introduction
With the continuous development of technology, the development of fixed broadband wireless access technology based on data services has been very mature, and mobile broadband wireless communication technology has not yet been widely used. Since the mobile broadband wireless access system needs to resolve the technical contradiction between bandwidth, mobility and coverage, it has to face major problems such as path loss and link budget, multipath transmission, and mutual interference between channels. The problem of channel resources is inevitably involved.
Daqing Oilfield adopts McWill wireless broadband communication technology based on SCDMA, and iSWAN1800 base station is the core unit in McWiLL technology, and it is the bridge connecting terminal wireless equipment and core network.
1 The channel structure of the base station
McWiLL wireless broadband access technology proposes a CS-OFDMA wireless access multiple access method based on the SCDMA technology system. It combines the technical advantages of OFDMA and SCDMA. Unlike traditional CDMA systems, McWiLL uses frequency domain spread spectrum rather than time domain spread spectrum. In the McWiLL system, in addition to the CS-OFDMA multiple access method, TDMA technology is also used, and 8 service slots are divided in each TDD cycle. The following uses 5Mhz radio frequency bandwidth as an example to introduce the channel resource structure of the base station.
The carrier with a bandwidth of 5Mhz is first divided into 5 subcarrier groups occupying a bandwidth of 1Mhz, each subcarrier group contains 8 time slots, that is, divided into 8 subcarrier sets, each set contains 16 consecutive subcarriers, carrier group 1, 5 Because on both sides, in order to avoid interference, two channels need to be vacated as protection channels, so there are only 14 channels.
In the modulation process, the consecutive N (1 ~ 8) symbols of each user modulation signal are first subjected to an orthogonal code spreading modulation with a spreading factor of 8, and the N signals after the spreading modulation are added to produce 8 codes片 ä¿¡å·ã€‚ Slice signal. Then select one subcarrier (1/16) from these 8 subcarrier sets respectively, and modulate the 8 chip signals onto the corresponding 8 subcarriers. The result of this modulation is that the energy of each symbol is dispersed and modulated into 8 subcarriers evenly distributed in the 1Mhz subcarrier group.
Combining CS-OFDMA and TDMA, the maximum number of physical channels of McWiLL system can be calculated as 5 & TImes; 16 & TImes; 8/2 = 320 (units).
2 Physical channel types of base stations
As mentioned earlier, the maximum number of physical channels of the McWiLL system is 320, which also includes some common channel overheads such as access channels. The following briefly introduces the channel types of the iSWAN1800 base station.
The McWiLL system has the following physical channel types:
â‘ Broadcast channel (BCH):
Contains two sub-channels. The main purpose is to broadcast base station information and terminal paging messages in the downlink time slot, broadcast IP / OAM broadcast packets and paging. The transmission energy of the broadcast channel is large, so as to ensure the normal reception of the terminal.
â‘¡Ranging channel (RG) / Ranging access response channel (RRCH)
The ranging channel is mainly used by the terminal to send ranging messages in the downlink and uplink protection time slots. After the terminal sends the ranging request, the base station sends a response using the ranging access response channel to adjust the terminal's uplink synchronization and power control.
â‘¢Uplink access channel (RACH) / access response channel (RARCH)
The uplink access channel is used for the terminal to send an access request message in the uplink time slot, and the access response channel is used for the base station to send the channel allocation message in the downlink time slot.
â‘£Traffic channel (TCH)
The terminal uses uplink and downlink time slots to send data, voice and control information, and uses downlink beamforming.
3 Base station wireless resources and allocation principles
3.1 Wireless resources
The resources of the iSWAN1800 base station include two parts, one is energy, and the other is the number of sub-channels. The total power of the upstream energy is 25dBm per TS, 400dBm is 24dBm, the total energy of the downstream is fixed, and the full power channel supported is inversely proportional to the coverage.
The number of sub-channels depends on two aspects. First, there are 8 time slots in the time domain; second, there are 5 carrier groups in the frequency domain, 14 sub-channels in the edge carrier group, and 16 sub-channels in the non-edge carrier group. These constitute all business concurrent channels.
In terms of bandwidth, the bandwidth occupied by each sub-channel varies with the channel modulation method. The details are as follows:
Table 1 Modulation and bandwidth
Modulation mode Sub-channel bandwidth Total terminal bandwidth Total base station bandwidth
QPSK 8Kbps 16 & TImes; 8 & TImes; 8Kbps≈1Mbps 76 × 8 × 8Kbps≈5Mbps
QAM8 12Kbps 16 × 8 × 12Kbps≈1.5Mbps 76 × 8 × 12Kbps≈7.5Mbps
QAM16 16Kbps 16 × 8 × 16Kbps≈2Mbps 76 × 8 × 16Kbps≈10Mbps
QAM64 24Kbps 16 × 8 × 24Kbps≈3Mbps 76 × 8 × 24Kbps≈15Mbps
3.2 Allocation and preemption principles
With abundant resources, how to allocate these resources to each user reasonably is also a very important issue. The iSWAN1800 base station allocates resources according to certain algorithm rules. The base station will select time slots based on information such as interference, signal-to-noise ratio, energy, etc. Generally, the time slot with the least interference will be preferentially allocated. The base station also allocates resources based on bandwidth requests. Information such as interference, signal-to-noise ratio, and energy will all become reference factors for allocation, and how many sub-channels can be allocated per time slot.
Each user tries to meet its minimum bandwidth. If a user is not available and the system has no resources, the preemption principle is triggered. After the trigger, other users in the carrier group where the user is located will only get their minimum bandwidth within a certain period of time. If this cannot meet their minimum bandwidth, priority preemption will be triggered. Each user belongs to a priority group. This group is pre-configured with the percentage of system resources it can occupy. Priority preemption is to allocate system users' bandwidth according to the percentage.
4 summary
With the development of data services, the demand for broadband wireless communication technology has become higher and higher. The iSWAN1800 broadband base station serves as the hub connecting the user terminal and the core network in McWiLL wireless broadband technology, rationally allocating wireless resources and improving the entire McWiLL The efficiency of the wireless communication system provides a reliable guarantee for the production of Daqing Oilfield.
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