit rate and here we provide some numerical results of the increase.

The measurement frame is shared among MA conventional frames. If the duration of a measurement frame is α compared to a conventional frame an "equivalent frame" can be defined which consists of the conventional frame plus the fraction of the measurement frame that can be related to this conventional frame, i.e., the duration of that equivalent frame is l+ /(MA) frames. This is the factor by which the channel bit rate increases by the introduction of a measurement frame (not considering different coding, guard time, etc., that might have to be changed). Since each frame consists of N time slots this scheme can be said to correspond to a N' -slot TDMA system, where N' is denoted as equivalent number of slots per frame. This value can be calculated as

a

N' = NM +- = N M-\ + a

M-V M-\

In Table 1 the equivalent number of slots per frame is shown as a function of multiframe length and number of slots per frame for a system with the same length ^' of measurement frame as a conventional frame ( = 1 ). It is clear that the increase in channel bit rate is quite modest for larger lengths of multiframes. For example, the equivalent number of slots per frame with 9 frames per multiframe is 2.25 for a 2-slot TDMA. I.e., a fair amount of measurements has been enabled at the cost of only a 12.5% increase in bandwidth.

12 TABLE 1

Equivalent number of time slots per frame

Multiframe Number of users (slots) i per frame length

(frames) 2 3 4 5 6 7 8

2 4.00 6.00 8.00 10.00 12.00 14.00 16.00

3 3.00 4.50 6.00 7.50 9.00 10.50 12.00

4 2.67 4.00 5.33 6.67 8.00 9.33 10.67

5 2.50 3.75 5.00 6.25 7.50 8.75 10.00

6 2.40 3.60 4.80 6.00 7.20 8.40 9.60

7 2.33 3.50 4.67 5.83 7.00 8.17 9.33

8 2.29 3.43 4.57 5.71 6.86 8.00 9.14

9 2.25 3.38 4.50 5.63 6.75 7.88 9.00

10 2.22 3.33 4.44 5.56 6.67 7.78 8.89

11 2.20 3.30 4.40 5.50 6.60 7.70 8.80

12 2.18 3.27 4.36 5.45 6.55 7.64 8.73

13 2.17 3.25 4.33 5.42 6.50 7.58 8.67

7.2 Number of channels that can be monitored

In order to estimate how the measurement capacity is improved when using the invention we study the number of channels K that can be monitored (reported) within T/_ seconds as a function of multiframe length and number of slots per frame. The conventional frame consists of N time slots and the measurement frame has ON time slots.

Assume that a user can make measurements of -Kjp channels during one time slot. Measurements can be made both during idle time slots in a frame and during all slots in the measurement frame. Thus, during a conventional frame and a measurement frame (N-2)Kp and NKp channels can be sampled, respectively. Therefore, during a multiframe 13

K_MF = {N- 2)(M- l)K_F + cNK_τ = [(N-2)(M-l) +aN]K_γ

channels will be sampled.

To measure K channels once takes KIK ^ multiframes. If the duration of one frame is Tp seconds the duration of a multiframe is T_MF = (M- l + ά)T_F seconds and it takes T_κ = K/K_m T_m = K/K^ (M- 1 + a)T_v seconds to measure K channels. Thus the number of channels that can be measured in T/_ seconds is obtained from the relation _κ < T_A . I.e., we have that

y _ r_A[(N-2)( - l) + QN] _F

K ≤ ( - 1 + a)T_¥ ^~ (M- l + a)T_τ

channels can be monitored within 7^ seconds.

In Table 2 and 3 the number of channels that can be monitored is shown as a function of multiframe length and time between reports are shown for 2-slot and 3- slot TDMA, respectively. The following parameters were used: Kp = 1 measurement per slot, = 1, and Tγ = 2 ms frame duration.

From Table 2 we see that for a system with 2.25 equivalent number of slots per frame ( =9)and a 100 ms report time it is possible to monitor 11 channels which may be sufficient in many cases.

14 TABLE 2

Maximum number of channels that can be monitored given the constraints above

(2-slot TDMA)

Multiframe Maximum number of channels length

(frames) τ_A - = 100 ms ?A = 50 ms T = 33.3 ms ?A^: = 25 ms

2 50 25 16 12

3 33 16 11 8

4 25 12 8 6

5 20 10 6 5

6 16 8 5 4

7 14 7 4 3

8 12 6 4 3

9 11 5 3 2

10 10 5 3 2

11 9 4 3 2

12 8 4 2 2

13 7 3 2 1

TABLE 3 Maximum number of channels that can be monitored given the constraints above (3-slot TDMA)

Multiframe Maximum number of channels length

(frames) τ_A = 100 ms A = 50 ms τ_A- -33.3 ms A = 25ms

2 100 50 33 25

3 83 41 27 20

4 75 37 25 18

5 70 35 23 17

6 66 33 22 16

7 64 32 21 16

8 62 31 20 15

9 61 30 20 15

10 60 30 20 15

11 59 29 19 14

12 58 29 19 14

13 57 28 19 14

The above mentioned is only to be considered as advantageous embodiments of the invention, and the scope of the invention is only defined by the following claims.

Claims

16

CLAIMS 1. A method for transmitting information between telecommunication terminals using multiframe TDMA, characterized in that a multiframe is used consisting of conventional user frames with at least one measurement frame.

2. A method as claimed in claim 1, characterized in that said at least one measurement frame can have an arbitrary length.

3. A method as claimed in claims 1 or 2, characterized in that during said at least one measurement frame for a given user, a terminal belonging to said user can make measurements on different carrier frequencies (F\, F2 ...._. Ff) in each time slot in said at least one measurement frame.

4. A method as claimed in claim 3, characterized in that the position of said at least one measurement frame in the multiframe is independently assigned for each user.

5. A method as claimed in claims 3 or 4, characterized in that during said at least one measurement frame said terminal performs measurements relating to interference levels of other channels or signal strength of broadcast signals from other base stations.

6. A method as claimed in claims 4 or 5, characterized in that frequency hopping is supported by said method.

7. A method as claimed in any of claims 1-6, characterized in that said terminals, in addition to said at least one measurement frame, make measurements during idle slots in said conventional frames provided that said conventional frames include three or more time slots.

8. A method as claimed in any of claims 1-7, characterized in that said terminals, in addition to said at least one measurement frame, steal conventional frames and make measurements in these conventional frames.

9. A method as claimed in any of claims 1-6, characterized in that for a 2-slot TDMA, all measurements are performed in said at least one measurement frame, which implies a low channel bit rate and a high capacity for measurements.

10. A method as claimed in claims 1 or 3, wherein users on the same carrier use the same measurement frame and users on different carriers use measurement 17 frames on different positions, characterized in that frames in a N-slot TDMA are grouped into multiframes consisting of M frames, wherein MA frames are used for transmission of data and one frame is used for measurements, and that if or less carriers (F\, F2, ...._. -E '. L Γëñ M ) are used a different frame is assigned for measurements for each carrier, and if there are more than M carriers ( L > M) a different frame is used for measurements in each multiframe to spread out the collisions of measurement frames.

11. A method as claimed in claim 10, characterized by varying the position of said measurement frame in said multiframe to minimize the risk of said collision of measurement frames .

12. A method as claimed in claim 10, characterized in that a base station transmit dummy- or other information during said measurement frame.

13. A method as claimed in claim 12, characterized in that said dummy information is transmitted in said measurement frame to inform other terminals that said measurement frame is not an ordinary frame which prevent said other terminals from occupying said measurement frame for traffic transmissions.

14. A method as claimed in any of claims 1-9, characterized in that the length of the multiframe is changed in time.

15. A method as claimed in any of claims 1-9, 14, characterized in that the length of the multiframe is different for different users.