When an IDC user transmits a WiFi waveform and receive data from the LTE network to provide real time traffic service, WiFi uplink data to AP will interfere with LTE receiving data from eNB, and LTE uplink data will also interfere with the WiFi receiving data from AP because LTE band 40 operates at TDD mode, and it will uplink and downlink data at the same frequency band. WiFi has 14 channels in ISM band, and each channel bandwidth is 22MHz. Channel 1 starts with 2401MHz, and channel 14 ends at 2495MHz.
Each channel separates from adjacent channel by 5MHz. There is an exception in channel number 14 where separation is 12MHz. Channel number 14 is defined beyond ISM band,
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and it is only used in Janpan. The transmitter of WiFi will affect receiver of LTE band 40 and vice-versa. Since LTE band 7 is a FDD band, it will affect WiFi receiver. However, WiFi doesn’t affect LTE receiver at LTE band 7 downlink.
Figure 1.4: LTE + WiFi portable router.
1.2.1.1 LTE + WiFi portable router
Fig. 1.4 can express this scenario. In this figure, it shows the situation for LTE downlink data from eNB to IDC-UE and for WiFi of IDC-UE downlink data to many stations, and the transmission of WiFi will interfere LTE receiver. UE uses LTE which is considered as a backhaul link to access the Internet to download data from network, and the UE shares the data by WiFi to other local WiFi stations which connect with it. Therefore, the UE becomes a portable AP which has full control on frequency chan-nel and transmitting power. The UE can move WiFi signal away from LTE band by itself. If this is not sufficient to solve IDCI problem, UE can inform eNB and require an IDCI solution. If we use TDM solution to solve the IDCI problem, it will allocate scheduled period and unscheduled period for LTE. During DL from eNB to UE, the worst case is that if a packet arrives at the eNB at the beginning of the LTE
unsched-uled period, the resulting delay is the sum of the LTE unschedunsched-uled period (waiting for LTE scheduling) and the LTE scheduling period (waiting for WiFi scheduling). It is the delay which begins with eNB receiving packet from Internet and ends with WiFi starting to transmit this packet to the station. The situation is similar to the UL. The scheduling/unscheduled periods can be made as small as 1 ms to minimize delay, but it is unusable because it doesn’t consider the impact on retransmissions and other timelines on both LTE and WiFi, and it also can’t satisfy WiFi transmission time which may excess 1ms. Therefore, the scheduling/unscheduled periods should be balanced between the timeline requirements and the needs of the specific Quality of service (QoS). The scheduling periods and unscheduled periods should use the following guidelines:
1. Scheduling periods and unscheduled periods should be typically not more than [20-60] ms.
2. The scheduling and unscheduled periods should be large enough to conform rea-sonable operation of the LTE and WiFi timelines.
3. Since LTE has typically lower data rate than the WiFi link, the LTE scheduling periods should be longer than the unscheduled periods in order to achieve roughly the same throughput on both links.
The coexistence interference case 1-3 of section 1.2 may happen in this scenario.
1.2.1.2 LTE + WiFi offload
Fig. 1.5 can express this scenario. It shows the situation for WiFi uplink data to AP and for LTE downlink data from eNB to UE, and the transmission of WiFi will interfere with LTE receiver. LTE UE can offload traffic from LTE to WiFi, and the WiFi transceiver of the UE operates as a terminal (not AP) in infrastructure mode.
For example, UE performs video conference. The video stream can be divided into image packets and VoIP packets. And it uses WiFi radio to transmit image packets for
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Figure 1.5: LTE + WiFi offload.
offloading load of LTE and uses LTE radio to transmit VoIP packets for guaranteeing Qos (delay). Because WiFi radio is not AP, it is difficult for the WiFi radio to change the configured frequency channel. In addition, the WiFi radio has to keep listening to the beacon signal transmitted from WiFi AP for maintaining connection. In this scenario, if we use time domain solution, the requirements for the scheduling period and the unscheduled periods will be analyzed as following three observations:
First observation is that UE in WiFi client mode must receive WiFi beacon. In order to receive beacons properly, the LTE unscheduled period needs to align with the WiFi beacons. Besides, in order to provide for beacon reception, the scheduling period of LTE should be no longer than 100ms.
Second observation is that the packet from network can choose one link (WiFi for offload packets, and LTE for non-offload packets) to transmit to UE. For offload packets, the largest delay will be scheduling periods, and vice versa. Comparing to WiFi portable router, the WiFi offload has larger scheduling periods and unscheduled periods with the
same delay requirements.
Third observation is that the traffic volume of the non-offloaded and offloaded traffic should be matched by the ratio of the scheduling and unscheduled periods.
Synthesizing the above observations, we conclude that the scheduling periods and the unscheduled periods shall find a balance between the QoS (delay) requirements and the requirements of the acknowledgement/timeline of LTE and WiFi (HARQ timer, beacon duration, and so on). In summary, the following guidelines are useful.
1. The scheduling and unscheduled periods should typically not be more than [40-100]
ms.
2. The scheduling and unscheduled periods should be large enough to conform rea-sonable operation of the LTE and WiFi timelines.
3. WiFi beacons are important messages, LTE unscheduled period should align with them.
4. The ratio of the scheduling and unscheduled periods should be aligned to the ratio of the volume of non-offloaded and offloaded traffic.
The coexistence interference case 1-3 of section 1.2 may happen in this scenario.