Multiple Access in Wireless Communication

Frequency Hopped Multiple Access (FHMA) is a digital multiple access technique in which the carrier frequency of each user changes (hops) pseudo-randomly over a wide band, following a PN code unique to that user. At any instant a user occupies only one narrow channel, but over time the signal is spread over the whole band. Different users hop with different codes, so they rarely collide.

 TRANSMITTER
 data->[FSK/BFSK mod]->(×)->[BPF]-> wideband channel
                        ^
            [freq. synthesizer]<-[PN code gen.]

 RECEIVER
 channel->(×)->[BPF/IF]->[FSK demod]-> data
           ^
  [freq. synthesizer]<-[PN code gen.] (synchronised)

Transmitter

1. Data modulates a narrowband carrier, usually with FSK.
2. A PN code generator gives a new code word every hop period; the frequency synthesizer converts it into a hop frequency.
3. The mixer moves the modulated signal to that frequency. The hop rate is the number of frequency changes per second.

Receiver

1. An identical PN generator, synchronised with the transmitter, drives a local synthesizer.
2. Mixing with this local signal de-hops the signal back to a fixed IF.
3. A conventional FSK demodulator recovers the data. Signals of other users, hopping on different patterns, fall outside the IF filter most of the time.

Example: a system has 8 channels f₀ … f₇ (3-bit PN code). User A's code gives the sequence f₃, f₇, f₁, f₄ …; user B's code gives f₅, f₂, f₆, f₀ …. In each hop interval they are on different frequencies, so both share the band. If both land on the same frequency in one hop (a collision), only that hop's bits are damaged and coding/interleaving corrects them. Bluetooth is a real example: 79 channels of 1 MHz, 1600 hops/s. GSM uses slow frequency hopping (217 hops/s) for diversity.

Types: slow hopping (several symbols per hop) and fast hopping (several hops per symbol).

Advantages: resistance to narrowband interference and fading, security, no near–far problem as severe as DS-CDMA. Disadvantage: fast synthesizers needed; collisions cause errors.

CDMA (Code Division Multiple Access) lets all users transmit at the same time on the same frequency band. Each user is given a unique spreading code (PN or Walsh code) that is (nearly) orthogonal to other users' codes. The receiver separates users by correlating with the wanted user's code.

Encoding (transmitter)

 data b(t)--(×)--(×)--> to channel  (sum of all users)
  ±1         ^     ^
       code c(t)   carrier cos(ωc t)
       chip rate Rc >> bit rate Rb

1. Each data bit is represented as +1 (for 1) or −1 (for 0).
2. Each bit is multiplied by the user's code of N chips, so one bit becomes N chips. Bandwidth spreads by the processing gain G = Rc/Rb = N.
3. The spread signal modulates the carrier (BPSK/QPSK) and is transmitted. In the air, all users' signals add up.

Decoding (receiver)

 r(t)--(×)--(×)--[∫ over Tb, Σ/N]--[decision]--> data
         ^     ^
   cos(ωc t)   c_k(t) (same code, synchronised)

1. The received composite signal is down-converted.
2. It is multiplied chip by chip with the wanted user's code and summed over the bit (correlation), then divided by N.
3. Because cᵢ·cᵢ = N and cᵢ·cⱼ = 0 for orthogonal codes, the wanted bit appears as ±1 and the others cancel.
4. A decision device gives 1 if the result > 0 and 0 if < 0.

Example

Two users with 4-chip Walsh codes:

• User A code c_A = (+1, −1, +1, −1); A sends bit 1 → +1
• User B code c_B = (+1, +1, −1, −1); B sends bit 0 → −1

 A signal = +1 × c_A = (+1, -1, +1, -1)
 B signal = -1 × c_B = (-1, -1, +1, +1)
 Channel sum r       = ( 0, -2, +2,  0)

 Decode A: r·c_A = 0(+1)+(-2)(-1)+2(+1)+0(-1) = 4
           4/4 = +1  -> bit 1
 Decode B: r·c_B = 0(+1)+(-2)(+1)+2(-1)+0(-1) = -4
           -4/4 = -1 -> bit 0

Both users' bits are recovered correctly from the same channel. Check of orthogonality: c_A·c_B = 1 − 1 − 1 + 1 = 0.

In practice (IS-95), Walsh codes separate users in the forward link, long PN codes separate users in the reverse link, and power control keeps all signals at equal strength to avoid the near–far problem.

In FDMA, the total band is divided into channels of equal width, after leaving a guard band at each edge of the allocated spectrum. The number of channels is

N = (B_t − 2·B_guard) / B_c

where B_t = total spectrum, B_guard = guard band at each edge, B_c = channel bandwidth.

Given: B_t = 12.8 MHz (per simplex band), B_guard = 10 kHz, B_c = 30 kHz.

 N = (12.8×10⁶ − 2 × 10×10³) / (30×10³)
   = (12 800 000 − 20 000) / 30 000
   = 12 780 000 / 30 000
   = 426

Answer: N = 426 radio channels (in each simplex band; these pair up to give 426 duplex channels in the forward and reverse bands).

This matches the AMPS (US analog) system, which uses 30 kHz channels with FDMA/FDD.

Advantages of CDMA over TDMA

Frequency Hopping Spread Spectrum (FHSS) multiple access

In FHSS, the carrier frequency of the narrowband modulated signal jumps pseudo-randomly among many channels spread over a wide band. The hopping sequence is set by a PN code; each user has its own code, so many users share the band with few collisions. The instantaneous bandwidth is narrow, but the total band W = (number of hop channels) × (channel width), and the processing gain is about the number of hop channels.

 TRANSMITTER
 data->[FSK modulator]->(×)->[BPF]-> antenna
                         ^
       [PN code gen.]->[frequency synthesizer]

 RECEIVER
 antenna->(×)->[BPF at IF]->[FSK demodulator]-> data
           ^
       [PN code gen.]->[frequency synthesizer]
       (same code, synchronised to Tx)

Working

1. Data modulates a carrier, usually with M-ary FSK.
2. The PN code generator outputs k bits every hop period; the frequency synthesizer maps them to one of 2ᵏ frequencies.
3. The mixer shifts the FSK signal to that hop frequency; the band-pass filter removes unwanted products.
4. At the receiver, an identical, synchronised PN generator and synthesizer produce the same hop sequence; mixing de-hops the signal to a fixed IF.
5. A normal FSK demodulator recovers the data. Other users' signals or jammers hit the same frequency only occasionally (a "hit"), and FEC with interleaving corrects those errors.

Example: Bluetooth hops over 79 channels of 1 MHz at 1600 hops/s.

Types of FHSS

• Slow frequency hopping (SFH): hop rate is less than the symbol rate – several symbols are sent per hop (Tₕ > Tₛ). Simple synthesizer; a hit damages several symbols. Example: GSM (217 hops/s).
• Fast frequency hopping (FFH): hop rate is greater than the symbol rate – each symbol is sent over several hops (Tₕ < Tₛ). Gives frequency diversity within each symbol and strong anti-jamming, but needs a very fast synthesizer and non-coherent detection.

Hybrid spread spectrum techniques combine two basic multiple access methods to get the advantages of both.

Hybrid spread spectrum multiple access techniques

1. Hybrid FDMA/CDMA (FCDMA)

• The available wideband spectrum is divided into several narrower sub-bands (FDMA); inside each sub-band, a separate CDMA system with its own PN codes serves a group of users.
• Advantages: the sub-bands need not be contiguous, so spectrum can be allocated flexibly; each sub-band has lower bandwidth, so cheaper hardware and lower chip rate; sub-bands can be switched on/off to avoid interference.
• Disadvantage: processing gain in each sub-band is lower than full-band CDMA, and frequency-selective fading is less averaged out.

2. Hybrid Direct Sequence / Frequency Hopped Multiple Access (DS/FHMA)

• The signal is first spread by a DS code, then the centre frequency of the DS signal hops pseudo-randomly. Each user has a unique DS code and hopping pattern.
• Advantages: avoids the near–far problem of pure DS-CDMA, since a near user rarely shares the same hop frequency; combines anti-jamming of FH with multipath resistance of DS.
• Disadvantage: complex transmitter and receiver; needs fast, accurate synthesizers and code synchronisation.

3. Time Division CDMA (TCDMA)

• Different spreading codes are assigned to different cells, and inside a cell only one user transmits in a given time slot (TDMA). So a user meets interference only from other cells.
• Advantages: no near–far problem within a cell; easier power control. Disadvantage: needs time synchronisation and has TDMA overhead.

4. Time Division Frequency Hopping (TDFH)

• A user hops to a new frequency at the start of each new TDMA frame (as in GSM slow frequency hopping).
• Advantages: avoids severe fades or co-channel interference on one channel for long; gives frequency diversity. Disadvantage: needs synchronised hopping across frames and more complex synthesizers.

Operation of FHMA

Frequency Hopped Multiple Access (FHMA) lets many users share a wide band by making each user's narrowband carrier hop pseudo-randomly among many channels. At any instant a user uses one channel, and different users follow different hop patterns.

 TRANSMITTER
 data->[FSK modulator]->(×)->[BPF]-> antenna
                         ^
       [PN code gen.]->[frequency synthesizer]

 RECEIVER
 antenna->(×)->[BPF at IF]->[FSK demodulator]-> data
           ^
       [PN code gen.]->[frequency synthesizer]
       (same code, synchronised to Tx)

1. Data modulates the carrier (usually FSK).
2. A PN code generator drives a frequency synthesizer, which sets a new carrier frequency every hop period.
3. The transmitted signal thus occupies a different channel in each hop.
4. The receiver, using the same PN code in synchronism, de-hops the signal to a fixed IF and demodulates it.
5. When two users hop to the same channel at the same time (collision), errors occur in that hop only; channel coding and interleaving correct them.

Example: with 8 channels, user A may follow f₂, f₆, f₁, f₇ … and user B f₅, f₀, f₃, f₄ …; they share the band without colliding in these hops.

Types of FHMA

• Slow FHMA: several symbols per hop (hop rate < symbol rate), e.g. GSM SFH at 217 hops/s.
• Fast FHMA: several hops per symbol (hop rate > symbol rate); better diversity and anti-jamming, more complex synthesizer.

FDMA gives each user a separate frequency channel for the whole call, while TDMA gives each user the whole carrier (shared with others) for a separate, repeating time slot.

Similarities: both are narrowband, orthogonal (non-interfering in ideal case) and channel-based; both are usually combined (GSM is FDMA/TDMA). TDMA allows variable rate by giving more slots, and saves battery because the transmitter is off between bursts.

Hybrid multiple access techniques combine two basic schemes (FDMA, TDMA, CDMA, FHMA) to get the benefits of both. Two common ones:

1. Hybrid FDMA/CDMA (FCDMA)

 |<------------ total spectrum ------------>|
 | CDMA band 1 | CDMA band 2 | CDMA band 3 |
   (users by code inside each sub-band)

• The wide spectrum is split into narrower sub-bands; each sub-band carries an independent CDMA system.
• Sub-bands need not be adjacent, so spectrum can be used flexibly; hardware works at lower chip rate.
• Drawback: lower processing gain per sub-band. IS-95 carriers of 1.25 MHz are an example.

2. Hybrid Direct Sequence / Frequency Hopped Multiple Access (DS/FHMA)

 data->(×PN code)->[modulator]->(×)-> hopping DS signal
                                 ^
               [PN]->[frequency synthesizer]

• The signal is direct-sequence spread, and the centre frequency of the spread signal hops pseudo-randomly.
• Advantage: avoids the near–far problem, since a strong nearby user seldom occupies the same hop frequency.
• Disadvantage: complex, needs fast synthesizers and tight synchronisation.

(Others: Time Division CDMA – different codes per cell, one user per time slot within a cell; Time Division Frequency Hopping – the user hops to a new frequency each TDMA frame, as in GSM.)

Non-linear effect in FDMA

In FDMA the base station transmits many carriers at once through a common power amplifier. To get high efficiency, the amplifier is run near saturation, where it is non-linear. A non-linear device produces intermodulation (IM) products: with carriers f₁ and f₂, third-order products at 2f₁ − f₂ and 2f₂ − f₁ fall inside the band, on other users' channels.

• IM products act as interference in other channels, and also spread the signal spectrum (spectral regrowth).
• Effects: lower signal-to-interference ratio and adjacent channel interference.
• Remedies: back off the amplifier into its linear region (lower efficiency), use linearisation, choose carrier frequencies so IM products do not fall on used channels, or use separate amplifiers per carrier.

Number of channels

Given: B_t = 25 MHz, B_guard = 100 kHz (at each edge), B_c = 200 kHz.

 N = (B_t − 2·B_guard) / B_c
   = (25×10⁶ − 2 × 100×10³) / (200×10³)
   = (25 000 000 − 200 000) / 200 000
   = 24 800 000 / 200 000
   = 124

Answer: N = 124 channels (this is the GSM-900 case: 25 MHz band with 200 kHz carriers).

CDMA design considerations are the factors that decide the capacity and quality of a CDMA system, since all users share the same band at the same time.

1. Spreading codes – Codes must have sharp autocorrelation (for synchronisation and multipath resolution) and low cross-correlation (for low multiple-access interference). Walsh codes (orthogonal) and long/short PN codes (m-sequences, Gold codes) are used.
2. Processing gain – G = W/R (chip rate/bit rate). Higher G gives more interference rejection and more users; e.g. IS-95: 1.2288 Mcps / 9.6 kbps = 128 (21 dB).
3. Power control – Because of the near–far problem, a nearby mobile can swamp a distant one. Fast closed-loop and open-loop power control keep all signals arriving at equal power (IS-95: 800 updates/s).
4. Interference-limited capacity – Capacity N ≈ 1 + (W/R)/(Eb/N₀); so lowering required Eb/N₀ (good coding), voice activity (factor ≈ 2.5), sectorization and frequency reuse factor 1 increase capacity.
5. Synchronisation and code acquisition – Receivers must acquire and track the code phase within a fraction of a chip.
6. Multipath handling – RAKE receivers combine resolvable paths (separated by more than one chip).
7. Soft handoff – Mobile communicates with several base stations together; needs network support.
8. Bandwidth – Wide band needed; must suit spectrum available (1.25 MHz in IS-95).

CDMA

Code Division Multiple Access (CDMA) is a spread-spectrum multiple access technique in which all users transmit simultaneously on the same frequency band, and each user is separated by a unique spreading code (PN or Walsh). The narrowband data is multiplied by a high-rate code, spreading it over a wide band; the receiver correlates with the same code to recover the user's data, while other users' signals remain spread and appear as low-level noise.

Characteristics

• All users share the full bandwidth at all times; no time or frequency division.
• Users separated by codes with low cross-correlation; message signal bandwidth ≪ transmitted bandwidth (processing gain G = W/R).
• Interference-limited, soft capacity: adding a user only raises the noise floor slightly.
• Power control is essential to overcome the near–far problem.
• Multipath is exploited using the RAKE receiver.
• Soft handoff between cells is possible.
• Frequency reuse factor of 1.
• Example: IS-95 (1.25 MHz, 1.2288 Mcps), WCDMA (5 MHz, 3.84 Mcps).

Advantages

1. Higher capacity than FDMA/TDMA, with soft (graceful) capacity limit.
2. Universal frequency reuse – no frequency planning.
3. Soft handoff reduces call drops.
4. Multipath diversity through RAKE receivers; resistance to fading.
5. Voice activity and sectorization directly raise capacity.
6. Inherent privacy and resistance to narrowband interference/jamming.
7. Lower average transmit power, longer battery life.

Limitations

1. Near–far problem – strong nearby signals mask weak distant ones; needs accurate, fast power control.
2. Self-jamming / multiple access interference – codes are not perfectly orthogonal (especially with asynchronous users and multipath), so each user adds interference.
3. Complex receivers: code acquisition, synchronisation, RAKE, power control.
4. Cell breathing – cell coverage shrinks as load increases.
5. Needs wide contiguous bandwidth.
6. Performance degrades as the number of users grows.

Features of CDMA

CDMA (Code Division Multiple Access) lets all users share the same carrier at the same time; each user is separated by a unique spreading (PN) code. Important features:

• Universal frequency reuse – every cell uses the same frequency (reuse factor N = 1), so no frequency planning is needed.
• Soft capacity – there is no hard limit on users; adding a user only raises the noise floor slightly (graceful degradation).
• Soft handoff – the mobile can talk to two or more base stations at once, giving fewer dropped calls.
• Multipath resistance – a RAKE receiver combines delayed copies of the signal (multipath diversity).
• Power control is essential – needed to solve the near-far problem.
• Privacy and anti-jamming – the signal looks like noise without the code.
• Voice activity gain – during silence the user transmits less, raising capacity.

Hybrid spread spectrum techniques

Hybrid techniques combine spread spectrum with FDMA, TDMA or frequency hopping to get the benefits of each.

1. Hybrid FDMA/CDMA (FCDMA) – The available wide band is divided into a number of narrower sub-bands. Each sub-band is a small DS-CDMA system with its own processing gain. A user is given a sub-band and a code. The sub-bands need not be contiguous, and different users can be given different bandwidths according to their needs.
   • Advantage: flexible use of fragmented spectrum; smaller sub-bands need lower chip rates.
   • Disadvantage: lower processing gain per sub-band than one full-band CDMA system.
2. Hybrid Direct Sequence / Frequency Hopping (DS/FHMA) – A DS-modulated signal (narrow PN-spread signal) is made to hop its centre frequency in a pseudo-random way. In each hop only a small part of the band is occupied, so a nearby strong user rarely sits on the same frequency at the same time.
   • Advantage: avoids the near-far effect.
   • Disadvantage: complex transmitter/receiver; not suited to soft handoff, because the base station cannot easily stay synchronised to many hopping signals.
3. Time Division CDMA (TCDMA) – Each cell uses a different spreading code. Inside a cell, users are given different time slots, so at any instant only one CDMA user transmits per cell. During handoff the user's code is changed to the code of the new cell.
   • Advantage: avoids the near-far effect (no two users of the same cell are received together).
   • Disadvantage: needs time synchronisation; capacity per cell is limited by slots.
4. Time Division Frequency Hopping (TDFH) – The subscriber hops to a new frequency at the start of each new TDMA frame. This gives frequency diversity against deep fades and spreads co-channel interference. GSM uses this (slow frequency hopping, 217 hops/s).
   • Advantage: avoids long deep fades and bursts of co-channel interference.
   • Disadvantage: needs a central hopping plan and tight frame synchronisation.

 DS/FHMA idea:
  freq
   ^   [DS]            [DS]
   |         [DS]
   |  [DS]                   [DS]
   +----------------------------> time
   each block = DS-spread burst at
   a pseudo-randomly chosen carrier

N-TDMA (narrowband TDMA, IS-54/IS-136 "US Digital Cellular") divides the band into 30 kHz radio channels, and each radio channel is time-shared by 3 full-rate users (assumed, as in IS-54/136).

Number of radio channels (guard band at both edges of the allocation):

N_ch = (B_t − 2·B_g) / B_c
     = (25 MHz − 2 × 20 kHz) / 30 kHz
     = (25 000 − 40) kHz / 30 kHz
     = 24 960 / 30
     = 832 radio channels

Number of simultaneous users (m = 3 TDMA slots per channel):

N_users = m × N_ch = 3 × 832 = 2496

Here B_t = 25 MHz one-way (forward or reverse) bandwidth, B_g = 20 kHz guard band, B_c = 30 kHz channel bandwidth. Since all these channels can be used inside one cluster (each cell gets a share of them), this is the number of users the cluster can serve at the same time.

Answer: 832 radio channels; with 3 users per channel, 2496 simultaneous users per cluster (if each radio channel carried only one user, it would be 832).

Comparison of FDMA and TDMA

Operation of FHMA

Frequency Hopping Multiple Access is a digital multiple access method in which the carrier frequency of each user changes (hops) in a pseudo-random way inside a wide band. The instantaneous signal is narrowband, but over time it covers the whole wideband channel.

 Transmitter
 data -> [Modulator] -> (x) -> [BPF] -> antenna
         (FSK)          ^
                  [Freq. synthesizer]
                        ^
                  [PN code generator]

 Receiver
 antenna -> (x) -> [IF filter] -> [Demod] -> data
             ^
       [Freq. synthesizer] <- [PN gen] (synchronised)

Steps:

1. Data modulates a carrier (usually BFSK or M-FSK).
2. A PN generator selects a frequency from the synthesizer; the mixer moves the signal to that hop frequency.
3. The hopping pattern is unique to each user, so two users rarely occupy the same frequency at the same time.
4. The receiver uses the same PN sequence, synchronised, to "de-hop" the signal back to a fixed IF, then demodulates it.
5. If two users hit the same frequency (a collision), errors occur; error-correcting codes and interleaving recover them.

• Slow FH: several symbols per hop (GSM, 217 hops/s). Fast FH: several hops per symbol.
• Advantages: immunity to narrowband interference and fading, no near-far problem as severe as DS-CDMA. Example: Bluetooth (1600 hops/s over 79 channels).

In FDMA the total band is split into equal channels, with a guard band at each edge of the allocated spectrum.

N = (B_t − 2·B_guard) / B_c
  = (12.5 MHz − 2 × 10 kHz) / 30 kHz
  = (12 500 − 20) kHz / 30 kHz
  = 12 480 / 30
  = 416

where B_t = 12.5 MHz total bandwidth, B_guard = 10 kHz, B_c = 30 kHz.

This is the classic AMPS case: each operator in the US had 12.5 MHz in each direction, giving 416 simplex channels each way (416 duplex channel pairs).

Answer: 416 channels

Disadvantages of spread spectrum systems

• Bandwidth inefficient for a single user – a signal of a few kHz is spread over MHz.
• Near-far problem – a nearby user's strong signal swamps far users; precise power control is needed.
• Self-jamming – codes are not perfectly orthogonal, so users interfere with each other.
• Complex receivers – code acquisition, tracking, RAKE receiver and high-speed chip processing.
• Strict synchronisation of PN codes is required.

Implementation of a CDMA system

In DS-CDMA every user transmits on the same carrier at the same time. Each user's data is multiplied by a unique high-rate PN (spreading) code. The receiver multiplies the received sum by the wanted user's code; only that user is despread back to narrowband, all others stay as wideband noise.

 Transmitter (user k)
 data d_k(t) --(x)--(x)--> sum of all users
               ^    ^         in the channel
             c_k(t) carrier
 Receiver for user 1
 r(t) --(x)--(x)--[Integrate over Tb]--> d1
          ^    ^
       carrier c_1(t)

Steps:

1. Data bit rate R_b is multiplied by code c_k(t) with chip rate R_c >> R_b. Processing gain G_p = R_c / R_b.
2. The spread signal modulates the carrier (BPSK/QPSK) and is sent.
3. The receiver gets r(t) = Σ d_k(t)·c_k(t) + noise.
4. Multiplying by c_1(t): since c_1·c_1 = 1, user 1's data is recovered; cross-terms d_k·c_k·c_1 remain spread because the codes have low cross-correlation.
5. Integrating over one bit gives d_1; interference is reduced by G_p.

Example (chip level): Let code of A = +1 −1 +1 −1, code of B = +1 +1 −1 −1 (orthogonal).

• A sends bit +1 → +1 −1 +1 −1; B sends bit −1 → −1 −1 +1 +1.
• Channel sum = 0 −2 +2 0.
• Receiver A: (0)(+1)+(−2)(−1)+(+2)(+1)+(0)(−1) = 4 → 4/4 = +1 (A's bit).
• Receiver B: (0)(+1)+(−2)(+1)+(+2)(−1)+(0)(−1) = −4 → −1 (B's bit).

Real systems (IS-95) use 1.2288 Mcps, Walsh codes for channel separation, PN short codes for cell separation, power control and a RAKE receiver.

Multiple access

Multiple access is the technique that lets many users share a limited radio spectrum (a common channel) at the same time without serious interference. The spectrum is divided in frequency, time, code or space, and each user gets a share.

Multiple access is combined with a duplexing method: FDD (separate frequencies for uplink/downlink) or TDD (separate time slots). Narrowband systems (FDMA, TDMA) give each user a small part of the band; wideband systems (CDMA) let all users use the whole band.

 freq|##|##|##|   FDMA       time| | | |  TDMA
     |  |  |  |                  |slots
 CDMA: all users fill whole freq-time plane,
       separated by codes

Near-far effect in CDMA

In the uplink, all mobiles transmit on the same frequency. A mobile near the base station is received with much higher power than a mobile far away (path loss differs by tens of dB). Because CDMA receivers treat other users as noise, the strong nearby signal raises the interference level so much that the weak far user cannot be despread. This is the near-far problem.

Mitigation techniques

1. Power control (main method): each mobile adjusts power so that all arrive at the base station with nearly equal power.
   • Open-loop: mobile sets power from received forward signal strength.
   • Closed-loop: base station sends up/down commands (800 times/s in IS-95, 1 dB steps).
   • Outer-loop: adjusts target Eb/N0 based on frame error rate.
2. Hybrid DS/FH (DS/FHMA) – a near user rarely shares the same hop frequency.
3. Time division CDMA (TCDMA) – only one user per cell transmits in a slot.
4. Multi-user detection / interference cancellation – the receiver detects strong users first and subtracts them.
5. Codes with low cross-correlation and higher processing gain.

Space division multiple access

Space Division Multiple Access (SDMA) separates users by their position in space. The base station uses directional or smart (adaptive) antennas that form narrow beams pointing at each user, so the same frequency and time slot can be reused for users in different directions within the same cell.

          beam 1 -> user A
 [BS]==== beam 2 -> user B   (same f, same t)
          beam 3 -> user C

• Advantages: higher capacity, lower interference, less transmit power, larger range.
• Ideal SDMA would give an infinitely narrow beam to each user; practical systems use sectoring or adaptive arrays.

Hybrid spread spectrum techniques that reduce the near-far effect

1. Hybrid DS/FHMA (direct sequence + frequency hopping)

• The data is first spread with a PN code (DS). The centre frequency of the DS signal then hops pseudo-randomly over the wide band.
• At any time a user occupies only a small part of the band. A strong nearby user is very unlikely to be on the same sub-band at the same moment, so it does not swamp the weak user. Hence the near-far problem is largely avoided.

 data->(x)PN code->(x)->[BPF]-> Tx
                    ^
          [Freq synthesizer]<-[hop PN gen]

• Disadvantage: complex hardware; poor support for soft handoff because the base station must follow many hopping patterns.

2. Time Division CDMA (TCDMA)

• Each cell is given a different spreading code (codes separate cells).
• Inside a cell, users are separated in time: each user gets a time slot, so only one user transmits at a time in a cell.
• Because there is never a near and a far user of the same cell received together, the near-far effect is avoided. On handoff, the user's code changes to that of the new cell.

 Cell 1 (code C1): |U1|U2|U3|U1|U2|U3|
 Cell 2 (code C2): |U4|U5|U6|U4|U5|U6|

• Disadvantage: needs slot synchronisation; per-cell capacity limited by number of slots.

Other hybrids (FCDMA, TDFH) also exist, but DS/FHMA and TCDMA are the two usually cited for near-far protection.

Principle of Frequency Hopping Multiple Access (FHMA)

In FHMA the carrier frequency of each user is changed pseudo-randomly over a wide band, following a PN hopping pattern unique to that user. At any instant each user occupies only one narrow channel, but over time it uses the whole band. Different users have different hopping patterns, so they rarely collide.

 freq ^
  f5  |      A           B
  f4  |  B         A
  f3  |            B          A
  f2  |      B           A
  f1  |  A                    B
      +-------------------------> time (hops)

• Transmitter: data → FSK modulator → mixer driven by a frequency synthesizer controlled by a PN generator → wideband channel.
• Receiver: a locally generated, synchronised hopping pattern "de-hops" the signal to a fixed IF before demodulation.
• Slow hopping: one or more symbols per hop (GSM). Fast hopping: several hops per symbol.
• A collision (two users on the same frequency at the same time) causes errors that are corrected by coding and interleaving.
• Advantages: resistance to narrowband jamming and frequency-selective fading, security. Example: Bluetooth hops 1600 times/s over 79 channels of 1 MHz.

Hybrid techniques that mitigate the near-far problem

1. Hybrid DS/FHMA

• The signal is first DS-spread by a PN code, then its centre frequency hops pseudo-randomly.
• A strong near user and a weak far user are rarely on the same hop frequency at the same time, so the near user cannot swamp the far user's despreading. Thus the near-far effect is avoided.
• Drawback: complex; soft handoff is hard since the base must synchronise to many hopping signals.

 d(t)->(x)c(t)->(x)->[BPF]->channel
                  ^
        [synth] <- [hop PN]

2. Time Division CDMA (TCDMA)

• Different cells use different spreading codes.
• Inside a cell, users are given time slots, so only one CDMA user per cell transmits at any time.
• Since near and far users of one cell are never received together, the near-far effect disappears. During handoff the user's code is switched to the code of the new cell.
• Drawback: needs time synchronisation; capacity per cell limited by slots.

 Cell A, code C_A: |u1|u2|u3|u1|u2|u3|
 Cell B, code C_B: |u4|u5|u6|u4|u5|u6|

Spread spectrum multiple access (SSMA)

SSMA uses signals whose transmission bandwidth is several orders of magnitude larger than the minimum required bandwidth. A pseudo-noise (PN) sequence spreads a narrowband message into a wideband, noise-like signal. Many users share the same wide band at the same time; each is separated by its own PN code or hopping pattern. The ratio of spread bandwidth to data rate is the processing gain, which gives immunity to interference, jamming and multipath.

There are two main variants: FHMA and DS-SS (CDMA), plus hybrid forms.

1. Frequency Hopped Multiple Access (FHMA)

• Carrier frequency of each user hops pseudo-randomly over the wide band; at any time the signal is narrowband.
• Slow FH: several symbols per hop; fast FH: several hops per symbol.

 data->[FSK mod]->(x)->[BPF]-> channel
                   ^
          [synth]<-[PN generator]

• Applications: Bluetooth, GSM slow frequency hopping, military radios.

2. Direct Sequence CDMA (DS-CDMA)

• Data bits are multiplied directly by a high-rate PN code (chips); all users use the same carrier at the same time.
• The receiver correlates with the wanted user's code; other users appear as noise.

 d(t)->(x)->(x)-> channel -> (x)->(x)->[∫]-> d
        ^    ^               ^    ^
      c(t) carrier        carrier c(t)

• Needs power control (near-far problem); uses RAKE receiver.
• Applications: IS-95 (cdmaOne), CDMA2000, WCDMA (3G UMTS), GPS.

3. Hybrid spread spectrum techniques

Advantages and applications of SSMA

• No frequency planning; universal reuse (CDMA).
• Soft capacity and soft handoff.
• Multipath diversity via RAKE receiver.
• Security and anti-jam capability (origin in military use).
• Used in cellular (IS-95, 3G), WLAN 802.11b (DSSS), Bluetooth (FHSS), satellite navigation (GPS).

(i) FDMA

• Advantages:
  1. Simple hardware; no complex synchronisation or framing needed.
  2. Narrowband channels with low ISI, so equalization is usually not needed.
• Disadvantages:
  1. Needs guard bands and costly, sharp RF filters; one transceiver per user at the base station.
  2. An idle channel cannot be used by others, wasting spectrum; capacity is low.

(ii) TDMA

• Advantages:
  1. One carrier is shared by several users, so fewer transceivers; duplexer not needed.
  2. Discontinuous transmission saves battery and allows mobile-assisted handoff in idle slots; variable bit rate by giving more slots.
• Disadvantages:
  1. Tight time synchronisation and guard times needed; overhead bits reduce efficiency.
  2. High burst rate causes ISI, so adaptive equalizers are needed.

(iii) CDMA

• Advantages:
  1. Universal frequency reuse (no frequency planning) and soft capacity, giving high capacity.
  2. Soft handoff and RAKE receiver use multipath, improving call quality; good privacy.
• Disadvantages:
  1. Near-far problem; needs fast and precise power control.
  2. Self-jamming due to imperfect code orthogonality; complex receivers and synchronisation.

Self-jamming problem in CDMA

Self-jamming arises because the spreading codes of different users are not exactly orthogonal (especially in the asynchronous uplink, where codes arrive with random delays). When the receiver despreads the wanted user, the other users' signals give non-zero cross-correlation and leave residual interference. This interference comes from users of the same system, so the system "jams itself"; it grows with the number of users and limits CDMA capacity.

Operation of FHMA

In FHMA the carrier frequency of each user hops pseudo-randomly within a wide band according to a PN code. At each instant the signal is narrowband; averaged over time, it covers the whole band.

 Transmitter
 data->[FSK modulator]->(x)->[BPF]->[PA]-> antenna
                         ^
                 [Frequency synthesizer]
                         ^
                 [PN code generator]

 Receiver
 antenna->[BPF]->(x)->[IF filter]->[FSK demod]-> data
                  ^
          [Frequency synthesizer]
                  ^
          [PN gen] (synchronised to Tx)

1. Data modulates a carrier (BFSK/M-FSK) to give a narrowband signal.
2. The PN generator picks one of many frequencies in the synthesizer; mixing shifts the signal to that hop frequency.
3. The receiver, with the same PN code synchronised, mixes the received signal down to a fixed IF (de-hopping), then demodulates.
4. Other users have different hopping patterns; occasional collisions are corrected by FEC and interleaving.

• Slow FH (GSM, 217 hops/s): many symbols per hop. Fast FH: several hops per symbol.

Two hybrid spread spectrum techniques

1. Hybrid DS/FHMA

• A DS-spread signal whose centre frequency hops pseudo-randomly.
• Advantage: avoids the near-far effect, since a strong near user is rarely on the same hop frequency.
• Disadvantage: complex; does not support soft handoff easily because the base station must synchronise to many hopping signals.

2. Time Division CDMA (TCDMA)

• Each cell uses a different spreading code; inside a cell, users take turns in time slots, so only one CDMA user per cell transmits at a time. On handoff, the user's code is changed to the new cell's code.
• Advantage: no near-far effect within a cell.
• Disadvantage: needs strict time synchronisation; capacity per cell limited by the number of slots.

(Other hybrids: FCDMA – sub-bands each running DS-CDMA, flexible spectrum use but lower processing gain; TDFH – hop each TDMA frame, gives diversity against fading but needs tight frame sync.)

Types of spread spectrum multiple access

Spread spectrum multiple access (SSMA) spreads each user's narrowband signal over a band much wider than needed, using a PN code. All users share the wide band; the code separates them. Processing gain G_p = (spread bandwidth)/(data rate) gives resistance to interference and multipath.

1. Frequency Hopped Multiple Access (FHMA)

• The carrier of each user hops pseudo-randomly over many narrow channels within the wide band.
• Slow FH: several symbols per hop; fast FH: several hops per symbol.
• Collisions (two users on the same frequency) are handled by error-correcting codes.
• Examples: Bluetooth, GSM slow hopping.

 data->[FSK]->(x)->Tx      synth driven by PN code
               ^           selects hop frequency
            [synth]<-[PN]

2. Direct Sequence Multiple Access (DS-SS / CDMA)

• Data is multiplied by a high-rate PN code (chip rate >> bit rate), spreading it over the full band. All users transmit together on the same carrier.
• The receiver correlates with the wanted code; others remain as wideband noise.
• Needs power control (near-far problem). Examples: IS-95, WCDMA.

 d(t)->(x)->(x)cos wct -> channel
        ^
      c(t) PN code

3. Hybrid spread spectrum techniques

• FCDMA: band split into sub-bands, each running DS-CDMA.
• DS/FHMA: DS signal whose centre frequency hops; avoids near-far effect.
• TCDMA: different code per cell, time slots per user inside a cell.
• TDFH: new hop frequency each TDMA frame (GSM), gives fading diversity.

FDMA vs CDMA

Multiple access

Multiple access is the method that allows many mobile users to share a limited radio spectrum simultaneously. The common resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA). Wireless systems often combine these in hybrid forms to get the advantages of each, for example GSM uses FDMA + TDMA, and hybrid spread spectrum schemes combine CDMA with TDMA or frequency hopping.

Time Division CDMA (TCDMA)

TCDMA (also called TDMA/CDMA) is a hybrid technique that combines code division between cells with time division inside a cell.

• A different spreading code is assigned to each cell.
• Within a cell, each user is given a time slot, so only one CDMA user transmits at a time in each cell.
• When the user hands off to another cell, its spreading code is changed to the code of the new cell.

 Cell 1, code C1: |U1|U2|U3|U1|U2|U3| ...
 Cell 2, code C2: |U4|U5|U6|U4|U5|U6| ...
          time ------------------------->

• Advantage: avoids the near-far effect, because a near and a far user of the same cell are never received at the same time. Code separation still lets neighbouring cells use the same frequency.
• Disadvantage: needs time synchronisation; capacity within a cell is limited by the number of slots.

Time Division Frequency Hopping (TDFH)

TDFH combines TDMA with slow frequency hopping.

• The subscriber hops to a new carrier frequency at the start of each new TDMA frame, following a hopping sequence set by the network.
• Within one frame (burst) the frequency is constant; it changes from frame to frame.

 freq ^
  f3  |        [U1]
  f2  |[U1]                [U1]
  f1  |               ...
      +---frame1--frame2--frame3--> time

• Advantages: frequency diversity – a user does not stay in a deep fade or on a heavily interfered channel for long; co-channel interference is averaged over many users, which helps in heavily loaded cells.
• Disadvantage: needs a central hopping plan and frame synchronisation between base station and mobile.
• Example: GSM uses TDFH (slow frequency hopping, about 217 hops/s, one hop per TDMA frame of 4.615 ms).

Multiple access technique

Multiple access is the technique by which many users share a common, limited radio spectrum at the same time without causing serious interference to each other. The shared resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA).

TDMA (Time Division Multiple Access)

TDMA divides one radio carrier into time slots; each user transmits only in its own slot, in bursts, using the full carrier bandwidth.

 |<-------------- one TDMA frame -------------->|
 |pre| slot1 | slot2 | slot3 | ... | slotN |
        U1      U2      U3            UN
 slot: |tail|data|training|data|tail|guard|

• Each frame has preamble (sync/address), slots and guard times.
• Transmission is discontinuous → battery saving and mobile-assisted handoff in idle slots.
• No duplexer needed; high burst rate needs equalization; tight synchronisation needed.
• Example: GSM (8 slots per 200 kHz carrier), IS-136 (3 users per 30 kHz).

CDMA (Code Division Multiple Access)

All users transmit on the same frequency at the same time; each user's data is multiplied by a unique PN spreading code with chip rate much higher than the bit rate.

 d1->(x)c1 --\
 d2->(x)c2 ---(+)--> channel --> (x)c1 --> d1
 d3->(x)c3 --/

• Receiver correlates with the wanted code; others remain as noise (reduced by the processing gain).
• Features: frequency reuse factor 1, soft capacity, soft handoff, RAKE receiver for multipath.
• Problems: near-far effect (needs power control), self-jamming.
• Example: IS-95 (1.25 MHz, 1.2288 Mcps), WCDMA.

SDMA (Space Division Multiple Access)

SDMA separates users by their spatial position. The base station uses directional or smart (adaptive) antennas that form a beam toward each user, so users in different directions can reuse the same frequency/time/code in the same cell.

           /--beam1--> user A
 [BS ant]--+--beam2--> user B
           \--beam3--> user C

• Advantages: higher capacity, less co-channel interference, lower transmit power, wider coverage.
• Simplest form: sectored antennas (120°); ideal form: adaptive arrays tracking each user.

Near-far effect

The near-far effect occurs in the CDMA uplink when a mobile close to the base station is received with much higher power than a mobile far away. Since all users share the same band and other users act as noise, the strong near signal drowns the weak far signal, which then cannot be despread correctly. It is controlled mainly by power control.

Hybrid technique: Time Division CDMA (TCDMA)

• Each cell is given a different spreading code.
• Inside a cell, users are assigned different time slots, so only one user per cell transmits at any time.
• A near user and a far user of the same cell are therefore never received together, and the near-far problem is avoided. On handoff the user's code is changed to the new cell's code.

 Cell code C1: |U1|U2|U3|U1|U2|U3|

(Hybrid DS/FHMA is another option: a DS signal whose carrier hops, so near and far users rarely share the same frequency at the same time.)

A TDMA cellular system shares one carrier among several users in time slots, while FDMA gives each user its own carrier. TDMA has these advantages over FDMA:

1. Fewer transceivers – one radio at the base station serves several users (8 in GSM), reducing cost and size.
2. No duplexer needed – transmit and receive happen in different time slots, so the handset is simpler.
3. Battery saving – the mobile transmits only in its slot (discontinuous transmission) and can switch off its transmitter between bursts.
4. Mobile-assisted handoff (MAHO) – during idle slots the mobile measures signal strength of neighbouring base stations, giving faster and more reliable handoff.
5. Flexible bit rate – a user can be given several slots for higher data rates (e.g. GPRS, HSCSD).
6. Higher capacity / spectral efficiency – digital coding, speech compression and better reuse give more users per MHz than analog FDMA.
7. Fewer guard bands and simpler filtering – wide carriers mean fewer channel edges; less adjacent-channel interference from intermodulation.
8. Digital features – easy encryption, error control coding and data services.

Principle of FHMA

Frequency Hopping Multiple Access is a spread spectrum method in which the carrier frequency of each user hops pseudo-randomly among many narrow channels inside a wide band. Each user follows its own PN hopping pattern; the receiver uses the same, synchronised pattern to de-hop the signal.

 Transmitter
 data->[FSK modulator]->(x)->[BPF]->[PA]-> antenna
                         ^
                 [Frequency synthesizer]
                         ^
                 [PN code generator]

 Receiver
 antenna->[BPF]->(x)->[IF filter]->[FSK demod]-> data
                  ^
          [Frequency synthesizer]
                  ^
          [PN gen] (synchronised to Tx)

• At any instant the signal is narrowband; over time it occupies the whole band.
• Slow FH: several symbols per hop (GSM); fast FH: several hops per symbol.
• Collisions between users are rare and are corrected by coding and interleaving.

Near-far effect in CDMA

In the CDMA uplink all mobiles share the same frequency. Path loss varies a lot with distance, so a near mobile arrives at the base station with much more power than a far mobile (often 60–80 dB difference without control). The receiver sees other users as noise, so the strong near signal swamps the weak far signal and the far user's call fails. Capacity drops sharply.

How it is solved

1. Power control – the main solution: every mobile's power is adjusted so all signals reach the base station at about the same level.
   • Open-loop: mobile estimates path loss from the received forward signal and sets its power (fast, rough).
   • Closed-loop: base station measures received Eb/N0 and sends power up/down bits (IS-95: 800 bps, ±1 dB steps).
   • Outer-loop: base station changes the target Eb/N0 to keep frame error rate at the desired level.
2. Hybrid DS/FHMA – near and far users seldom share a hop frequency.
3. TCDMA – only one user per cell transmits at a time.
4. Multi-user detection / successive interference cancellation – strong signals are detected and subtracted first.

Multiple access

Multiple access is the technique by which many users share a common, limited radio spectrum at the same time without causing serious interference to each other. The shared resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA).

Merits of CDMA

1. Universal frequency reuse (N = 1): all cells use the same carrier; no frequency planning.
2. Soft capacity: more users can be added with a gradual drop in quality.
3. Soft handoff: the mobile connects to two base stations at once; fewer dropped calls.
4. Multipath advantage: RAKE receiver combines multipath components (diversity).
5. Voice activity and sectorisation gains directly increase capacity.
6. Privacy and interference rejection due to PN spreading and processing gain.
7. Wide bandwidth reduces the effect of frequency-selective fading.

Demerits of CDMA

1. Near-far problem – needs fast and accurate power control.
2. Self-jamming – codes not perfectly orthogonal; interference from other users limits capacity.
3. Complex receivers – code acquisition, tracking and RAKE processing.
4. Strict timing – IS-95 base stations need GPS synchronisation.
5. Cell breathing – coverage shrinks as load increases.

GSM frame efficiency

Given: tail (trailing) bits = 6, guard bits = 8.25, training bits = 26, data = 2 × 58 = 116 bits.

Bits per slot   = 6 + 8.25 + 26 + 2×58 = 156.25 bits
Bits per frame  b_T  = 8 slots × 156.25 = 1250 bits
Overhead/slot   = 6 + 8.25 + 26 = 40.25 bits
Overhead/frame  b_OH = 8 × 40.25 = 322 bits

η_f = (1 − b_OH / b_T) × 100%
    = (1 − 322 / 1250) × 100%
    = (1 − 0.2576) × 100%
    = 74.24 %

(Equivalently, useful data per slot = 116/156.25 = 0.7424.)

Answer: Frame efficiency η = 74.24 %

Multiple access technique

Multiple access is the technique by which many users share a common, limited radio spectrum at the same time without causing serious interference to each other. The shared resource is divided in frequency (FDMA), time (TDMA), code (CDMA) or space (SDMA). Narrowband systems (FDMA, TDMA) give each user a small part of the spectrum; wideband systems (CDMA) let every user occupy the whole band. Each is combined with FDD or TDD duplexing.

FDMA vs CDMA