CN1274160C - Link selection in a communication system - Google Patents

Abstract

In a wireless communication system, at least one active link for packet data communication is selected. Reverse link measurements are performed for a plurality of active transmitting base stations (102-104) serving a mobile station (108). A subset of active links with the largest signal measurements are selected for forward transmission with at least one packet data communication.

Description

Link Selection in Communication Systems

technical field

The present invention relates to forward channel control, and more particularly to a method and apparatus for selecting a forward link with reduced overhead.

Background technique

In a Code Division Multiple Access (CDMA) system, forward link communications occur over multiple channels from different base transceiver stations (BTS). The forward link is used for BTS to mobile station (MS) communication. The reverse link is used for MS to BTS communication. The channel where communication occurs in the forward direction is usually called an "active set", and the active set of this channel is the channel for demodulation by the receiver of the mobile station. A so-called "neighbor set" of the channel is monitored, although not demodulated, for soft handoff purposes.

Signals are transmitted to the mobile station through channels, and the more such channels are, the better the diversity performance will be. However, this performance improvement comes at the cost of reducing the overall system capacity, since fewer channels will be available to other mobiles, and at some load points, the diversity gain will be lower than at all soft handover positive The additional power transmitted onto the link. Therefore, in a CDMA system, there is a trade-off between performance and capacity.

For packet data, the bit error rate is obtained by a forward error correction scheme such as automatic repeat request (ARQ). The target for the forward error rate (FER) is expected to be in the range of 10% to 15% for packet data. When compared with the target of 1% forward error rate in speech transmission, or 0.1% forward error rate target in circuit data transmission, and taking this higher forward error rate as the target, due to the soft The diversity benefit of multipathing due to switching will be particularly small.

Contents of the invention

Accordingly, there is a need for improved forward channel selection for packet data communications in CDMA systems.

According to the present invention, there is provided a method of selecting at least one active link for packet data communication in a wireless communication system, comprising the steps of: measuring the reverse link among a plurality of active transmitting base stations; A subset of the active links with the largest signal measurements is selected for forward transmission of the at least one packet data communication based on the measurements of the forward links instead of the measurements of the forward links.

The method provides performance improvements for packet data and maintains flexibility for circuit data or voice by selecting the best forward link subset for transmission to preserve the diversity benefits of soft handover.

Description of drawings

Figure 1 is a block diagram illustrating a cellular communication system;

Figure 2 illustrates a functional block diagram of network operation.

Detailed ways

In a wireless communication system, at least one active link is selected for packet data communication. The reverse link is measured for a plurality of active transmitting base stations (102-104) serving the mobile station (108). A subset of active links having the greatest signal measurement is selected for forward transmission with at least one packet data communication.

A forward transmit scheme is suggested for transmitting data on the optimal forward link, or on the optimal subset of forward transmit links. Link performance results for static and high Ricean channels (as experienced by fixed wireless terminals) indicate that it is optimal to transmit data on the best forward link among all active serving transceivers. Transmitting on the best or best two forward links rather than all forward links reduces the required communication bandwidth between the base transceiver station and the infrastructure (network) responsible for controlling, locating and initializing the base transceiver station (Backhaul bandwidth), the infrastructure described is typically called a Cellular Base Station Controller (CBSC), or a Radio Network Controller (RNC), or a Selection/Distribution Unit (SDU). When only n or fewer forward links are used, from n+1 (e.g., n+1=3) forward links associated with soft handover or softer handover to obtain the required FER for a given user Transmitting on the best two forward links can also increase system capacity when the total power of the forward links exceeds the required power.

As used herein, a link is a set of channels used for communication between mobile stations and base transceiver stations. Channels include dedicated control channels, pilot channels, auxiliary channels, paging channels, etc.

FIG. 1 discloses a cellular system 100 . The illustrated cellular system 100 is a code division multiple access system that includes a plurality of base transceiver stations (BTS) 102-104 that communicate with a mobile station MS 108 via respective wireless communication paths. Those skilled in the art will recognize that typically more than three base transceiver stations and more than one mobile station will be provided in a system. The transmitting base stations 102-103 are connected to the mobile switching system network 110. Such CDMA cellular systems are well known.

In the CDMA system 100, the speech target frame error rate (FER) is 1%, and the circuit target FER is 0.1%. For these target FERs, soft handover provides diversity gain. Therefore, it is best to use all provided soft handover links. However, for third generation (3G) packet data applications, since the target FER is expected to be in the range of 10% to 15%, the desired bit error rate (BER) can be achieved with automatic repeat request (ARQ). When targeting these higher FERs compared to the 1% target in speech or the 0.1% target in circuit data, the multipath diversity benefit due to soft handover is small.

Each communication path between terminal base stations 102-104 and mobile station 108 has a forward link and a reverse link. Network 110 will select the forward link or the link with the least transmission loss. If the mobile station measures the forward transmission link from the terminal base station 102-104, then the measured value measured by the mobile station (for example, as in the IS 95 and IS2000 standards using the Pilot Energy Measurement Message (PSMM) sent SNR (Ec/lo) measurements) must be transmitted back to the base transceiver station and then to the network 110. This requires message overhead, which is not ideal. Said overhead can be substantially eliminated by utilizing reverse channel signal measurements to detect and determine the optimal subset of active channels, said subset being used for forward channel packet data communications.

For example, the forward channel packet data channel can be determined by the reverse link signal-to-noise ratio (SNR), which is the total interference measured by each base transceiver station 102-104 from the reverse link signal and The reverse link signal received from mobile station 108 is obtained by adding noise power (RSSI). An advantage of using the reverse link signal is that the mobile station does not need to use messages to communicate the forward link signal-to-interference ratio (SIR) to the base transceiver stations 102-104. The reverse link channel SIR can be estimated from the reverse channel pilots (IS2000 standard) or derived Walsh symbol energies (IS95A, B standards), and is proportional to Ew/Nt or pilot Ec/Nt, respectively. The resulting measurement (SNR) is the signal to (thermal) noise plus interference ratio, calculated from the reverse link SIR and RSSI.

Each serving base transceiver station (BTS) 102-104 sends its reverse signal-to-noise ratio (SNR) to a network selection distribution unit (SDU) 110, which is typically located at a radio network controller (RNC) or centrally located on a frame basis. Inside the base station controller (CBSC). If a BTS currently serving (i.e., a BTS in the active cell of the mobile station) involves more than one sector in the call (this is known as a typical softer handover, where more than one station serves the mobile station), the best SNR of the softer handover sector is selected as the SNR of the serving transmitting base station. Network (SDU) 110 selects either one best forward link or the best subset of active forward links for packet data transmission. A threshold determined by the current service option and/or the target FER can be used to select the best channel. Based on the threshold, either the best forward link is selected, or the best subset of forward link channels is selected. The SDU synchronizes the BTS and MS to transmit and receive forward data.

FIG. 2 is a functional flow chart illustrating the work of the network 110. For example, the network may be a mobile switching center and a serving transceiver station (BTS2 103 shown in the figure). As known to those skilled in the art, at step 216 the base transceiver station 103 receives lock-in filtered rake finger information. In step 214, the BTS calculates a signal-to-interference ratio for each of the lock-filtered rake pointer information signals in step 214. This SIR information is provided to a signal-to-noise ratio estimator 212 . The total power of the signal plus noise is also estimated in the base transceiver station (RSSI), and the interference rise (RISE) over the receiver thermal noise field is calculated as follows: RISE = RSSI - RSSInoload, where each quantity The unit is dB. The RISE result value is also input to the signal-to-noise ratio estimator 212 as shown in step 220 . As is known to those skilled in the art, at step 218 a sliding filter filters the baseband input. As shown in step 212, the filtered signal is also used for SNR estimation. The SNR is communicated to the network 110 along with other signal measurements from other active units or serving BTS 102, 104.

In step 206, the network 110 responds to the desired FER in the network server (as shown in step 204) to set a threshold. In the preferred embodiment described, the threshold specifies how close in dB the weaker link must be to the strongest link. If the threshold value is 5dB, then the base transceiver station with the SNR or signal strength (S) within 5dB of the strongest transceiver is part of the reduced active cell, so it will be assigned to the forward link in the next transmission . The higher the FER target value, the lower the threshold value (eg, for a higher target value, the threshold value may be 3dB). In step 208, the best forward link selection is made. In this step, the network controller (which may be a computer or microcontroller, or other suitable system) selects a subset of active units or serving BTSs for data packet transmission. Once a BTS is selected, packet data is transmitted by the selected serving BTS. A new BTS may be selected for each packet communication, or a serving BTS may be selected at a predetermined interval.

One way to calculate the SNR is to calculate the Signal to Interference Ratio (SIR) and the Reverse Link Interference Rise (RISE) over the noise field. The SNR can be calculated as follows:

SNR(i)=SIR(i)+RISE(i)(dB), i=1,...,N (1)

In the formula, i represents the i-th serving base transceiver station (BTS), and N represents N soft handover branches. SIR can be calculated by accumulating filtered RAKE pointer energy values. The SIR energy may be based on the reverse link pilot or the demodulated symbol energy of the reverse link signal received by the BTS over a control or data channel (e.g., in IS95, IS2000, the channel may be the fundamental channel (FCH), Dedicated Control Channel (DCCH), or Supplementary Control Channel (SCH)), as follows:

$\mathrm{<span\; class="notranslate">\; SIR</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; j</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

In the formula, E(j) represents the j-th filtered pointer energy value, and M represents M pointers. The reverse link interference rise (RISE) associated with the noise field is calculated as:

RISE(i)＝(RSSI(i)-RSSInoload(i)) (3)

In the formula, RSSI is the received signal strength indicator value of the base transceiver station (BTS), as known to those skilled in the art, it is updated every frame. RSSInoload(i) is the signal strength received by the BTS when the BTS is not loaded with any service. It is determined by field calibration or can be calculated based on the expected nominal noise value of the BTS, as is known to those skilled in the art. RSSI can be obtained by filtering out baseband front-end signal sampling values within a certain period of time (for example, 2 seconds) by low-pass or queuing or adjusting means. Note that it is possible to calculate the signal energy (S) and use it in Equation 4 below instead of SNR. Specifically, S is calculated using RSSI, where S(i)=SIR(i)*RSSI(i). Those skilled in the art will recognize that in this technical field, the formula S(i)=SIR(i)+RSSI(i) in dB. SIR can also be estimated from a decoder metric, such as the decoder total metric, the Walsh (data) symbol energy produced in an uncorrelated receiver of IS95A and IS95B compliant communication equipment.

The SDU selection function selects the best forward link Flink(k), or the best subset of forward links, based on the threshold given by:

Flink(k)＝Best(SNR(1), SNR(2),..., SNR(N)) (4)

In the formula, k=1, 2, . . . , K, represents the total number of forward links selected to pass the threshold. The Best( ) function selects the best link, or subset of links, for the soft handoff branch from the provided SNR measurements. Furthermore, in other preferred embodiments, the Best( ) function is based on the aforementioned signal power measurement S(i).

It is foreseeable that updates to the best forward link will be made frequently. With the frequent updates obtained, the delay will be smaller and the selection of the best link for the next packet transmission more accurate. The more link options there are, the higher the probability that the pulses will be received without error and with the smallest possible power, which in turn increases the capacity of the system.

In this way, a selection scheme for the optimal forward link is proposed, which does not depend on the forward link measurement value Ec/lo (e.g., the Ec/lo measurement value sent by the pilot PSMM), Ec/lo in the already known mobile stations used for speech transmission, but only rely on reverse link signal measurements, such as RSSI or SNR measurements in the BTS. Each serving base transceiver station (BTS) provides its reverse link SNR to the network SDU. The SDU selects the forward link, or several forward links, whose reverse link SNR exceeds a predetermined threshold and has the best signal level. The SDU then synchronizes the BTS to transmit and the mobile station (MS) to receive data bursts. The scheme provides performance improvements for packet data and maintains flexibility for circuit data or voice by selecting the best forward link subset for transmission to preserve the diversity benefits of soft handover.

Claims (12)

1. A method of selecting at least one active link for packet data communication in a wireless communication system, comprising the steps of: Measuring the reverse link at multiple active transmitting base stations; and A subset of active links having a maximum signal measurement is selected for forward transmission of at least one packet data communication based on the reverse link measurements and not based on the forward link measurements. 2. The method of claim 1, wherein said subset comprises a single channel. 3. The method of claim 1, wherein said measured value is a function of reverse link RSSI. 4. The method of claim 3, wherein said measured value is a function of reverse link signal-to-noise ratio. 5. The method of claim 1, wherein said measured value is a function of reverse link signal-to-noise ratio. 6. The method of claim 1, wherein said measured value is a function of RSSI and SIR. 7. The method according to claim 1, wherein said measured value is RSSI+SIR, and the unit is dB. 8. The method of claim 1, wherein said measurements are taken and sent back to the network equipment to determine the best forward link for each frame time interval. 9. The method of claim 1, wherein said measurements are sent from each serving base transceiver station back to the network device in each frame time interval. 10. The method of claim 1, further comprising the step of using measurements from all BTSs serving a mobile station to determine the best forward link. 11. The method of claim 1, wherein said measured value is a function of a decoder metric. 12. The method of claim 1, wherein the subset of active links used for packet communication is less than the full set of active links.

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