Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1999-01-25
2002-04-16
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S321000
Reexamination Certificate
active
06373833
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a time division cellular mobile communications system in which the radio interface between a mobile station and a base station consists of traffic frames comprising of several time slots in which the mobile stations can transmit in one time slot of an uplink frame to the network a request in an access burst to be provided with a radio connection for traffic.
BACKGROUND OF THE INVENTION
A radio system based on time division multiple access (TDMA) features frame structures so that the transmission and reception frames consist of time slots. The transmission occurs in a certain frequency and in a certain time slot, and the reception occurs in a certain frequency and in a certain time slot. The transmission and reception frequencies can be the same frequency, as in, for example, a DECT system, or they can be different frequencies, as in, for example, GSM and DCS systems, which are thereby both time division and frequency division systems. As an example of a TDMA system the following examines a GSM/DCS system. In this system the length of one TDMA frame is 4.615 ms and it consists of eight time slots, numbered from 0 to 7. The number of a time slot is referred to by the abbreviation TN (Time Slot Number). The duration of one time slot has been defined as (576+12/13) &mgr;s or the duration of (156+1/4) bits. A full speed traffic channel TCH consists of every eighth time slot in a cyclic manner, so from the viewpoint of the network one carrier can be used to form eight traffic channels. The traffic from the mobile station to the base station (uplink direction) and the traffic from the base station to the mobile station (downlink direction) has been arranged in such a manner that the reception in the base station occurs three burst durations later than the transmission. In this case the transmission time slot number of a transmission frame TN and the time slot number of a reception frame (TN) are identical. This is illustrated in
FIG. 1
the upper part of which shows the consecutive time slots of a certain transmission carrier and the lower part the consecutive time slots of the reception frequency related to this carrier. The frequencies are generated by using a transceiver TRX. A TRX unit can change its frequency, in which case a different frequency can be in use during each time slot. There must be several TRX units so that it is possible to use several frequencies within one time slot.
The consecutive time slots TN=0 of reception frames of a certain frequency in a base station form the RACH (Random Access Channel) and in this channel the network receives the requests transmitted by mobile stations to be given access to channel resources. The RACH is an uplink channel only. If the request is accepted, the network transmits on the PAGCH (Paging and Access Grant Channel) the acknowledgement of the request and the information as to which channel the mobile station must switch to. The PAGCH is a downlink channel only and it consists of the consecutive time intervals TN=0 of the frames of a certain transmission frequency.
The following bursts can be distinguished from one another: access burst, F and S bursts and normal burst. The difference between them is in their time-amplitude profile.
The normal burst is the longest burst of all; its duration is 148 bits and it is used in the traffic channel and in most signalling cases. It includes two sequences of 58 bits which are separated by an training sequence of 26 bits, and, at the beginning and the end of the burst there are three tailing bits. The duration of normal bursts must be slightly smaller than that of a time slot so that when a base station receives, the bursts transmitted in adjoining time slots do not overlap. The transmission of the normal burst in a mobile station starts by the amount of the timing advance TA before the reception time slot in the base station begins, in which case the burst arrives in the time slot right at the beginning of the slot and the entire burst fits in the slot. F and S bursts are only transmitted in the downlink direction in frequency correction and synchronization channels and they are used when the mobile stations synchronize themselves to the base station and to correct frequency errors caused by movement.
The access burst is only used in the uplink direction at the beginning of a connection when the propagation delay between the base station and mobile station is not known. This is the situation when the mobile station contacts the network via the RACH and in some cases in handover situations when the mobile station moves to a new cell. The access burst includes an training sequence of 41 bits, 36 information bits and 7 tailing bits at the beginning and 3 tailing bits at the end, or a total of 87 bits (the length of a normal burst is 148 bits). The access burst is thereby very short and no other bursts are used in the RACH. The base station receives access bursts in the RACH, in other words, in the time slot TN=0 and, if the network simultaneously receives several bursts, it rejects them all. The mobile station retransmits the access burst until the request is accepted and a traffic channel is assigned to the mobile station. The training sequence of the burst is longer than that of a normal burst so that the probability of success in the demodulation of the burst is high. This is important because the receiver does not know the level of the burst, frequency error, or time of arrival within the time slot. Because the propagation delay between the mobile station and base station is not known when the access burst is used, the arrival of the access burst to the base station features a time error compared to the reception time slot, the length of which is two times the length of the propagation delay. To compensate this the duration of the access burst is short so the mobile station can progress as far as 35 km before the access burst misses the reception time slot.
The aforementioned 35 km is at the same time in theory the maximum cell radius in the network. When it is desirable to expand the system to sparsely populated or uninhabited regions by arranging the radio coverage of the system to cover at least the main roads, complete radio coverage can only be achieved by placing fully equipped base stations at intervals of 70 kilometers. This is a rather expensive solution because a fully equipped base station contains a great deal of expensive components and the base station link mast must be extremely high. However, the distance can be increased, especially on flat terrain, by building the base station masts even higher and by using only every other time slot of a frame. The time slots used are the even ones, because the time slot TN=0 is reserved for access bursts. When only every other time slot is used, the timing advance values achieved are tremendous, so the cell radius can be expanded to a much higher value than 35 km, at the expense of channel efficiency.
On the other hand, when it is desirable to have good radio coverage in a densely populated region, fully equipped base stations must be placed very close to one another. This must be done, of course, because of the great traffic density, but especially when it is desirable to arrange coverage in indoor spaces, such as car parks, department stores, subway stations, etc., in other words, places where there are a lot of people but where radio wave penetration is poor. More base stations must also be established if it is desirable to arrange radio coverage to shadow regions between and behind tall terrain features. When the base stations are added, the number of Abis interfaces also increases which include the interfaces between the base station controller BSC and the base stations controlled by it.
The patent application FI-933091 describes a method to expand the cell radius to a value considerably in excess of 35 km. The application suggests that the timing of the receiver of one transceiver unit is delayed in relation to the transmitter. This is impleme
Suonvieri Jukka
Uola Risto
Altera Law Group
Nguyen Brian
Nokia Telecommunications Oy
Ton Dang
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