关于手机集成WLAN的文章，来一起学习

HWRD · 发表于 2005-10-19 20:28:00

When GSM is transmitting, the WLAN subsystem can''t receive WLAN packets (Fig. 1). Likewise, when GSM is in receive or monitor mode, the WLAN can'' transmit, as it will desensitize the GSM receiver. In addition, one radio chain must be turned off while the other transmits, due to interaction between the two transmitters. In most cases, the GSM transmitter will be given precedence and the WLAN transmitter will be disabled, as the existing infrastructure limits any changes to the GSM standard. What results is the need for some type of traffic management, or scheduling within the multi-mode solution. This is often achieved in the upper levels of the architecture. This scheduling, for instance, may exist within the application software or top-level baseband protocol stacks. While the result is a functional multi-mode solution, only one standard is ever instantaneously active at any time. As a consequence, only one of the two existing radio chains would ever be used at one time.
One vendor has developed multi-mode intellectual property (IP) that maximizes scheduling. By efficiently synchronizing GSM transmissions and receptions with those of WLAN, a single radio chain can be used for a multi-mode solution. This allows for a simple architecture, as the silicon content, complexity, component count, and overall size can be reduced. However, this IP has another advantage, as it reduces the overall time-averaged power consumption of the multi-mode handset.
To avoid desensing the GSM receiver, the IP schedules WLAN transmission at times when GSM doesn''t need the radio channel. Likewise, the scheduling algorithms can synchronize access point transmissions to GSM radio activity. This technology results in WLAN receptions that are never corrupted by GSM transmissions, or vice-versa.
While both handsets have two basebands and an application processor, the multi-mode handset needs only one and one antenna. This is due to the IP that allows one radio be shared by multiple standards concurrently (Fig. 2). The fundamental advantage the multi-mode IP is that it eliminates the interference between the WLAN and GSM. A second advantage is that basic scheduling and contention mechanisms are integrated into the radio. This why the GSM and WLAN baseband can share a radio transceiver with minimal changes to the baseband interfaces or upper level software and protocol stacks.

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<CENTER>2. The interaction between the WLAN and GSM is shown using a proprietary architecture.</CENTER>
During GSM bursts, the WLAN system isn''t permitted to use the radio chain. Once the GSM finishes its radio activity, the WLAN subsystem can seize the radio, tune to the ISM band, and initiate a transmission or reception. Due to the turnaround times required to switch between the GSM and WLAN bands, the available time to transmit or receive packets for WLAN is less than that of a dual-radio solution. However, Quorum Systems'' has developed multi-mode synchronization IP that improves the handset''s performance with minimal impact to excess WLAN bandwidth. Using this architecture, three service links (GSM voice data, three-way VOIP conferencing, and two-way video conferencing) can be sustained simultaneously using one transceiver and two basebands.
Typically when implementing multi-mode IP, the probability of a successful WLAN transaction corresponds to the length of the WLAN packet. As the WLAN packets increase in duration, the likelihood that they will overlap with a competing GSM burst also increase. This will cause the WLAN packet to be dropped, requiring it to be retransmitted at a later time (Fig. 3). WLAN downlinks tend to be more robust as the WLAN receiver can operate during both GSM idle times and receive bursts.

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<CENTER>3. This figure illustrates the performance degradation of a dual radio solution.</CENTER>
The Quorum Systems'' solution employs proprietary scheduling algorithms as the baseline reference. On the downlink, an access point transmits a data packet to the handset. Upon successful reception of the packet, the handset replies with an acknowledgement (ACK) burst transmission. This additional overhead is required when sustaining a link to any access point or base station. Because the ACK is of fixed length, the probability of it interfering with a GSM burst is independent of the data length. However, the probability of a successful data reception is inversely proportional to length of the data packet.
For short DATA packets, the probability of a successful data-ACK transaction is dominated by the ACK success. For longer transmissions, it''s dominated by the probability of a successful data packet reception. In the case of the uplink, a successful data-ACK transaction is almost solely dependent on the transmission''s length.
Because transmissions have a smaller contention window to deal with, the probability of a successful transmission, data-ACK falls off more quickly than the probability of a successful reception. These issues are somewhat mitigated be the fact that VoIP data traffic consists inherently of shorter data packets (on the order of 100 bytes or so). But even in this case, dual-radio solutions are still less optimal than those which employ a scheduling algorithm.

HWRD · 发表于 2005-10-19 20:29:00

Multi-mode handsets are designed to work in conventional wireless LAN (WLAN) hotspots as well as in GSM cellular networks. Multi-mode communication is important to consumers for applications such as GSM/WLAN voice-over-IP (VoIP) VoIP conference calling or GSM Voice/WLAN Internet access. 
Significant gains are achieved over other multi-mode implementations by coordinating the transmissions and receptions between the wireless standards. By scheduling radio resources in a prioritized round-robin manner, both standards have fair access to the radio channel while eliminating cross interference. This reduction in cross interference removes the need for expensive filter components and increases link reliability.
As with any mass-market consumer application, there''s merit in examining the market driven factors that will affect the system architecture and solution. For example, convergence handsets can''t sacrifice performance. The wireless consumer has come to expect an acceptable performance level from their cellular handset. Overall link range and voice quality can''t suffer when WLAN and VoIP functionality is added.
Users also demand competitive talk and standby times. A multi-mode handset operates in both cellular and WLAN/VoIP networks. For widespread consumer acceptance, its time-averaged power draw must be equal to or better than the single-mode cellular device.
Third, the devices must have small form factors. VoIP will achieve mass-market deployment in feature phones, or devices geared predominantly towards voice-centric applications. Such phones don''t employ large touch screens or alphanumeric keypads and are often driven by market competition towards the smallest possible form factor. These form factors must be maintained even when WLAN/VoIP capability is added.
Issues with concurrent operation
It''s been established that in small, low-cost, handsets GSM and WLAN interfere with one another, and thus degrade the performance of both systems. There are several conditions where the interference can be problematic. These include when both GSM and WLAN are simultaneously transmitting, when GSM is receiving and WLAN is transmitting, and when GSM is receiving and WLAN is transmitting.
When both GSM and WLAN are transmitting, the interaction of the two transmitters cause intermodulation distortions due to power supply and local oscillator pulling. This interaction distorts both transmissions such that the waveforms are no longer Wi-Fi or GSM compliant. Furthermore, it causes spurious emissions that violate FCC regulations for GSM. The broadband noise floors of both power amplifiers can also lead to cross-spectral emission violations. This can be somewhat mitigated in traditional approaches by using more selective (and costly) filtering and RF shielding and isolation.
When GSM is transmitting at full power (30 to 33 dBm), its out-of-band leakage will desense the WLAN receiver, causing a Wi-Fi multi-mode device to miss incoming packets. The WLAN access point is thus burdened with additional retransmissions to the multi-mode device. Conversely, when WLAN is transmitting at full power (18 to 20 dBm), its out-of-band leakage will also desense the GSM receiver. For GSM subscribers, dropping even a few data packets during a conversation will lead to unacceptable audio quality. In addition, this desense will cause the GSM receiver to fail GSM type acceptance criteria for overall system sensitivity.
There are two predominant methods for dealing with interference. Several multi-mode devices significantly reduce the output power levels for both the GSM and WLAN radios. One device measured in a lab scaled the GSM transmit power back to 26 dBm, with the maximum measured WLAN power of only 13 dBm. With increased front-end filtering and significant separation between the GSM and WLAN antennae, some simultaneous operation was possible with the device.
There are several major drawbacks to such a solution. These include increased cost and size and reduced range. The increased cost is due to the increased selectivity required from the front-end filtering, while the increased size results from physical separation required between the WLAN and GSM antenna and transceiver chain circuitry. The reduced range is directly caused by the lower transmit power levels for both GSM and WLAN. This severely reduces the operating range.
Due to these limitations, most traditional multi-mode GSM/WLAN systems have resorted to non-simultaneous operation. The WLAN transmitter is turned off whenever the GSM radio is active, thus preventing any degradation to GSM. Furthermore, if a GSM transmission interferes with a WLAN reception, the WLAN subsystem relies on WLAN access point retransmissions to receive the packet (Fig. 1).

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<CENTER>1. The interaction between WLAN and GSM is shown.</CENTER>

htyoung · 发表于 2005-11-14 16:44:00

wlan 和PLMN接合是一个复杂的问题这时一篇好文

aquasnake · 发表于 2005-11-21 13:26:00

理论性的科学研究，2个协议要整合不太现实

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