jov-metropolitan-beacon-system-icd-01.txt

Internet DRAFT - draft-jov-metropolitan-beacon-system-icd

draft-jov-metropolitan-beacon-system-icd

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Previous Versions:	draft-jov-metropolitan-beacon-system-icd-00.txt (diff) - 12-Apr-2014

Internet Engineering Task Force J. Vogedes
Internet-Draft NextNav, LLC
Intended status: Informational April 24, 2014
Expires: October 25, 2014

Metropolitan Beacon System (MBS) ICD
draft-jov-metropolitan-beacon-system-icd-01.txt

Status of this Memo

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Internet-Draft Metropolitan Beacon System (MBS) ICD April 2014

This document is subject to BCP 78 and the IETF Trust.s Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date
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may not be created, except to format it for publication as an
RFC or to translate it into languages other than English.

Abstract

This document describes the air interface of the Metropolitan Beacon
System (MBS) system.

MBS provides a high precision, reliable, consistent positioning system
indoors and in urban canyons, where GNSS solutions are degraded or denied.

In addition to the high 2-D accuracy, the MBS system architecture also
provides for high resolution and accuracy in the vertical dimension, with
the aid of embedded sensors.

MBS technology provides a very fast time to first fix (TTFF), on the order
of ~6 seconds under cold start conditions.

Similar to GNSS, MBS technology allows computation of the location on the
device without any network dependence thus enabling a wide variety of
standalone applications.

Table of Contents

1. Introduction...................................................4
2. Conventions used in this document..............................4
4. MBS M1 Signal Structure........................................5
4.1. MBS M1 Signal Generation..................................5
A timing view of the data that is being sent at the output of
the XOR gate is in the accompanying pdf document..................6

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4.2. Spectral Characteristics..................................6
4.3. MBS Signal Temporal Characteristics.......................7
A sample of the slotted mode MBS signal transmission is
shown in the accompanying pdf document.........................7
5. Databurst Format...............................................9
5.1. Slot Structure............................................9
5.2. Error-correcting code and CRC check......................10
5.3. Modulation...............................................12
5.4. Packet Types . MAC Layer.................................14
5.4.1. Overall Packet Structure............................14
5.4.2. Packet Structure for Packet Type 1 (Full
Trilateration Information).................................16
5.4.2.1. Descriptions of the fields of packet type 1....17
5.4.2.1.1. Latitude..................................17
5.4.2.1.2. Longitude.................................17
5.4.2.1.3. Altitude..................................17
5.4.2.1.4. Tx Correction.............................17
5.4.2.1.5. Tx Quality................................17
5.4.2.1.6. Pressure..................................18
5.4.2.1.7. Temperature...............................18
5.4.2.1.8. Weather Information (Optional)............18
5.5. Packet Structure for Packet Type 2 (Tx ID and GPS
time along with Partial Trilateration Info)...................19
5.5.1. Descriptions of the fields of packet type 2.........20
5.5.1.1. Transmitter ID.................................20
5.5.1.2. Tx Correction..................................20
5.5.1.3. Pressure, Temperature, and Weather info........20
5.5.1.4. GPS time . Week number & TOW...................21
5.5.1.5. MBS time offset relative to GPS................21
5.5.1.6. Slot Index.....................................21
5.5.1.7. UTC time offset from GPS.......................22
5.6. Additional Packet Types..................................23
5.7. Periodicity of Packet Type Transmission..................23
5.8. Transmit Filter Taps (at 4 samples per chip).............23
5.9. PN Codes that may be used by MBS.........................23
6. Security Considerations.......................................24
7. IANA Considerations...........................................24
8. Conclusions...................................................24
9. References....................................................25
9.1. Normative References.....................................25
9.2. Informative References...................................25

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1. Introduction

This document and the accompanying pdf document describe the
air interface of the Metropolitan Beacon System (MBS) system.

MBS provides a high precision, reliable, consistent
positioning system indoors and in urban canyons, where GNSS
solutions are degraded or denied.

In addition to the high 2-D accuracy, the MBS system
architecture also provides for high resolution and accuracy
in the vertical dimension, with the aid of embedded sensors.

MBS technology provides a very fast time to first fix (TTFF),
on the order of ~6 seconds under cold start conditions.

Similar to GNSS, MBS technology allows computation of the
location on the device without any network dependence thus
enabling a wide variety of standalone applications.

2. Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as
described in RFC-2119 [RFC2119].

In this document, these words will appear with that
interpretation only when in ALL CAPS. Lower case uses of
these words are not to be interpreted as carrying RFC-2119
significance.

In this document, the characters ">>" preceding an indented
line(s) indicates a compliance requirement statement using
the key words listed above. This convention aids reviewers in
quickly identifying or finding the explicit compliance
requirements of this RFC.

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3. High Level Architecture

The high level system architecture diagram is represented in
the accompanying pdf file.

MBS beacons are an overlay network used to cover a
metropolitan area. One implementation uses licensed wireless
spectrum in the M-LMS band. Various components are described
below:

Beacon: The beacons denote the MBS beacons broadcasting the
MBS signal. The beacons may be housed on roof tops or towers
(typically pre-existing cell/broadcast sites), or in any
other location deemed appropriate by the operator of the MBS
network.

Cell Phone: An example device that needs location information
is shown as a cell phone under GNSS-challenged conditions
such as urban canyons and indoors where GNSS signals from
satellites may not be received reliably or may provide poor
performance. The cell phones shown in the figure would be
capable of receiving and processing MBS signals. Note that
any device equipped to process MBS signals would work under
these scenarios. A data or a voice connection is not required
for a device to compute its location using the MBS
technology.

Location Server: In certain applications, it may be useful
for a centralized server to compute the location with
information it receives from the mobile because of the
additional information that may be available to the server
device at the time of location determination.

GPS Satellite: Shown for illustrative purposes that it is
blocked by buildings in an urban canyon.

4. MBS M1 Signal Structure

4.1. MBS M1 Signal Generation

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The MBS signal SHALL be generated from a PN sequence and BPSK
spreading. The chipping rate SHALL be 1.023 m/2 Mchips/sec,
where m is an integer greater than or equal to 2, and the
length of the PN sequence SHALL be 1024 n-1, where n is an
integer greater than or equal to 1.

The various blocks in the Signal Generator are described
below:

PN Code Generator: Generates binary waveforms of length 1024
n-1. The PN code generator generates chips at the rate of
1.023 m/2 Mchips/sec (period of each chip is 1/1.023/(m/2)
microseconds).

Data Generator: Collects information from sensors and other
information such as tower Latitude, Longitude, Height (LLH)
and other information and formats them into frames and sub-
frames.

FEC: Adds forward error correction. See Figure in the
accompanying pdf file for detailed block diagram.

Pilot/Preamble Sequence: During some periods (preamble and
ranging periods) MBS beacons transmit a known sequence of
bits. During the preamble, they transmit the preamble bits,
which help with acquisition. During ranging periods, they
transmit pilot bits, which enable long coherent integration
to improve ranging performance.

A timing view of the data that is being sent at the output of
the XOR gate is in the accompanying pdf document.

4.2. Spectral Characteristics

The transmit spectrum SHALL have the following
characteristics:

Tx transmission type: Spread spectrum transmission using BPSK
spreading

RF BW (null-to-null): 1.023m MHz, where m = 2,3,4,...

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The Tx center frequency MAY be in any band. In the USA, one
frequency allocation for MBS is in the LMS band, in the range
920.773 MHz to 926.277 MHz.

The transmit filter taps for the USA LMS band are in Appendix
A, and the frequency response of the transmit filter is shown
in the accompanying pdf document, for sample values of m and
n, where m=2, n=1.

4.3. MBS Signal Temporal Characteristics
The MBS architecture SHALL use an access scheme where each
beacon transmits its data for a specified duration within
each transmission period.

A sample of the slotted mode MBS signal transmission is shown
in the accompanying pdf document.

System parameters:

o Each transmission period SHALL be 1 sec long

o Transmission periods SHALL be deltaT seconds apart, where
deltaT SHALL be an integer greater than or equal to 1

o There SHALL be ten 100ms slots in each transmission period

o The MBS signal SHALL be generated from a PRN sequence and
BPSK spreading

o Each transmitter SHALL be assigned:

o One of the ten slots as its primary slot
o One PRN code

o Additional optional transmitter parameters include

o A primary slot pattern
o This is a sequence of slot indexes (each one in the
range 1 to 10), that determine which slot the
transmitter will transmit in successive seconds of
transmission.
o The sequence MAY be as basic as a simple repetition
of the primary slot, or may be any sequence of slot

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indexes, with each transmitter potentially having a
different periodicity in their slot pattern.
o Secondary slot patterns
o Each beacon MAY have up to nine secondary slot
patterns.
o These MAY have the same or different PRN as the
primary slot pattern of that transmitter, and will
have a transmit power that SHOULD be between 0dB to
50dB lower than the transmit power of the primary
slot pattern.
o Frequency offset

o The chipping rate SHALL be 1.023 m/2 Mcps, where m is an
integer greater than or equal to 2.
o Each PN code SHALL have 1024n-1 chips and lasts (n+(n-
1)/1023)/(m/2) ms.
o Every 100ms slot includes (100 m/2)/(n+(n-1)/1023)
PN code symbols
o One PN code symbol must be used as a guard time
between slots, therefore there are (100 m/2)/(n+(n-
1)/1023)-1 PN code symbols available for ranging
and data transmission in each 100ms.
o For example, when m=2 and n=1, the system can fit
100 PN code symbols in 100ms, out of which 99 are
available for ranging and data transmission.
o Each beacon transmits a preamble using a PN code reserved
only for preambles.
o Ranging slots (described in the next section) have
a preamble of length p^R PN codes (leaving (100
m/2)/(n+(n-1)/1023)-1-p^R PN codes for pilot
symbols)
o Hybrid slots (described in the next section) have a
preamble of length p^H PN codes (leaving (100
m/2)/(n+(n-1)/1023)-1-p^H PN codes for pilot and
data symbols)
o A list of possible PN Codes used by MBS is shown in
Appendix B.

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5. Databurst Format
MBS uses the concept of databursts in order to be able to
transmit all the data required for trilateration (such as
latitude, longitude, etc.) in a short amount of time, and
also be able to perform long coherent integrations to enable
high ranging accuracy. An optional implementation would be to
divide the time available to a transmitter into ranging
portions and data portions. During the ranging part,
transmitters transmit pilot symbols that enable long coherent
integration, and during the data part, transmitters transmit
data symbols at a physical-layer rate of 1 bit per PN code
period. An optional slot structure, implementing the above
methodology, is presented below.

5.1. Slot Structure

1. Separate slots for ranging and data

o One slot uses BPSK pilot symbols for ranging
o This MAY be followed by one or more slots that are hybrid
(ranging & data slots)

2. Use error-correcting codes & CRC for the data portions

In general, an MBS deployment MAY have zero or more hybrid
slots for each ranging slot. In scenarios where there are
zero hybrid slots, receivers must obtain assistance data via
another channel in order to perform trilateration.

One possible implementation, for the sample scenario of
m=2,n=1, which results in 99 PN code symbols per 100ms being
available for ranging and data transmission, uses the
following settings:

o Slot structure consists of one ranging slot followed
by two hybrid slots
o This structure is referred to as RH1H2 and is
depicted in the Figure in the accompanying pdf
document

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o Ranging slots:
o 7 PN codes for preamble
o 92 PN codes for pilot symbols
o Hybrid ranging & data slots:
o 4 PN codes for preamble
o 14 PN codes for pilot symbols
o 81 PN codes for data transmission using BPSK at 1
PN code/symbol

Using the RH1H2 slot structure and sample implementation from
above, MBS is able to support 102 information bits in one
data packet. These information bits are used for transmitting
information required for trilateration (such as Tx
lat/long/altitude).

In terms of alignment of above slot structure to GPS time,
MBS physical slot 1 of the R frame (see Figure 5) starts at
.GPS time in seconds. modulo 3 = 0, plus .GPS time offset.
(from MBS packet type 2, described in a later section)

5.2. Error-correcting code and CRC check
MBS SHALL use error-correcting codes to ensure operation at
low SNRs and uses CRC to ensure that the decoded bits are
valid. The error-correcting codes and CRC polynomials chosen
for MBS may vary from implementation to implementation.

The remainder of this section describes the implementation
with the RH1H2 slot structure and the sample scenario of m=2,
n=1, which uses a convolutional code with constraint length 7
and a 16-bit CRC polynomial.

A block diagram of the encoding process is shown in the
figure in the accompanying pdf file.

The CRC check is accomplished using a length-N(CRC) CRC code.
The value of N(CRC) is 16, and the CRC polynomial used is
x^16 + x^15 + x^12 + x^7 + x^6 + x^4 + x^3 + 1.

Each of the two hybrid slots is encoded and decoded
separately, though the CRC is common to both slots. That is,
the transmitter takes the 102 information bits, calculates
the 16 bits of CRC, resulting in 118 bits. It then divides

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these 118 bits into two parts of length 59 bits, and it is
these 59 bits which are encoded and transmitted using the 81
available PN code symbols in each hybrid slot.

The error-correcting code used is a convolutional code. The
code has constraint-length 7 and is a rate-1/2 code that is
punctured to ensure that the encoded bits fit within the 81
available PN code symbols in each hybrid slot. The
transmitter adds 6 all-zero tail bits to the information bits
before encoding, due to the nature of convolutional coding
and decoding.

The encoding process and what is described above can also be
visualized in the figures in the accompanying pdf file.

The encoding process for this sample scenario can be
summarized as:

1. Take 102 info bits as inputs

2. Add 16 CRC bits, to end up with 118 bits

3. Split into two groups of 59 bits (first 59 for H1 slot
last 59 for H2)

4. For each group of 59 bits

a. Add 6 tail bits, to end up with 65 bits

b. Encode using the rate 1/2 encoder, to end up with 130 bits

c. Puncture the output of the encoder, to end up with 81 bits

d. Interleave the above bits, and send the result to the
modulator, to be transmitted over-the-air to the receiver.

Encoder information:

o Convolutional encoder of rate: 1 / 2
o Constraint-length: 7
o Encoder polynomials: [171 133] (in octal)
o Puncturing pattern: Of the 130 encoder output bits,
select 81, according to bpunct[k] =
benc[idx_pass[k]], k = 0 to 80 where

idx_pass[] = {
1,2,4,6,7,9,10,12,14,15,17,18,20,21,23,25,26,28,29,31,33,34,3

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6,37,39,41,42,44,45,47,49,50,52,53,55,57,58,60,61,63,64,66,68
,69,71,72,74,76,77,79,80,82,84,85,87,88,90,92,93,95,96,98,100
,101,103,104,106,107,109,111,112,114,115,117,119,120,122,123,
125,127,128 };

o Interleaving pattern: From the input bit sequence
bpunct[k] where k = 0 to 80, calculate the output bit
sequence bout[k] according to

bout[k] = bpunct[idx_permute[k]], k = 0 to 80

where idx_permute is the following length-81 array:

idx_permute[] = {

4,21,80,65,39,35,6,32,8,47,45,25,23,76,41,16,30,7,46,11,9,51,
2,43,71,79,69,74,50,70,78,10,62,17,60,15,13,5,68,36,27,72,75,
40,38,54,24,52,64,58,55,20,63,59,26,67,31,49,0,56,42,61,53,66
,3,18,48,22,34,57,12,33,19,37,73,28,1,29,77,44,14 };

(The receiver demodulates the signal in each slot, de-
interleaves the resulting soft bits and passes them through
the decoder. The receiver concatenates the output of the
decoder from the two hybrid slots H1 and H2 and does a CRC
check to ensure that the block of data was sent successfully)

5.3. Modulation

In ranging slots, after the preamble, MBS SHALL use BPSK
modulation to transmit (100 m/2)/(n+(n-1)/1023)-1-p(sub R) pilot
bits over the same number of PN code periods. These are the
pilot bits that enable the long coherent integration times. The
pilot bit sequence during ranging slots is described below.

In hybrid slots, after the preamble, there are (100 m/2)/(n+(n-
1)/1023)-1-p(sub H) PN code periods left in the slot. MBS uses
BPSK modulation to transmit pilot bits over a subset of these
code periods, and then uses DBPSK (differential BPSK) modulation
to transmit data bits over the remaining PN code periods. The
transmitter uses the last pilot bit as the first DBPSK data bit
so that it can maximize the number of data bits it can transmit,
even though it is using DBPSK. The pilot bit sequence is
different for H1 and H2 slots.

The pilot bit sequences for ranging and hybrid slots depend on
the MBS network configuration and MAY be in one of two modes.

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The following are the two modes for the RH1H2 slot structure for
the sample scenario of m=2,n=1 :

Pilot Sequence Mode 1:

Ranging (R) slot:

0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

H1 pilot sequence: 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0

H2 pilot sequence: 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1

Pilot Sequence Mode 2:

R slot:

1,1,1,1,1,0,1,0,1,1,0,1,0,0,0,1,1,0,1,1,0,0,1,1,1,0,0,0,0,1,0
,1,1,0,1,1,0,1,0,1,1,1,0,0,1,0,0,1,1,1,0,0,1,1,0,0,0,1,1,0,1,1,0
,0,1,0,1,1,0,0,0,1,0,0,1,1,0,1,0,0,0,0,0,1,0,1,1,1,1,1,0,1

H1 pilot sequence: 0, 1, 0, 0, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0

H2 pilot sequence: 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 1, 0

In all sequences above, a .0. is mapped to .-1., and a .1. is
mapped to .1. during modulation.

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5.4. Packet Types . MAC Layer

MBS supports various packet types, such as one that carries
trilateration information and one that carries GPS time
information. For each packet type, MBS could support
encryption of the payload, and MBS service providers may
choose to encrypt or may choose not to encrypt the various
packets.

The remainder of this section describes the implementation
corresponding to the RH1H2 slot structure and the sample
scenario of m=2, n=1, which is able to carry 102 information
bits per data packet.

The various packet types supported are listed in Table 1 (see
accompanying pdf document for additional details).

Table 1: Packet types

+------+---------------+-------------------+------------+

+------+---------------+-------------------+------------+

| 0 | Reserved | TBD | TBD |

| 1 | LLA, etc | 99 | 2 |

| 2 | TxID, etc | 96 | 2 |

| 3-6 | Reserved | TBD | any |

| 7 | Extensions | TBD | any |

+------+---------------+-------------------+------------+

This section specifies how many bits are required to be
transmitted for each field of each packet type listed above.

5.4.1. Overall Packet Structure

Since there is more than one data packet type, there is a need
for an indicator to denote which one the Rx is seeing at any
given time.

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Three bits are allocated to describe the packet type. In future
versions of MBS, extension packet types MAY be supported by
using .111. as the base packet type (to denote .more packet type
information to come.), and then have a few bits after that to
denote more packet types.

The total payload of the RH1H2 scheme is 102 information bits
per RH1H2 triplet of slots. Out of those 102 bits, 3 are for
packet type index, leaving 99 bits for the data payload and any
other framing overhead.

If some data to be transmitted is more than can be carried in
one RH1H2 packet, the Tx sends the data over more than one
packet. In that case, there is a need for a scheme to identify
how the bits from the current data packet fit into the overall
set of data bits that are to be transmitted. In order to have
unambiguous understanding by the receiver on what is being
transmitted in each data packet, the scheme is visually
presented in the accompanying pdf document. Below are some of
the principles of the data packet structure:

o In every packet of 102 bits, the first three bits are
the packet type
o For packet types 0 and 1:
o The next 99 bits contain the main packet payload
o For packet types other than 0 and 1:
o The fourth bit is a reserved bit.
o The fifth bit is the start bit, and denotes whether
this frame begins a new packet (1) or the
continuation of a previous packet (0).
o The sixth bit is the stop bit, and denotes whether
this is the last frame of a packet (1) or a
continuation frame of a packet (0).
o The next 96 bits contain the packet payload

Summary: 3 bits of framing overhead for packet types 0 and 1,
and 6 bits of framing overhead for packet types other than 0 and
1.

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5.4.2. Packet Structure for Packet Type 1 (Full Trilateration
Information)

Table 2: Packet Info for Packet Type 1

+-------------+-----------+----------+

| Field | bit_index | num_bits |

+-------------+-----------+----------+

| Packet type | 1 - 3 | 3 |

| Payload | 4 - 102 | 99 |

+-------------+-----------+----------+

Table 2: Payload for Packet Type 1

+-------------------------+----------+-----------+------------+

+-------------------------+----------+-----------+------------+

| Latitude | 1 | 1 - 26 | 26 |

| Longitude | 2 | 27 - 53 | 27 |

| Altitude | 3 | 54 - 68 | 15 |

| Tx correction | 4 | 69 - 73 | 5 |

| Tx quality | 5 | 74 - 77 | 4 |

| Pressure | 6 | 78 - 87 | 10 |

| Temperature | 7 | 88 - 94 | 7 |

| Weather info(optional) | 8 | 95 - 99 | 5 |

+-------------------------+----------+-----------+------------+

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5.4.2.1. Descriptions of the fields of packet type 1

Individual MBS service providers SHOULD map the raw values of
the bits for each field to a range and resolution they feel
best meets their requirements. Below are descriptions and
sample ranges for each field.

5.4.2.1.1. Latitude

Latitude of the Tx antenna. Sample range: [-90, 90] degrees.

5.4.2.1.2. Longitude

Longitude of the Tx antenna. Sample range: [-180, 180]
degrees.

5.4.2.1.3. Altitude

Altitude of the Tx antenna. Sample range: [-500, 9000]
meters.

5.4.2.1.4. Tx Correction

Tx correction is the residual timing error left over after
the Tx adjusts its transmission to account for the various
delays in the system, such as cable delays. The receiver
needs to take the Tx correction into account to fine-tune the
pseudorange estimate from each transmitter (the Tx correction
value for a given beacon needs to be subtracted from the
receiver time stamp of the time-of-arrival estimate for that
beacon).

Sample range: [0,31] ns.

Note: A bit sequence of all ones for the Tx Correction bit
field denotes an invalid Tx Correction value, i.e. the
transmitter has not been able to determine the Tx Correction
value.

5.4.2.1.5. Tx Quality

Each beacon transmits some bits that denote to the receiver
some relative quality metric about that particular beacon.

Sample range: [0, 15].

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5.4.2.1.6. Pressure

The transmitter SHALL transmit pressure information to the
receiver.

One option is to transmit the pressure measured at the
beacon. Another option MAY be to transmit a transformation of
the pressure measured at the beacon. As a sample
transformation, the transmitter MAY convert the pressure
measured at the beacon to an estimated pressure at a
reference altitude level.

Sample range: [94500, 106776] Pa.

5.4.2.1.7. Temperature

The temperature measured at the beacon, which represents
ambient atmospheric temperature.

Sample range: [228, 330] Kelvin.

5.4.2.1.8. Weather Information (Optional)

Each transmitter MAY transmit some bits that denote to the
receiver some extra information about the weather and/or
weather equipment, to enable improved altitude calculation.

Some examples of such information are:

o Wind speed
o Quality of the weather data
(pressure/temperature/etc)
o Additional weather/atmospheric extensions

Sample range: [0,31]

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5.5. Packet Structure for Packet Type 2 (Tx ID and GPS time
along with Partial Trilateration Info)

Table 4: Packet Info for Packet Type 2

+--------------+-----------+----------+

| Field | bit_index | num_bits |

+--------------+-----------+----------+

| Packet type | 1 - 3 | 3 |

| Reserved bit | 4 | 1 |

| Start bit | 5 | 1 |

| Stop bit | 6 | 1 |

| Payload | 7 - 102 | 96 |

+--------------+-----------+----------+

Table 5: Payload for Packet Type 2

+--------------------------+----------+-----------+------------+

+--------------------------+----------+-----------+------------+

| Tx ID | 1 | 1 - 15 | 15 |

| Tx correction | 2 | 16 - 20 | 5 |

| Pressure | 3 | 21 - 31 | 11 |

| Temperature | 4 | 32 - 39 | 8 |

| Weather info | 5 | 40 - 46 | 7 |

| GPS time - Week Number | 6 | 47 - 56 | 10 |

| GPS time - TOW in seconds| 7 | 57 - 76 | 20 |

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| Time offset relative/GPS | 8 | 77 - 86 | 10 |

| Slot Index | 9 | 87 - 90 | 4 |

| UTC time offset from GPS | 10 | 91 - 96 | 6 |

+--------------------------+----------+-----------+------------+

5.5.1. Descriptions of the fields of packet type 2

5.5.1.1. Transmitter ID

The Tx ID field MUST be a unique ID that identifies each
transmitter within one major deployment area, such as within
North America. With 15 bits, up to 32,768 unique transmitters
can be identified. The Tx ID should be used, along with an
almanac on the receiver, to extract the lat/long/height of
each transmitter, as well as the Tx quality information for
each transmitter.

Sample range: [0, 2^15-1]

5.5.1.2. Tx Correction

Tx correction is as described in Section 5.4.2.

Sample range: [0,25] ns, 1ns resolution

5.5.1.3. Pressure, Temperature, and Weather info
Pressure, Temperature, and Weather info are as described in
Section 5.4.2.

Pressure - Sample range: [94500, 106776] Pa, with 6 Pa
resolution

Temperature - Sample range: [228, 329.6] Kelvin, with 0.4
degrees Kelvin resolution

Weather info - Sample range: [0,124]

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5.5.1.4. GPS time . Week number & TOW

This represents the GPS time of the R frame immediately
preceding the H1/H2 frames in which this packet was carried.
GPS time is represented as time of week (TOW) and GPS week
number.

TOW is the number of seconds since the beginning of the GPS
week, which runs from zero to 604799 at the end of week. The
TOW second count returns to zero coincident with the
resetting of the GPS PRN codes.

The GPS week number represents the GPS weeks (modulo 1024)
since week 0 which started at 00:00:00 Sunday 6th January,
1980.

Week number - Range: [0,2^10-1] weeks, with 1 week
resolution

TOW seconds - Range: [0, 604799] sec, with 1 sec resolution

5.5.1.5. MBS time offset relative to GPS

This is the offset of MBS system time relative to GPS time.
Note that MBS system time is always delayed relative to GPS
time by the number of nano-seconds specified in this field
and is expected to be a constant.

Sample range: [0,1000] ns, with 1ns resolution.

5.5.1.6. Slot Index

This is the physical time slot in which a transmitter is
transmitting.

Range: [0,9].

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5.5.1.7. UTC time offset from GPS

This is the UTC time offset from GPS time. The UTC offset
field can accommodate 63 leap seconds (six bits).

Range: [0,63] sec, with 1 sec resolution.

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5.6. Additional Packet Types

Additional packets using packet type greater than 2 MAY be
defined as required for the MBS system.

5.7. Periodicity of Packet Type Transmission

The periodicity and the associated time offset of the
transmission for various packet types is MBS service provider
specific. The packet transmissions of a particular type MAY
be staggered relative to other beacons.

As an example, in the beacon with Tx ID 1 occupying slot 1,
the packet with type 2 MAY be transmitted once in 30 seconds
starting at GPS TOW second (modulo 30)=0 and packet type 0
MAY be transmitted at all other times. Whereas, in the beacon
with Tx ID 2 occupying slot 2, packet type 2 may be
transmitted once in 30 seconds starting at GPS TOW second
(modulo 30)=3 and packet type 0 may be transmitted at all
other times.

5.8. Transmit Filter Taps (at 4 samples per chip)

See accompanying pdf document, appendix A.

5.9. PN Codes that may be used by MBS

In general, any family of PN codes MAY be used for MBS. For
example, the GPS family of Gold Codes MAY be used, as shown
in the accompanying pdf document (Appendix B). Note that the
G2 delay and G2 code initial state in the table are specified
in the same way as in the GPS interface specification IS-GPS-
200 Revision E.

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Sample PN Codes used by MBS, based on GPS family of Gold
Codes are shown in the accompanying pdf document.

The .G2 delay. referred to in the table above is the delay of
the G2 code used in the standard GPS PN Code generation of
length 1023. In pseudocode:

y1 = standard_gps_m_sequence1_G1;
y2 = standard_gps_m_sequence2_G2;
PN_code = xor(y1, circular_shift(y2,delay));

6. Security Considerations

The MBS ICD does not itself create a security threat.

7. IANA Considerations

There are no IANA considerations for the MBS ICD.

8. Conclusions

Metropolitan Beacon System (MBS) consists of a network of
terrestrial beacons broadcasting signals for positioning
purposes. Terrestrial Beacon Systems can be designed to
facilitate UE positioning in areas where in-orbit satellite
based systems are most challenged, such as indoors, or in
dense urban environments and extends UE positioning
capabilities in these environments. In addition, MBS enables
the delivery of an accurate UE altitude for emergency or
commercial services.

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9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.

9.2. Informative References

[GPS ICD] IS-GPS-200, Revision D, Navstar GPS Space Segment Navigation User Interfaces, March 7th, 2006

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Authors' Addresses

Jerome Vogedes
NextNav, LLC
484 Oakmead Parkway
Sunnyvale, CA 94085
jvogedes@nextnav.com

Ganesh Pattabiraman
NextNav, LLC
484 Oakmead Parkway
Sunnyvale, CA 94085

Arun Raghupathy
NextNav, LLC
484 Oakmead Parkway
Sunnyvale, CA 94085

Andrew Sendonaris
NextNav, LLC
484 Oakmead Parkway
Sunnyvale, CA 94085

Norman Shaw
NextNav, LLC
484 Oakmead Parkway
Sunnyvale, CA 94085

Madhu Shekhar
NextNav, LLC
484 Oakmead Parkway
Sunnyvale, CA 94085

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