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Fully Centralized Wireless Network based on Analog Bloom
Filter and Contention-Free Multi-Bit Simultaneous Query |
In
a typical wireless network, a base station or a router may be associated with
multiple user devices such as mobile phones.
When the network operates in a centralized manner, the base station or
the router dictates the transmission schedule, i.e., which devices should
transmit, at what time, for how long, etc. The base station or the router needs
to know the set of active devices in the network, based on which it can
calculate the data transmission schedule to meet various performance
requirements and avoid collision.
However,
learning the set of active devices may incur very high overhead. The challenge
is not with any devices that the base station or the router is currently
communicating with, because the base station or the router already knows they
are active. Instead, the main challenge is with the devices that were
previously idle and just turned active, i.e., just received data from the upper
layer. To find which devices just turned active, it is infeasible for the base
station or the router to query all devices one by one, because the number of
query messages will be large. Therefore, some random access method is typically
employed to allow the devices to inform the base station or the router.
Analog
Bloom Filter (ABF) is a novel solution for this problem. With ABF, the base
station or the router may periodically designate some resources, i.e., some
subcarriers in one or a few OFDM symbols. The resources are used to support a set
of orthogonal bases, such as the original OFDM subcarriers or the Zadoff-Chu
sequence modulated on the subcarriers. All
devices that just turned active may transmit signals on such resources simultaneously on multiple randomly selected bases;
all other devices remain silent. The base station or the router then runs a
demodulation algorithm to learn which devices transmitted signals in the ABF
symbol period.
The key novelty of ABF is to
allow a device to transmit multiple bases, while the current solution in LTE, i.e., the LTE PRACH, allows a
device to transmit only one base. With more bases, the chance of collision loss is actually
lower when the number of associated devices is very high but the
number of new active devices is low, which is likely to be true in the 5G
scenario.
The
main research challenge in ABF is to design the demodulation algorithm, such
that the base station or the router can decode the information from the ABF
signal, even when there is collision due to random base selection, without any
channel state information because the devices just turned active. The proposed
solution is based on belief propagation, and achieved good performance, such as
reducing the error ratio by more than an order of magnitude than the current
LTE PRACH.
A
version of ABF which uses the OFDM subcarriers as the bases has been
implemented on Microsoft SORA software-defined radio and has been
experimentally proven to outperform existing solutions significantly. A new
Medium Access Control (MAC) protocol is implemented based on ABF and has been
shown to achieve a MAC layer data rate more than 75% of the physical layer data
rate, even under very challenging data traffic, in which many devices alternate
between the active node and the idle mode, with many small data packets such as
40 bytes.
This research was supported by my NSF
CAREER grant: CAREER: Addressing Fundamental Challenges for Wireless Coverage
Service in the TV White Space. 1149344.
Publication:
1.
Z. Zhang,
“Analog Bloom Filter
and Contention-Free Multi-Bit Simultaneous Query for Centralized Wireless
Networks,” IEEE Transactions on
Networking, vol. 25, no. 5, pp. 2916-2929, 2017.
2. Z. Zhang, “Novel PRACH scheme for 5G
networks based on Analog Bloom Filter,” IEEE Globecom 2018. Abu Dubai,
UAE. IEEE
Code:
Will be made available
Slides:
Analog Bloom Filter (ABF) for Ultra-low Latency
Random Access in Wireless Networks
Provisional patent
application in 2018:
·
U.S. Provisional Application No. 62/671,719, “PHYSICAL RANDOM
ACCESS CHANNEL SCHEME FOR 5G NETWORKS USING AN ANALOG BLOOM FILTER"