What Is Spectrum Efficiency?
The link spectrum efficiency of a digital communication system is defined as the net bit rate (rate of useful information, excluding error correction codes) or the maximum throughput divided by the bandwidth of the communication channel or data link (unit: hertz). Modulation efficiency is defined as the net bit rate (including error correction code) divided by the bandwidth.
The link spectrum efficiency of a digital communication system is defined as the net bit rate (rate of useful information, excluding error correction codes) or the maximum throughput divided by the bandwidth of the communication channel or data link (unit: hertz). Modulation efficiency is defined as the net bit rate (including error correction code) divided by the bandwidth.
- Chinese name
- Spectral efficiency
- Foreign name
- Spectrum Effectiveness
- Attributes
- Digital communication system
- Definition
- Net bit rate
Introduction to Spectrum Efficiency
Spectrum Effectiveness
When comparing the effectiveness of different communication systems, it is not enough to just look at their transmission rates. You should also look at the width of the channel occupied by such transmission rates. So what really measures the transmission efficiency of a digital communication system should be the symbol transmission rate in the unit band, that is,
Spectrum Effectiveness = R / B (Symbol Rate / Bandwidth) Unit is Bd / HZ (Baud per Hertz)
The transmission bandwidth B of a digital signal depends on the symbol rate R, and the symbol rate and the information rate Rb have a certain relationship. In order to compare the transmission efficiency of different systems, the spectral efficiency can be defined as:
Spectrum Effectiveness = Rb / B (information rate / bandwidth) in bps / HZ (bits per second per hertz)
Spectral efficiency link spectral efficiency
The unit of Link spectral efficiency of digital communication systems is bit / s / Hz , or (bit / s) / Hz (less used, but more accurate). It is defined as the net bit rate (rate of useful information, excluding error correction codes) or the maximum throughput divided by the bandwidth of the communication channel or data link (unit: hertz). Modulation efficiency is defined as the net bit rate (including error correction code) divided by the bandwidth.
Spectral efficiency is often used to analyze the efficiency of digital modulation methods, and sometimes also considers forward error correction (FEC) and other physical layer overheads. In the latter case, one "bit" specifically refers to one user bit, and FEC overhead is always excluded.
Example 1: A technology that can transmit a 1000-bit leap second in a 1 kHz bandwidth has a spectral efficiency or modulation efficiency of 1 bit / s / Hz. Example 2: The V.92 modem of the telephone network transmits on the analog telephone network at a downlink rate of 56,000 bit / s and an uplink rate of 48,000 bit / s. After filtering by the telephone exchange, the frequency is limited to 300Hz to 3,400Hz, and the corresponding bandwidth is 3400 300 = 3100 Hz. The spectral or modulation efficiency is 56,000 / 3,100 = 18.1 bit / s / Hz (downlink) and 48,000 / 3,100 = 15.5 bit / s / Hz (uplink).
The maximum spectral efficiency that can be achieved by using the FEC rack air conditioner can be obtained by using the specimen theorem. The alphabet of the signal (computer science) is combined by the number of symbols M, and each symbol is represented by N = log2 M bit. In this case, the spectrum efficiency cannot exceed the efficiency of 2N bit / s / Hz without using inter-code interference. For example, if there are 8 types of symbols and each of them has 3 bits, the maximum spectral efficiency does not exceed 6 bit / s / Hz.
Spectral efficiency is reduced when using forward error correction coding. For example, when using FEC with 1/2 coding rate, the coding length will be 1.5 times, and the spectral efficiency will be reduced by 50%. While the spectral efficiency is reduced, FEC can improve the SN ratio of the signal (not necessarily an improvement).
For a certain SN ratio communication, when there are no transmission errors at all, and the encoding and modulation methods are in an ideal situation, the upper limit of its spectral efficiency can be obtained by Hartley's theorem. For example, when the SN ratio is 1 or the decibel is 0, no matter how the coding and modulation methods change, the spectral efficiency will not exceed 1 bit / s / Hz.
Goodput (the amount used by the application layer intelligence) is smaller than the throughput generally calculated here, which is caused by the overhead of packet retransmission and over-transmission protocol.
The term "spectrum efficiency" has a larger value, which can cause more effective misunderstanding of the frequency spectrum. For example, mobile phones have low spectrum efficiency due to spectrum spreading and the use of FEC technology, but sometimes the SN ratio is not good and can still communicate normally. Therefore, it is possible to use more links than the frequency of the frequency bandwidth, and the overall effect can make up for the shortcomings of low spectral efficiency. As will be mentioned later, bit / s / Hz with a more suitable scale representing the "unit bandwidth utilization" unit exists. This is a technology that belongs to the division code multiplexing (CDMA) technology and has become the basic constituent technology of digital mobile phones. However, telephone lines and cable television networks, etc., do not have the problem of mutual interference between channels, and they are basically used for the maximum spectrum efficiency under the SN ratio.
Spectral efficiency
Wireless networks are quantified by the number of customers and services that can be simultaneously supported by the system's spectral efficiency within a limited wireless frequency bandwidth. Its unit is bit / s / Hz / area unit , bit / s / Hz / cell , bit / s / Hz / site, etc. for measurement. It is possible to express the total throughput and goodput of the system that can simultaneously support users as the bandwidth (Hz) of the communication loop. This does not only affect the technology that uses a single communication loop. The multiple connection methods and radio resource management technologies are also affected. In particular, dynamic radio resource management can be improved. When the maximum goodput is defined, mutual interference and conflicts between communication loops are eliminated, and overhead of high-level communication protocols is also ignored.
The capacity of the mobile phone network is also expressed by the maximum number of simultaneous connection lines on a 1 MHz cycle number bandwidth, ie, Erlang / MHz / cell, Erlangs / MHz / sector, Erlangs / MHz / km & sup2; and other units. This value also affects the message encoding technology (data compression) and is also used in analog telephone networks.
Example: When a mobile phone system based on frequency division multiple access (FDMA) and fixed channel allocation (FCA) has a frequency reuse factor of 4, each base station can use 1/4 of all spectrum. According to this calculation, the maximum system spectral efficiency (bit / s / Hz / site ) is 1/4 of the link spectral efficiency . When each base station uses 3 sector antennas to divide the signal into 3 sectors, it is called 4/12 reuse mode. Each part can use 1/12 of the full spectrum, so the system's spectral efficiency (bit / s / Hz / cell or bit / s / Hz / sector ) is 1/12 of the link's spectral efficiency.
Even if the link spectral efficiency (bit / s / Hz) is low, judging from the system spectral efficiency point of view, it does not necessarily mean that the coding efficiency is not good. For example, when the spectrum division multiplexing (CDMA) spectrum spreads into a single communication loop (that is, only users), the spectral efficiency is not good, but because there are multiple communication loops in the same bandwidth, the spectral efficiency of the secondary system very good.
Example: For a W-CDMA 3G mobile phone system, a maximum compression of 8,500 bit / s when making a call will cause a spread of 5 MHz bandwidth. At this time, the throughput of this connection is 8,500 / 5,000,000 = 0.0017 bit / s / Hz. In this case, the same sector can have 100 calls (with sound) at the same time. Since each base station is divided into 3 sectors by sector antennas in 3 directions, the frequency reuse coefficient will become smaller than 1 after the spectrum is diffused. At this time, the system spectral efficiency is 1 · 100 · 0.0017 = 0.17 bit / s / Hz / site or 0.17 / 3 = 0.06 bit / s / Hz / cell (also converted into bit / s / Hz / sector).
Spectrum efficiency can be improved using fixed / dynamic channel allocation, power control, or radio resource management technology called Link Adaptatio.
LTE Spectrum efficiencySpectrum efficiency in LTE
While E-UTRA continues to maintain the original site location, it must greatly improve spectrum utilization while increasing the cell's edge bit rate.
Downlink spectrum efficiency
In a high-load network, the target of spectrum efficiency (bits / sec / Hz / site) is 3 to 4 times that of R6HSDPA. This goal is to assume that the reference performance of R6 is based on a single transmitting antenna of NodeB. The UE has a type1 receiver with enhanced performance. At the same time, E-UTRA may use up to two transmitting antennas at NodeB and two receiving antennas at UE. .
Uplink spectrum efficiency
In a high load network, the spectral efficiency (bit / (s · Hz · site) target is 2 to 3 times that of 3GPPR6E-DCH (a single transmitting antenna at the UE and two receiving antennas at the NodeB end). The UE uses one largest single transmitting antenna and the NodeB has two receiving antennas, which can achieve the above goals.
Table 11-12 shows the throughput of the LTE system, and Table 11-13 shows the spectral efficiency.
Table 11-12 LTE system user throughput simulation evaluation results
| Target performance | Evaluation results and conclusions |
Down | Average throughput | 3 to 4 times of R6 | Most of the results meet or are close to the target (within 10%), which may be satisfied by considering inter-cell interference coordination. |
Cell edge throughput | 2 to 3 times of R6 | All results are met |
Upward | Average throughput | 2 to 3 times of R6 | All results are met |
Cell edge throughput | 2 to 3 times of R6 | All results are met |
Table 11-13 Simulation evaluation results of LTE system spectrum efficiency
| Target performance | Evaluation results and conclusions |
Down | 3 to 4 times of R6 | Most of the results meet or are close to the target (within 10%), which may be satisfied by considering inter-cell interference coordination. |
Upward | 2 to 3 times of R6 | All results are met |
The above results show that the LTE system can fully meet the requirements of TR25.913 on the uplink, that is, the cell and user throughput is increased by more than 3 times. However, the downlink evaluation results have not fully met the requirements. For example, it is still difficult to achieve 3 to 4 times the sector / average user throughput improvement and 2 to 3 times the cell edge user throughput improvement. According to the results provided by some companies, the above indicators can be achieved by adopting longer TTI, smaller control overhead, and enhanced technology.
According to the results of the external field test, when the system is 70% loaded and the uplink IOT is 6 dB, the measured LTE uplink and downlink spectral efficiency are shown in Table 11-14.
Table 12-14 Field measured LTE spectrum efficiency
Each point refers to "near, middle, and far points" | Throughput rate Mbit / s | Spectrum efficiency bit / (Hz · sector) |
Downlink 1UE per point | 10M-TM2 | 18.43 | 1.84 |
10M-TM3 | 21.54 | 2.15 |
20M-TM2 | 33.05 | 1.75 |
20M-TM3 | 27.44 | 1.37 |
Downlink 2UE per point | 10M-TM2 | 15.8 | 1.58 |
10M-TM3 | 19 | 1.9 |
20M-TM2 | 32.92 | 1.65 |
20M-TM3 | 41.01 | 2.05 |
One UE per point | 10M | 16.6 | 1.66 |
20M | 38.03 | 1.8 |
Two UEs per point | 10M | 17.9 | 1.79 |
20M | 33.68 | 1.68 |
